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UNIT 1 INTRODUCTION TO ORGANIC CHEMISTRY
Key unit competency
Apply IUPAC rules to name organic compounds and explain types of isomers for
organic compounds
Learning objectives
• Classify organic compounds as aliphatic, alicyclic and aromatic
• Determine different formulae for given organic compounds
• Describe the common functional groups and relate them to the homologous series
• Use IUPAC rules to name different organic compounds
• Describe the isomers of organic compounds
Introductory activity
Consider the following substances: Sodium chloride, starch, table sugar,
magnesium carbonate, glucose, sodium hydrogen carbonate, water.
1. Heat a small sample of each ( 5g for solids, 5ml for liquids) in a crucible
2. Record your observations.
3. From the observations, classify the substances listed above.
4. What criterion do you use for that classification?
5. Interpret your observations
Organic chemistry is defined as the study of the compounds mainly composed by
carbon and hydrogen atoms, and sometimes oxygen, nitrogen, phosphorus, sulphur
and halogens atoms. The study of the rest of the elements and their compounds falls
under the group of inorganic chemistry. However, there are some exceptions such
as carbonates, cyanides, carbides, carbon oxides, carbonic acid, carbon disulphide
which are considered as inorganic compounds. Since various organic compounds
contained carbon associated with hydrogen, they are considered as derived from
hydrocarbons. Thus, a more precise definition of organic chemistry is: “the study of
hydrocarbons and the compounds which could be thought of as their derivatives’’.
The organic and inorganic compounds can be differentiated based on some of their
properties as summarised in the following table.
Why to study organic chemistry as a separate branch?
The organic chemistry involves the study of all chemical reactions that are commonly
used in industries and many other organic reactions that take place in living systems.
Materials used in everyday life, food processing and other manufacturing objects are
obtained based on organic chemistry. Some other reasons are highlighted below.
• Large number of compounds: up to now, no one knows exactly the number
of organic compounds that are present in nature.
• Built of relatively few elements: The elements frequently encountered
in organic compounds are carbon, hydrogen, oxygen, nitrogen, sulphur,
phosphorous, and halogens;
• Unique characteristic of carbon to undergo catenation: carbon atom is
unique among other elements whose atoms possess the capacity to unite
with each other by the covalent bonds resulting in a long chain of carbons ( i.e:
polysaccharides, proteins, polyesters, polyamides…).
Isomerism is the existence of compounds that have the same molecular formula
but different arrangements of atoms; these compounds are called “isomers”.
• Functional groups as basis of classification: Organic molecules contain
active atoms or groups of atoms which determine their chemical behaviour.
These are called functional groups joined in a specific manner. Therefore,
organic compounds with similar functional groups display similar properties and form a class
• Combustibility: organic compounds are combustible.
• Nature of chemical reactions: organic compounds being formed by covalent
bonds, they are slow and often have a low yield.
Importance of organic chemistry
The organic chemistry is a subject that plays an important role in modern life. In
general, there is no art, science or industry where knowledge of organic chemistry
is not applied.
Examples where organic chemistry is applied:
1)Application in daily life.
In our day-to-day life, we find many substances or materials that are commonly used
and the later are made of organic compounds.
• Food: starch, fats, proteins, vegetables,...
• Clothes: cotton, wool, nylon, dacron, ....
• Fuels: petrol, diesel oil, and kerosene
• Dyes of all kinds
• Cosmetics (body lotion,…)
• Soaps and detergents
• Medicine: cortisone, sulphonamide, penicillin,…
• Drugs: morphine, cocaine,...
• Stationery: pencils, paper, writing ink,…
• Insecticides,rodenticides,ovicides …
2)Applications in industry
The knowledge of organic chemistry is required in many industries such as
manufacture of food, pharmacy, manufacture of dyes and explosives, alcohol
industry, soil fertilisers, petroleum industry, etc.
3)Study of life processes
Organic chemistry in other words is the chemistry of life. For example the vitamins, enzymes, proteins and hormones are important organic compounds
produced in our body to ensure its proper development.
(https://chemistry.tutorvista.com/organic-chemistry/hydrocarbons.html)
1.1.1. Aliphatic compounds
Aliphatic compounds are organic compounds in which the carbon atoms are
arranged in a straight or branched chain.
1.1.2. Alicyclic compounds
Alicyclic compounds are organic compounds that contain one or more carbon rings
that may be saturated or unsaturated
1.1.3. Aromatic compounds
Aromatic compounds are compounds that contain a closed ring that consists of
alternating single and double bonds with delocalised pi electrons.
Aromatic compounds are designated as monocyclic, bicyclic and tricyclic if they
contain one, two or three rings, respectively.
Examples:
Note: Heterocyclic compounds: Are also classified as cyclic compounds which
include one or two atoms other than carbon (O, N, S) in the ring.Thus furan, thiophene
and pyridine are heterocyclic compounds.
1.2. Types of formulas for organic compounds
Atoms bond together to form molecules and each molecule has a chemical formula.
In organic chemistry, we can distinguish empirical, molecular and structural formulas.
1.2.1. Empirical formula
The empirical formula is the simplest formula which expresses the ratio of the number
of atoms of each element present in a particular compound. The empirical formula
is determined using the percentage composition according to the following steps.
i. The percentage of each element, considered as grams of that element in 100g
of the compound, is divided by its atomic mass. This gives the number of moles
of the element in 100g of the compound.
ii. The result in i. is then divided by the lowest ratio (number of moles in 100g of
the compound), seeking the smallest whole number ratio.
iii. If the atomic ratios obtained in ii. are not the whole number, they should be
multiplied by a suitable common factor to convert each of them to the whole
numbers (or approximatively equal to the whole numbers). Minor fractions are
ignored by rounding up or down (ex: 7.95 = 8).
1.1.2. Molecular formula
The molecular formula is a formula expressing the exact number of atoms of each
element present in a molecule.
Molecular formula = Empirical formula x n
Example 1:
An organic compound contains 31.9% by mass of carbon, 6.8% hydrogen and 18.51%
nitrogen and the remaining percentage accounts for oxygen. The compound has a
vapour density of 37.5. Calculate the molecular formula of that compound.
Vapour density = a half molecular mass
Molecular mass = 2 x vapour density = 2 x 37.5 = 75g/mol
Note: From the above calculations, we can extend our generalized expression: : From the above calculations, we can extend our generalized expression:
% of Oxygen = 100 – (% hydrohen + % carbon)
1.2.3. Structural formulas
Structural formula shows how the different atoms in a molecule are bonded (i.e.
linked or connected)
There are three types of structural formulas: displayed, condensed and skeletal
(stick) formulas.
1.3. Functional groups and homologous series
1.3.1 Functional groups
A functional group is an atom or group of atoms in a molecule which determines
the characteristic properties of that molecule. Examples of some fuctionnal groups
are indicated in the Table 1.2.
1.3.2. Homologous series
When members of a class of compounds having similar structures are arranged in
order of increasing molecular mass, they are said to constitute a homologous series.
Each member of such a series is referred to as a “homologous” of its immediate
neighbours. For example, the following sequence of straight chain of alcohols forms
a homologous series.
Characteristics of a homologous series
1. Any member of the series differs from the next by the unit –CH2-(methylene group)
2. The series may be represented by a general formula of alcohols which is
CnH2n+1 OH where n =1,2,3, etc.
3. The chemical properties of the members of a homologous series are
similar, though in some series the first members show different behaviour.
4. The physical properties such as density, melting point and boiling point
generally increase within the molecular mass.
1.4. General rules of nomenclature of organic compounds according to IUPAC
The organic compounds are named by applying the rules set by the International
Union of Pure and Applied Chemistry (IUPAC). The purpose of the IUPAC system of
nomenclature is to establish an international standard of naming compounds to
facilitate the common understanding.
In general, an IUPAC name has three essential parts:
• A prefix that indicates the type and the position of the substituents on the
main chain.
• The base or root that indicates a major chain or ring of carbon atoms found
in the molecule’s structure. e.g. Meth- for one carbon atom, eth- for 2 carbon
atoms, prop- for 3 carbon atoms, hex- for five carbon atoms, etc.
• The suffix designates the functional group.
Example -ane for alkanes, -ene for alkenes, -ol for alcohols, -oic acid for carboxylic
acids and so on.
Steps followed for naming organic compounds:
1. Identify the parent hydrocarbon:
It should have the maximum length, or the longest chain
3. Identification of the side chains.
Side chains are usually alkyl groups. An alkyl group is a group obtained by a
removal of one hydrogen atom from an alkane. The name of alkyl group is obtained
by replacing -ane of the corresponding alkane by –yl (Table 1.3).
4.If the same substituent occurs two or more times, the prefix di, tri,tetra, ...is
attached to substituent’s name. Its locants separate the prefix from the name of the
substituent.
5.Identify the remaining functional groups, if any, and name them. Different side
chains and functional groups will be listed in alphabetical order.
The prefixes di, tri, tetra,...are not taken into consideration when grouping
alphabetically. But prefixes such iso-, neo- are taken into account.
Example:
Identify the position of the double/triple bond.
Example:
The sum of the numbers which show the location of the substituents is the possible smallest
The correct name will be the one which shows the substituents attached to the third
and fifth carbon, respectively and not to the fourth and the fiveth carbon atom.
Numbers are separated by commas Hyphens are added between numbers and
words. Successive words are merged in one word.
1.5. Isomerism in organic compounds
Isomerism is the existence of compounds that have the same molecular formula
but different arrangements of atoms; these compounds are called “isomers”.
Isomers have different physical or/and chemical properties and the difference may
be great or small depending on the type of isomerism.
There are two main classes of isomerism: Structural isomerism and stereoisomerism.
1.5.1. Structural isomerism
1. Position isomerism
Position isomers are compounds with the same molecular formula but different
positions of the functional group or substituent(s).
2. Chain isomerism
Chain isomers are compounds with the same molecular formula, belonging to the
same homologous series, with chain of carbon atoms of different length.
3. Functional isomerism
Functional (group) isomers are compounds which have the same molecular formula
but different functional groups.
Examples:
1.5.2. Stereoisomerism
1. Geometrical isomerism
Geometrical isomers or cis-trans isomers are compounds with the same molecular
formula, same arrangement of atoms but differ by spatial arrangements.
This type of isomers is mainly found in alkenes due to the restricted rotation around
the carbon-carbon double bond.
Note: For more information, visit the website below. (https://www.youtube.com/
watch?v=7tH8Xe5u8A0).
The necessary condition for an alkene to exhibit geometrical isomerism is that each
carbon doubly bonded has two different groups attached to it.
2. Optical isomerism
Optical isomers are compounds with the same molecular formula and arrangements
of atoms but have different effect on the plane polarised light.
• A compound that rotates the plane polarised light is said to have an optical activity.
• This type of isomerism occurs in compounds containing an asymmetric
(asymmetrical) carbon atom or chiral centre1.
.
• When a molecule has chiral centre, there are two non superimposable isomers
that are mirror images of each other.
• Such compounds are called enantiomers
In a mirror, the left hand is the image of the right hand and they are non
superimposable, i.e. they are enantiomers. An achiral object is the same as its mirror
image, they are nonsuperimposable.
Activities: 5 -
UNIT 2: ALKANES
Key unit competency
Relate the physical and chemical properties of the alkanes to the preparation
methods, uses and isomerism.
Learning objectives
• Name straight chain alkanes up to carbon-20
• Define homologous series
• Use IUPAC system to name straight and branched alkanes
• Describe the preparation methods of the alkanes
• Prepare and collect methane gas
• Respect of procedure in experiment to carry out preparation of methane or propane
• Describe and explain the trend in physical properties of homologous series ofalkanes
• Be aware of the dangers associated with combustion reactions of the alkanes
• Write reaction for free radical mechanism for a photochemical reaction
• State the chemical properties of the alkanes
• Develop practical skills,interpret results make appropriate deductions.
• Appreciate the importance of the alkanes in daily life
• Appreciate the dangers caused by the alkanes to the environment as major
sources of air contaminants
• State the uses of the alkanes
Alkanes are the simplest class of organic compounds. They are made of carbon and
hydrogen atoms only and contain two types of bonds, carbon-hydrogen (C-H) and
carbon-carbon (C-C) single covalent bonds. They do not have functional groups.
Alkanes form a homologous series with the general formula CnH2n+2 where n is the
number of carbon atoms in the molecule. The first member of the family has the
molecular formula CH4 (n=1) and is commonly known as methane and the second
member with molecular formula is C2H6 (n=2) is called ethane.
These compounds are also known as saturated hydrocarbons. This name is more
descriptive than the term “alkane’’ because both their composition (carbon and
hydrogen) and the fact that the four single covalent bonds of each carbon in their
molecules are fully satisfied or ‘’saturated’’.
The name alkane is the generic name for this class of compounds in the IUPAC
system of nomenclature. These hydrocarbons are relatively unreactive under
ordinary laboratory conditions, but they can be forced to undergo reactions by
drastic treatment. It is for this reason that they were named paraffins (Latin parum
affinis = little activity).
2.1. Nomenclature of alkanes
IUPAC Rules for the nomenclature of alkanes
a. Find and name the longest continuous carbon chain.
b. Identify and name groups attached to this chain.
c. Number the chain consecutively, starting at the end nearest a substituent
group.
d. Designate the location of each substituent group by an appropriate
number and name.
e. Assemble the name, listing groups in alphabetical order. The saturated
hydrocarbon form homologous series (series in which members have similar
chemical properties and each differs from the preceding by a methylene
group –CH2-).
The first four members are known by their common names, from C5 and
above the Roman prefixes indicating the number of carbon atoms is written
followed by the ending “ane” of the alkanes.
Note: Alkyl groups are obtained when one hydrogen atom is removed from
alkanes; therefore their names are deduced from the corresponding alkanes
by replacing “ane” ending with “yl” desinence (Table 2.1)
Prefixes di, tri, tetra, sec, tert, are not considered when alphabetizing.
f. In case of chains of the same length, the priority is given for part where
many branched of alkyl groups appear.
g. For cyclanes or cycloalkanes, the prefix “cyclo” is recommended, followed
by the name of the alkanes of the same carbon number.But in case of ramified cyclanes, the priority is for the ring.
Note: If there are more than one substituent, the numbering is done so that the
sum of the numbers used to locate the locants is minimum. This is the lowest sum
rule.
2.2. Isomerism
Alkanes show structural isomerism. The easiest way to find isomers is to draw the
longest chain of carbon atoms first and then reduce it by one carbon first until
repetition begins to occur.
d. Substitute one hydrogen on carbon (3) by the mathyl group
e.Longest chain reduced further to 4 carbon atoms by cutting 2 methyl group
-Putting the methyl group on position 1 or 5 gives you the same straight chain
isomer.
2.3 Occurrence of Alkanes
1. The alkanes exist in nature in form of natural gases and petroleum. Natural
gas and petroleum existence are the results of decomposition of died
bodies after many years ago.
2. The most natural gas is found in lake Kivu as methane gas but in form of
traces like ethane, propane and butane
3. Petroleum is the most world energy, it is formed by decomposition by
bacteria for millions of years died marine living things and as the last
product is petroleum and natural gases which are separated in fractional
distillation of their crude oil and the results are obtained according to their
boiling point.
2.4. Preparation of alkanes
Note: The reaction is practically used to reduce by one carbon the length of carbon chain. It is referred as decarboxylation of sodium carboxylates.Other reactions used for the preparation of alkanes are the following:
1. Addition reaction of hydrogen to alkenes and alkynes in the presence of
catalyst like Nickel, Palladium or platinum produces alkanes: this reaction
is called hydrogenation reaction of alkenes and alkynes; it is also called a
reduction reaction of alkenes and alkynes.
2. From halogenoalkanes or Alkyl halides
On reduction of alkyl halides with Zn and concentrated hydrochloric acid, alkyl
halides are converted to alkanes.
b) Alkyl halides when heated with sodium metal in ether solution give higher alkanes (alkanes with more carbon atoms) (Wurtz reaction).
c) When Alkyl halides are treated with Zn-Cu couple, in the presence of ethanol,
alkanes are formed.
Note: Zn-Cu couple is obtained by adding Zinc granules in aqueous copper (II)
sulphate solution where copper is deposited on the Zn pieces.
3. From carbonyl compounds
Reduction of carbonyl compounds, with amalgamated Zinc (alloy made of zinc
and mercury) and HCl. This is the Clemmensen reduction).
2.5. Physical properties of alkanes
The above Table shows that the boiling and melting points of homologue alkanes
increase with the number of carbon i.e. molecular mass.
Explanation:
The boiling and melting points depend on the magnitude of the Van Der Waal’s
forces that exist between the molecules. These forces increase in magnitude with
molecular mass.
Note: Branched chain isomers have lower boiling and melting points than their
straight chain isomers, because straight chain isomers are closer packed than the
branched chain isomers.
2.6. Chemical properties of alkanes
Generally, alkanes are quite inert towards common reagents because:
• The C-C bond and C-H bonds are strong and do not break easily.
• Carbon and hydrogen have nearly the same electronegativity valuehence
• C-H bond only slightly polarized; generally C-H bond is considered as covalent.
• They have unshared electrons to offer.
They, however, undergo the following reactions.
1. Reaction with oxygen
Alkanes react with oxygen to produce carbon dioxide (if oxygen is enough to
burn all quantity of hydrocarbons), or carbon monoxide or carbon if oxygen is in
insufficient quantity, and water. This reaction is called “combustion”
Carbon dioxide (CO2) produced from the burning of alkanes or fossil fuels for heating, transport and electricity generation is the major atmospheric pollutant that increases the green house potential of the atmosphere .Carbon dioxide is the major Green House Effect (GHE) gas.
Burning wood and forests produce also carbon dioxide and lead to the increase of that gas in the atmosphere. Methane as another GHE gas is produced by human activities, agriculture (Rice), and cattle-rearing.
There are many natural ways of reducing atmosphere carbon dioxide:
i. Water in seas dissolves millions of tonnes of gas (but less now than it did inthe past, since the average ocean temperature has increased by 0.5 oC in the last 100 years, and gases are less soluble in hot than in cold water).
ii. Plankton can fix the dissolved carbon dioxide into their body mass by photosynthesis
iii. Trees fix more atmospheric carbon dioxide than do grass and other vegetation through photosynthesis according to the equation below.
There are other ways than natural ways of reducing GHE gases and among them
there are the use of technologies that reduce the green house gas emissions, the
recycling of the GHE.
Notice: (i) Br2
reacts as Cl2
but slowly while iodine reacts hardly or does not.
Notice: (i) Br2 reacts as Cl2 but slowly while iodine reacts hardly or does not
,Fluorine, the most electronegative element of the periodic table reacts with
alkanes to give coke,
i.e. a decomposition reaction:
A mechanism of a reaction is a description of the course of the reaction which
shows steps of the reaction and the chemical species involved in each step.
The mechanism for the reaction between methane and bromine is the following:
(ii) Due to radical formation involved, the main product of reaction is the one from
the most stable radical, starting with tertiary, secondary, primary and methyl in
decreasing order of stability.
A tertiary free radical is better stabilised by the electron donating methyl groups
than the secondary, primary and methyl ones where the carbon atom is attached
to more hydrogen atoms
3. Dehydrogenation of alkanes gives alkenes under heat and a catalyst like V2O5
4. Cracking
On heating or in the presence of a catalyst, large molecules of alkanes are
decomposed into smaller alkanes and alkenes. If the cracking is performed on
heating, it is referred as themocracking.
If the cracking is performed using a catalyst; it is referred as catalytic cracking and
many products result from one reactant as shown below.
2.7. Uses of alkanes
1. Methane (CH4)
Methane finds many uses:
• It is used as a fuel at homes, ovens, water heaters, kilns and automobiles as it
combusts with oxygen to produce heat.
• Highly refined liquid methane is used as rocket fuel.
• Methane is used as fuel for electricity generation.
• It is used as a vehicle fuel in the form of liquefied natural gas (LNG).
• Methane can be used as raw material in the production of urea, a fertilizer.
In general, methane is more environmental friendly than gasoline/petrol and
diesel.
2. Butane (C4H10)
• Butane is a key ingredient of synthetic rubber.
• It is used as fuel in cigarette lighters.
• When blended with propane and other hydrocarbons, it may be referred to
commercially as LPG, for liquefied petroleum gas.
• Butane gas cylinders are used in cooking.
• Also used in aerosol spray cans.
3. Propane (C3H8)
• Propane is used as a propellant for aerosol sprays such as shaving creams
and air fresheners.Used as fuel for home heat and back up electrical
generation in sparsely populated areas that do not have natural gas
pipelines.
• Propane is commonly used in movies for explosions
4. Ethane (C2H6)
• Ethane is used in the preparation of ethene and certain heavier
hydrocarbons.
• Ethane can be used as a refrigerant in cryogenic refrigeration systems.
5. Pentane (C5H12)
• Pentane is used in the production of polystyrene foams and other foams.
• Used in laboratories as solvents.
• It is also an active ingredients of pesticides.
• Used as solvent in liquid chromatography
6. Hexane (C6H14)
• It is used in the formulation of glues for shoes, leather products, and roofing.
• It is also used to extract cooking oils such as canola oil or soy oil from seeds.
• Hexane is used in extraction of pyrethrine from pyrethrum; e.g. Horizon
SOPYRWA (a pyrethrum factory in Musanze District).
• Also for cleansing and degreasing a variety of items, and in textile
manufacturing.
7. Heptane (C7H16)
• Heptane is used as solvent in paints and coatings.
• Pure n-heptane is used for research, development and pharmaceutical
manufacturing
• Also as a minor component of gasoline.
• It is used in laboratories as a non-polar solvent.
Activities: 0 -
UNIT 3: ALKENES AND ALKYNES
Key unit competency
Relate the physical and chemical properties of alkenes and alkynes to their
reactivity and uses
Learning objectives
• Explain the reactivity of alkenes in comparison to alkanes
• Explain the existence of geometrical isomerism in alkenes
• Describe the industrial process of preparing alkenes and alkynes
• Apply IUPAC rules to name alkenes and alkynes
• Carry out an experiment to prepare and test ethene gas
• Outline the mechanisms for electrophilic addition reactions for alkenes and
alkynes
• Write the structural formulae of straight chain alkenes and alkynes
• Apply Markovnikov’s rule to predict the product of hydrohalogenation of
alkenes
• Classify alkynes as terminal and non-terminal alkynes using their different
structures
• Appreciate the combustion reaction as source of fuels.
• Appreciate the uses and dangers of addition polymers (polythene used for
polythene bags, polypropene for plastic bottles etc.)
Introductory activity
Observe the following picture and answer the questions that follow.
1. What is the collective name of the substances used to manufacture the
items showed in the above picture?
2. a. What are the raw materials used in the manufacture of the substances
identified in 1)?
b. These raw materials may be obtained from different sources. Discuss
this statement.
c. Do you expect these raw materials be soluble or not in water? Justify
your answer.
3. Even though the items which appear in the picture above are interesting,
they also present some disadvantages. Discuss this statement.
3.1. Definition, structure and nomenclature of alkenes
Alkenes are a homologous series of hydrocarbons which contain a carbon-carbon
double bond. Since their skeleton can add more hydrogen atoms, they are referred
as unsaturated hydrocarbons.
The general formula of alkenes is CnH2n.
Alkenes are abundant in the nature and play important roles in biology. Ethene,
for example, is a plant hormone, a compound that controls the plant’s growth and
other changes in its tissues.
Ethene affects seed germination, flower maturation, and fruit ripening.
They are described as unsaturated hydrocarbons because they can undergo
addition reactions.
The double bond in alkenes is made of one sigma bond and one pi bond. This gives rise to the impossibility of rotation around the double bond. The hybridization state in alkenes is sp2 and the structure around each carbon doubly bonded is trigonal planar with a bond angle value of 120o
.IUPAC names of alkenes are based on the longest continuous chain of carbon
atoms that contains the double bond.
The name given to the chain is obtained from the name of the corresponding
alkane by changing the suffix from –ane to –ene.
If the double bond is equidistant from each end, number the first substituent that
has the lowest number. If there is more than one double bond in an alkene, all of
the bonds should be numbered in the name of the molecule, even terminal double
bonds. The numbers should go from lowest to highest, and be separated from one
another by a comma.
The chain is always numbered from the end that gives the smallest number for the
location of the double bond.
In naming cycloalkenes, the carbon atoms of the double bond are numbered
1 and 2 in the direction that gives the smallest numbers for the location of the
substituents.
If a compound contains two or more double bonds, its location is identified by a
prefix number. The ending is modified to show the number of double bonds:
• a diene for two double bonds,
• a triene for two three bonds
• a tetraene for four double bonds
3.2. Isomerism in alkenes
Alkenes exhibit two types of isomerism: structural isomerisms and
stereoisomerism.
1. Structural isomerism
Alkenes show as well position isomerism, chain isomerism and functional isomerism.
In position isomerism, the position of the double bond changes but the length
of the chain remains the same.
Alkenes and cycloalkanes have two fewer hydrogen atoms than alkanes. That is
why, they have the same molecular formula. However, they belong to different
homologous series. Therefore, they are functional group isomers. This isomerism
that relates open chain compounds to ring chain compounds is referred to as ring
isomerism.
2. Stereoisomerism
Due to the impossibility of rotation around the double bond, alkenes give rise to
cis-trans or geometrical isomerism.
3.3. Preparation of alkenes
Different methods are used for the preparation of alkenes. Most of them are
elimination reactions.
1. Dehydration of alcohols
An alkene may be obtained by dehydration of an alcohol. The reaction involves
the loss of H and OH (water) from adjacent carbons of an alcohol to form an alkene. The dehydration is carried out by heating an alcohol with concentrated sulphuric acid or 85% phosphoric acid.
If two or more alkenes may be obtained, the one having more substituents on
the double bond generally predominates. This is the Zaitsev’s rule.
This is due to the stability of the intermediate carbocation. The carbocation
produced in step 2 may undergo a transposition (rearrangement) of a hydride ion
or a methyl group giving a more stable carbocation and therefore a more stable
alkene.
Mechanism
2. Dehydrohalogenation of halogenoalkanes
alkenes. The reaction follows the Zaitsev’s rule.
3. Dehalogenation of dihalogenoalkanes
When a compound containing two halogen atoms on the adjacent carbon
atoms is treated with magnesium or zinc it transforms to an alkene.
3.4. Laboratory preparation and chemical test for ethene
Activity 3.4
Preparation of ethene Set up the apparatus as shown in the Figure below (Figure 3.1) and follow the instructions to perform the experiment on the preparation ofethane
Requirements:
Chemicals:
• Ethanol, aluminium oxide, lime water, mineral wool, bromine water,
acidified potassium permanganate solution (very dilute), water.
Additional apparatus:
• Boiling tube
• Rubber stopper with hole
• Delivery tube
• Trough
• Test- tube rack
• 5 test tubes
• 5 rubber stoppers for test tubes
• Spatula Procedure and setting
• Bunsen burner
• Glass rod
• Splint
• Matches
1. Preparation of ethene:
- Pour some ethanol into the boiling tube to a 3 cm depth
- Add some glass wool to soak up the ethanol, using a glass rod to push the wool
down the tube.
- Clamp the boiling tube in a horizontal position using a retort stand.
- Put a small amount of aluminium oxide about half way along the boiling tube.
- Complete the set up of the apparatus as shown in the diagram above.
- Light the Bunsen burner, adjust it to a blue flame and heat the aluminium
oxide. (Make sure the test tube is filled with water when you start to collect the
gas produced.)
- As the aluminium oxide gets hot the heat reaches the ethanol at the end of the
tube. The ethanol then changes to vapour, passes over the hot aluminium oxide
and is dehydrated to produce ethene gas.
- Collect 5 test tubes of the gas and put a stopper on each tube when it is filled.
- When the test tubes have all been filled, loosen the retort stand and raise the
apparatus so that the delivery tube no longer dips into the water. This avoids
suck back of water as the tube begins to cool which could cause the boiling
tube to crack. Turn off the Bunsen burner.
2. Testing the properties of ethene
Addition of bromine:
- Taking great care, add about 1ml of the test tube of bromine water to one of
the test tubes of ethene.
- Replace the stopper and shake the tube a few times.
- Record your observations.
- Write down your conclusions
- Addition of acidified potassium permanganate:
- Add about 1ml of very dilute potassium permanganate solution to one of the
test tubes of ethene and shake the tube a few times.
- Record your observations.
- Write down your conclusions
Combustion:
- Remove the stopper of one of the tubes filled with ethene and apply a light to
the mouth of the test tube using a lighted splint.
- Allow the gas to burn and when it has stopped burning add a small amount of
lime water to the test tube, stopper it and shake the tube a few times.
- Write down your observations.
Interpretation
When ethanol is heated in the presence of aluminium oxide, a gas is produced. This
gas does not react with lime water. This means that the produced gas is not carbon
dioxide. The equation of the reaction is:
The gas decolourises bromine water. Bromine water is a test used to identify the
presence of a carbon-carbon double bond or triple bond. The bromine adds across
the double bond and a dibromoalkane is formed. The reaction between alkene and
bromine water is shown below:
If you shake an alkene with bromine water (or bubble a gaseous alkene through
bromine water), the solution becomes colourless. Alkenes decolourise bromine
water.
The Figure 3.2 shows Bromine water added to ethene: before the reaction (left)
the color of bromine appears, and after the reaction (right) the colour of bromine
disappears.
When ethene reacts with acidified potassium manganate (VII), the purple colour of
the permanganate solution turned to colourless or light pink indicating the presence
of the carbon – carbon double bond.The reaction is the following:
The gas burns with a smoky flame producing carbon dioxide and heat energy. The
carbon dioxide produced turns into milky lime water.
3.5. Physical properties of alkenes
• Alkenes which have less than 5 carbon atoms are gaseous at ordinary
temperature, the other are liquid up to 18 while others are solids as the
number of carbon atoms increases.
• Boiling points and melting points of alkenes are less than those of alkanes
but also increase as the molecular weight increase.
• Alkenes are insoluble in water but soluble in most organic solvents.
• Cis-alkenes have a slightly higher boiling point than the trans-isomers
because the dipole moments in trans structures cancel each others----.
3.6. Chemical properties
3.6.1. Addition reactions
3.6.1.1. Electrophilic additionsAlkenes are far more reactive than alkanes due to the carbon-carbon double bond.
These compounds are unsaturated and they can easily undergo addition reactions
to yield saturated products.
The double bond in alkenes is a region of high density of electrons. Therefore, this
region is readily attacked by electrophiles. An electrophile is an atom, a molecule
or an ion which is electron-deficient; i.e. it is a Lewis acid or an electron pair acceptor.
1. Addition of hydrogen halides
Hydrogen halides (HCl, HBr, HI) react with alkenes to yield halogenoalkanes. The
reaction is carried out either with reagents in the gaseous state or in inert solvent
such as tetrachloromathane.
When hydrogen halides add to unsymmetrical alkenes, the reaction leads to the
formation of two products in two steps. The first step leads to the formation of
two different carbocations with the major product formed from the more stable carbocation. This is the Markownikov’s rule. That is “The electrophilic addition of an unsymmetric reagent to an unsymmetric double bond proceeds by involving the most stable carbocation.
2. Addition of water
The hydration of alkenes catalysed by an acid is an electrophilic addition. Ethene
can be transformed into ethanol. The first step consists of adding concentrated
sulphuric acid. The second step consists of the hydrolysis of the product of the
first step.
In industry the reaction is carried out at approximately 300 °C in the prence of
phosphoric acid as a catalyst.
3. Addition of cold concentrated sulphuric acid
When cold concentrated sulphuric acid reacts with alkene, an alkyl hydrogen
sulphate is obtained. If the starting alkene is unsymmetrical, two different alkyl
hydrogen sulphates are obtained. If the alkyl hydrogen sulphate is warmed in
the presence of water, an alcohol is obtained.
4. Addition of halogens
The addition of halogens (halogenation) on alkenes yields vicinal
dihalogenoalkanes. The reaction takes place with pure reagents or by mixing
reagents in an inert organic solvent.
When a chlorine or bromine molecule approaches an alkene, the pi electrons
cloud interact with the halogen molecule causing its polarisation.
3.6.1.2. Hydrogenation
In the presence of a catalyst (Pt, Ni, Pd), alkenes react with hydrogen to give
alkanes.
Alkenes are readily oxidised due to the presence of the double bond.
1. Reaction with oxygen
i. Transformation to epoxides
Ethene react with oxygen in the presence of silver as a catalyst to yield epoxyethane.
3. Reaction with potassium permanganate
Alkenes react with dilute potassium permanganate solution to give diols. The
reaction takes place in the cold.
The colour change depends on the medium of the reaction.
4. Hydroformylation
The hydroformylation is a process by which alkenes react with carbon monoxide
and hydrogen in the presence of rhodium catalyst to give aldehydes.
3.6.3. Addition polymerisation
Alkenes undergo addition polymerisation reaction to form long chain polymers.i.e
a polymer is a large molecule containing a repeating unit derived from small unit
called monomers. A polymerisation reaction involves joining together a large number of small molecules to form a large molecule.
Many different addition polymers can be made from substituted ethene
compounds.
Each polymer has its physical properties and therefore many polymers have wide
range of uses.
Mechanism for the polymerisation of ethene.
1. Initiation
It is a free radical initiation.
2. Propagation
3. Termination
where the part between brackets indicates a unit of the formula of the polymer
that repeats itself in the formula; n indicates the number of the units in a formula of
a polymer and is a very large number.
Summary of most alkene polymers obtained from alkenes as monomers and their
uses (Table 3.1)
3.7. Structure, classification and nomenclature of alkynes
A triple bond consists of one sigma bond and two pi bonds. Each carbon of the triple
bond uses two sp orbital to form sigma bonds with other atoms. The unhybridised 2p
orbitals which are perpendicular to the axes of the two sp orbitals overlap sideways
to form pi bonds.
According to the VSEPR model, the molecular geometry in alkynes include bond
angle of 180o around each carbon triply bonded.Thus, the shape around the triple
bond is linear.
There are two types of alkynes: terminal alkynes and non-terminal (internal)
alkynes
A terminal alkyne has a triple bond at the end of the chain e.g.: : R-C ≡ C − H
A non-terminal alkyne has a triple bond in the middle of the chain: R − C ≡ C − R'
Examples
Alkynes are named by identifying the longest continuous chain containing the
triple bond and changing the ending –ane from the corresponding alkane to –yne
3.8. Laboratory and industrial preparation of alkynes
1. Preparation of ethyne
Activity: 3.8
Set up the apparatus as shown in the diagram below.
Procedure:
• Place 2g of calcium carbide in a conical flask
• Using the dropping funnel, add water drop by drop.
• Collect the gas produced in the test tube.
• Remove the first tube and connect a second test tube.
• To the first test tube add two drops of bromine water. Record your observations
• To the second tube add two drops of potassium manganate (VII). Record your observations.
Ethyne (acetylene) can be prepared from calcium carbide which is obtained by
reduction of calcium oxide by coke at high temperature.
When bromine water is added to acetylene, the red colour of bromine is discharged.
The solution becomes colourless.
The decolourisation of bromine water is a test for unsaturation in a compound.
When potassium manganate (VII) is added to acetylene, its purple colour is
discharged.
2. Alkylation of acetylene
The hydrogen atom of ethyne as that of other terminal alkynes is slightly acidic and therefore it can be removed by a strong base like NaNH2 or KNH2.The products of the reaction are acetylides. Acetylides react with halogenoalkanes to yield higher alkynes.
3. Dehydrohalogenation
The dehydrohalogenation of vicinal or geminal dihalogenoalkanes yields alkynes
4. Dehalogenation
The dehalogenation of a tetrahalogenoalkane yield an alkyne.
3.9. Physical properties of alkynes
Alkynes are non-polar compounds with physical properties similar to those of
alkenes with the same number of carbon atoms. Their linear structure gives them
greater intermolecular forces than alkenes
3.10. Chemical reactions of alkynes
Addition reactions
As unsaturated hydrocarbons, alkynes are very reactive. Because they are unsaturated
hydrocarbons, alkynes undergo addition reactions. Alkynes can add two moles of reagents.
Even though they have a higher electron density than alkenes, they are in general less reactive because the triple bond is shorter and therefore the electron cloud is less accessible.
1. Addition of hydrogen halides
Alkynes react with hydrogen halides to yield vicinal dihalogenoalkanes, the reaction follows the Markownikov’s rule. The reaction takes place in four steps.
2. Addition of water
Alkynes react with water in the presence of sulphuric acid and mercury sulphate at 60o C to give carbonyl compounds.
3. Hydrogenation
The hydrogenation of alkynes in the presence of palladium catalyst gives alkanes
The reaction requires two moles of hydrogen for a complete saturation.
In the presence of Lindlar catalyst, the alkynes are partially hydrogenated giving alkenes4
A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on
calcium carbonate and poisoned with different lead derivatives such as lead oxide
or lead acetate. A heterogeneous catalyst is the one which is in the phase different
from that of the reactants.
4. Reaction with metals
Terminal alkynes react with active metals to yield alkynides and hydrogen gas.
Internal alkynes do not react as they do not have an acidic hydrogen atom.
5. Reaction with metal salts
When a terminal alkyne is passed through a solution of ammoniacal silver nitrate, a white precipitate of silver carbide is formed.
When a terminal alkyne is passed through a solution of ammoniacal copper(I)
chloride, a red precipitate of copper(I)carbide is formed.
The reactions above are used to:
• Differentiate between terminal and non-terminal alkynes.
• Differentiate ethene and ethyne
The reaction shows that hydrogen atoms of ethyne are slightly acidic, unlike those
of ethene.
3.11. Uses of alkenes and alkynes
Activity 3.11
Look at the picture below and appreciate the importance of alkenes and alkynes.
• Alkenes are extremely important in the manufacture of plastics which have
many applications such as: packaging, wrapping, clothing, making clothes,
artificial flowers, pipes, cups, windows, ...
• Ethene is a plant hormone involved in the ripening of fruits, seed germination,
bud opening;
• Ethene derivatives are also used in the making of polymers such as polyvinylchloride (PVC), Teflon,...
• Alkenes are used as raw materials in industry for the manufacture of alcohols, aldehydes, ...
• Alkynes are used in the preparation of many other compounds. For exampleethyne is used in the making of ethanal, ethanoic acid, vinyl chloride, trichloroethane, ...
• Ethyne (acetylene) is used as a fuel in welding and cutting metals.
• Propyne is used as substitute for acetylene as fuel for welding.
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UNIT 4: HALOGENOALKANES (ALKYL HALIDES)
Key unit competency
The learner should be able to relate the physical and chemical properties of halogenoalkanes to their reactivity and their uses
Learning objectives
• Define halogenoalkanes and homologous series.
• Explain the reactivity of halogenoalkanes.
• Explain the physical properties of halogenoalkanes.
• Describe preparation methods for halogenoalkanes.
• Explain different mechanisms in halogenoalkanes.
• Explain the uses and dangers associated with halogenoalkanes.
• Draw displayed structural formulae of halogenoalkanes and give names
using IUPAC system.
• Classify halogenoalkanes according to developed formula as primary,
secondary and tertiary.
• Write reaction mechanisms of halogenoalkanes as SN1, SN2, E1 and E2.
• Test for the presence of halogenoalkanes in a given sample organic
compound.
• Appreciate the uses and dangers of halogenoalkanes in everyday life.
• Develop the awareness in protecting the environment.
• Develop team work approach and confidence in group activities and
presentation sessions.
4.1. Definition and nomenclature of halogenoalkanes
1. Definition
Halogenoalkanes compounds are com
pounds in which the halogen atoms like
chlorine, bromine, iodine or fluorine are attached to a hydrocarbon chain. When
the halogen atom is attached to a hydrocarbon chain the compound is called a halogenoalkane or haloalkane or an alkyl halide.
Halogenoalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group.
2. Nomenclature of halogenoalkanes
Halogenoalkanes are organic compounds that contain a halogen atom: F, Cl, Br, I.
They are named using the prefixes fluoro-, chloro-, bromo- and iodo-.
Numbers are used if necessary to indicate the position of the halogen atom in the
molecule.
4.2. Classification and isomerism
4.2.1. Classification of halogenoalkanes
There are three types of halogenoalkanes:
A primary halogenoalkane has a halogen atom attached to the ended carbon atom
of the chain. A secondary halogenoalkane has a halogen atom attached to a carbon
bonded to two other carbon atoms while a tertiary halogenoalkane has a halogen
atom attached to a carbon bonded to three other carbon atoms.
4.2.2. Isomerism
Halogenoalkanes exhibit both chain and position isomerism.
Example: Molecular formula C4H9Br
a. Chain isomerism: This arises due to arrangement of carbon atoms in chains of
different size.
b. Position isomerism: This arises due to the different positions taken by the
halogen atom on the same carbon chain.
The following compounds are position isomers: CH3 CH2 CH2 CH2-Br and CH3 CH2 CH Br CH3; because the atoms of bromine are on different positions of the chain.
Hence, all isomers of the compound with molecular formula C4H9 Br are the following.
4.3. Physical properties of halogenoalkanes
1. Volatility
Volatility is a property that shows if a substance transforms easily or not into vapour
or gaseous form. This property depends on the nature of the bonds that make up
the molecule of the substance. Generally non polar covalent compounds are more
volatile than polar covalent compounds. We know that halogens when bonded to
other atoms form polar bonds because they possess high electronegativities: F =
4.0, Cl = 3.0, Br = 2.8, I
= 2.5, and C = 2.5.
The more the difference of electronegativities of the atoms that form the bond,
the more polar is the bond. This explains the high polarity of C-F bond with an
electronegativity difference of 1.5, and the low polarity of C-Cl and C-Br bonds where
the electronegativity differences are 0.5 and 0.3 respectively.
The presence of polarity or charge distribution results into more attraction between
polar molecules called dipole-dipole attraction forces, one type of Van der Waals
forces, as shown below:
The dashed line represents the attraction forces between the polar molecules or
dipoles.
Therefore, more energy must be supplied to separate polar molecules and this
explains why melting and boiling temperatures of fluoroalkanes and chloroalkanes
are higher than those of alkanes of similar molecular mass.
As we have already learnt, molecules of organic halogen compounds are generally
polar. Due to the greater polarity as well as higher molecular mass as compared
to the parent hydrocarbons, the intermolecular forces of attraction (dipole-dipole
and Van der Waals) are stronger in the halogen derivatives. That is why the boiling
points of chlorides, bromides and iodides are considerably higher than those of the
hydrocarbons of comparable molecular mass (Table 4.1).
Chloromethane, bromomethane, chloroethane and some chlorofluoromethanes
are gases at room temperature. Higher members are liquids or solids.
The attractions get stronger as the molecules get bigger in size. The pattern of
variation of boiling points of different halides is depicted in Figure 4.1. For the same
alkyl group, the boiling points of alkyl halides increase in the order: RF <RCl < RBr, <
RI This is because with the increase in size and mass of halogen atom, the magnitude of Van der waal forces increases.
2. Solubility
The solubility is the capacity of a substance to dissolve in a given solvent; in chemistry the most common solvent we refer to is water. It is a result of the interaction between the molecules of the substance, a solute, and the molecules of the solvent.
Polar molecules can interact with water molecules, but the attractive forces set
up between water molecules and molecules concerned are not as strong as the
hydrogen bonds present in water. Halogenoalkanes therefore, although they
dissolve more than alkanes, are only slightly soluble in water.
3. State
The state of matter is the physical appearance of that matter: solid, liquid and
gaseous.
Chloromethane, bromomethane, chloroethane and chloroethene are colourless
gases at room temperature and pressure. The higher members are colourless
liquids with a sweet pleasant smell
4. Density
The density is a measure of the quantity of matter by volume unit. Cotton wool is
less dense than sand because if you compare the quantity of matter cotton wool
and sand contained in for instance 1m3, you find that there more matter in sand than in cotton wool.
The density of halogenoalkanes increases in the order RCl < RBr < RI, since the
atomic weight of halogens increases in order Cl < Br < I. Iodo, bromo and polychloro
derivatives are denser than water but chloro derivatives are less dense than water.
4.4. Preparation methods of halogenoalkanes
1. From alkenes and alkynes
Direct halogenation of alkanes in the presence of ultraviolet light gives alkyl halides and a hydrogen halide.
Activities: 0 -
UNIT 5: ALCOHOLS AND ETHERSKey unit competency:To be able to compare the physical and chemical properties of alcohols and ethers to their preparation methods, reactivity and uses.Learning objectives:• Distinguish between alcohols from other organic compounds by representing the functional group of alcohols• Classify primary, secondary and tertiary alcohols by carrying out the method of identification• Write the name of alcohols by using IUPAC system• Describe the physical properties of alcohols to other series of organic compounds• Carry out the method of preparation of alcohols• Describe the local process of making alcohol by fermentation.• Explain the effect of oxidation on urwagwa when it overstays• Compare the physical, chemical and the method of preparation of alcohols to ethers• State the use of ethers
5.1. Definition and nomenclature
5.1.1. DefinitionAlcohols are organic compounds that are derivatives of hydrocarbons where one or more hydrogen atoms of hydrocarbon is or are replaced by hydroxyl (-OH) group. They are represented by the general formula: CnH2n+1OH or ROH where R is a radical: alkyl group made by a chain of carbon atomsAlcohols are called monohydric if only one hydroxyl group is present (eg: CH3CH2-OH). Dihydric alcohols are those with two hydroxyl group (diol: vicinal and gem), trihydric (triols) and polyhydric are those with many – C-OH groups. The functional group attached is –OH group to any atom of carbon.5.1.2. NomenclatureAccording to IUPAC system, alcohols are named by replacing the final ‘‘e’’ of the parent hydrocarbon with ‘‘ol’’, then specify the position of -OH group before ending by ol5.2. Classification and isomerism
Alcohols are classified as:Primary alcohols: These have only one alkyl group attached to the carbon carrying the –OH.• Functional isomers:Except methanol which has one carbon, other alcohols are isomers with ethersanother chemical function of general formula R-O-R’ where R and R’are alkyl groups or aryl groups but not hydrogen.5.3. Physical properties
a.Boiling pointsThe chart shows the boiling points of some simple primary alcohols and alkanes with up to 4 carbon atoms.• The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.• The boiling points of the alcohols increase as the number of carbon atoms increases.• The boiling point of alcohols with branches is lower than that of unbranched alcohols with the same number of carbon atoms. This is because increased branching gives molecules a nearly spherical shape and the surface area of contact between molecules in the liquid. This results in weakened intermolecular forces and therefore in lower boiling points.• Tertiary alcohols exhibit the lowest boiling point than secondary and primary alcohols:• Primary alcohol > Secondary alcohol > Tertiary alcoholHighest boiling point lowest boiling pointThe patterns in boiling point reflect the patterns in intermolecular attractions: In the case of alcohols, there are hydrogen bonds set up between the slightly positive hydrogen atoms and lone pairs on oxygen in other molecules.b.Solubility of alcohols in waterThe lower members of alcohols are completely soluble in water because mixed hydrogen bonds between water and alcohol molecules are formed. As the length of hydrocarbon group of the alcohol increases, the solubility decreases.c. VolatilityAlcohols are volatile and the volatility decreases as the molecular mass increases. Compared to alkyl halides, alcohols are less volatile. Polyalcohol are viscous or solids. Example: propane-1, 2, 3-triol (glycerine). This is due to stronger intermolecular forces than those of monoalcohols.5.4. Alcohol preparations
b. From alkenes
Alkenes react with water in the presence concentrated sulphuric acid to yields
alcohols
c. From carbonyl compounds
When aldehydes and ketones are reduced by hydrogen in the presence of a
suitable catalyst like Pt, Ni or Pd, they form primary and secondary alcohols
respectively.
d. From esters
Esters on hydrolysis in the presence of mineral acid or alkalis produce alcohols and
carboxylic acids.
e. From Grignard reagents
The reaction between carbonyl compound and Grignard reagent (alkyl magnesium
halides) produces an alcohol with more carbon atoms. The reaction is a nucleophilic addition on a carbonyl compound.
f. From primary amine to give primary alcohol
Primary amines react with nitrous acid to produce primary alcohols.
5.5. Preparation of ethanol by fermentation
This method is mainly used to prepare ethanol industrially. Ethanol is prepared from starch(e.g. maize, cassava, millet, sorghum) and sugar(e.g. banana juice, molasses) by fermentation process.Fermentation can be defined as any of many anaerobic biochemical reactions in which enzymes produced by microorganisms catalyse the conversion of on substance into another.The ethanol obtained by fermentation process is only about 11%. This is made concentrated by distillation which converts it to about 95% ethanol. This on further distillation yields a constant boiling mixture whose composition does not change (an azeotropic mixture). Therefore, 100% ethanol is obtained by either:i. Adding quick lime which removes waterii. Distilling with of benzene as a third componentNote: Methanol can be prepared industrially by the reaction of carbon monoxideand hydrogen at 300 °C and a pressure of 200 atmospheres.5.6.1. Oxidation
Aldehydes formed by oxidation of primary alcohols tend to undergo further oxidation to carboxylic acid.Ketones formed by oxidation of secondary alcohols are not further oxidised, unless if the oxidising agent is hot and concentrated in which case bonds around the –CO_ group are broken and two smaller carboxylic acids are formed.Tertiary alcohols resist oxidation because they have no hydrogen atom attached on the functional carbon atom. Oxidation also occurs when the alcohol is in gaseous phase by used of silver or copper catalyst under 500 °C and 300 °C respectively; and the vapour of the alcohol is passed with air (oxygen) over heated silver.An acidified potassium dichromate solution is turned from orange to green when it reacts with primary and secondary alcohols.Secondary alcohols having the following structure R-CHOH-CH3 only undergo oxidation, on treatment with iodine solution in the presence of sodium hydroxide to give yellow precipitate of tri-iodomethane.Note: This is a reaction which is characteristic of methyl ketones, CH3-CO-R’; but iodine here acting as an oxidizing agent first oxidizes the CH3-CHOH-R’toCH3-CO-R’ ; then the methyl ketone formed then gives the yellow precipitate of CHI3 (Iodoform). From the reaction involved we have the Iodoform test.5.6.2. Reaction with sulphuric acid
5.6.4. Reaction with strong electropositive metals and metal hydroxides5.6.5 Action of hydrohalic acids (HX)Notice:i. Reaction with concentrated hydrochloric acid is catalyzed by anhydrous zinc chloride.ii. This reaction is called LUCAS test and is used to distinguish between simple primary, secondary or tertiary alcohols. In this reaction, the alcohol is shaken with a solution of zinc chloride in concentrated hydrochloric acid.Observations: Immediate cloudiness indicates presence of a tertiary alcohol. If the solution becomes cloudy within 5 minutes then the alcohol is a secondary one. Primary alcohol would show no cloudiness at room temperature since the reaction is very slow.For example all alcohols which are isomers of C4H10O can be distinguished by the LUCAS test.Alcohols are also transformed into halogenoalkanes using phosphorus halides and thionyl chloride.5.7. Uses of alcohols
Ethanol is the alcohol found in alcoholic drinks. Alcoholic fermentation converts starch sugar into ethanol. For example grapes are used to produce wine, ripe banana to produce urwagwa, honey for spirits are obtained by distilling the ethanol –water product obtained when sugar is fermented.Drinking alcohol, i.e. the ethylic alcohol also called ethanol, is a normal social activity; but excess of it is dangerous for our health. Hence excess of alcoholic consumption must be avoided.For non-adult youth, consumption of alcohol in any form is illegal in Rwanda and many other countries.There are some alcoholic drinks produced in Rwanda and in the Region that are prohibited to be sold in Rwanda. However, alcohols have many other applications in daily life as indicated in the Table 5.1.Ethanol produced by sugar cane fermentation has been used as alternative fuel to gasoline (petrol). It has been mixed with gasoline to produce gasohol.1. Ethers are sparingly soluble in water but are soluble in organic solvents.2. The polar nature of the C-O bond (due to the electronegativity difference of the atoms) results in intermolecular dipole-dipole interactions.3. An ether cannot form hydrogen bonds with other ether molecules since there is no H to be donated (no -OH group).4. Their melting and boiling points increase with the increase in molecular mass because of increasing the magnitude of Van der Waal’s forces with size.5. The boiling points of ethers are much lower than those of alcohols of similar molecular mass. This is because of the intermolecular hydrogen bonding which are present in alcohols but are not possible in ethers.Since they are saturated compounds and non-polar, they are relatively chemically inert reason why their chemical reactions are very few.a. Ethers can act as the Lewis base due to the two non-bonded electron pair on oxygen to form coordinative bonds with Grignard reagent. This explains clearly why organ magnesium compounds are manipulated in ether solvent but not in water since in water, there is a reaction which generate alkanes.Lower ethers are used as anesthesia since they produce inert local cooling when sprayed on a skin, ether are also used as local anesthesia for minor surgery operation.Lower ethers are volatile liquid which on evaporation produce low temperature they are therefore used as refrigerants.Ether itself is one of the most important organic solvents for fats, oils, resins, and alkaloids.Activities: 0 -
UNIT 6: CARBONYL COMPOUNDS: ALDEHYDES AND KETONES
Key unit competency
To be able to compare the chemical nature of carbonyl compounds to their
reactivity and uses.
Learning objectives
• Describe the reactivity of carbonyl compounds
• State the physical properties of aldehydes and ketones
• Describe the preparation reactions of ketones and aldehydes
• Explain the mechanism of nucleophilic addition reactions of carbonyl
compounds
• Prepare ketones from secondary alcohols by oxidation reactions
• Compare aldehydes and ketones by using Fehling’s solution and Tollens’
reagent
• Write and name carbonyl compounds and isomers of ketones and aldehydes
• Write equations for the reactions of carbonyl compounds with other
substances
• Compare the physical properties of carbonyl compounds to those of alcohols
and alkenes
• Differentiate the methyl ketones from other ketones by using the iodoform
test
• Carry out an experiment to distinguish between carbonyl compounds and
other organic compounds
• Carry out an experiment to distinguish between ketones and aldehydes
• Carry out an experiment to prepare ethanol and propan-2-one.
6.1. Definition and nomenclature of carbonyl compounds
Introductory activity
Many fruits such as mangoes and honey contained sugar.The following images
represent mangoes, honey and some sugars such as fructose and glucose.
6.1.1 Definition
Carbonyl compounds are compounds that contain carbon-oxygen double bond
(C=O). Carbonyl compounds are classified into two general categories based on the
kinds of chemistry they undergo. In one category there are aldehydes and ketones;
in the other category there are carboxylic acids and their derivatives. This unit looks
on category of aldehydes and ketones.
Aldehyde molecules
For aldehydes, the carbonyl group is attached to hydrogen atom and alkyl group as
shown in the molecule of propanal below. Methanal is the smallest aldehyde, it has
two hydrogen atoms attached to carbonyl group.
If you are going to write this in a condensed form, you write aldehyde as –CHO,
don’t write it as -COH, because that looks like an alcohol functional group.
Ketone molecules
6.1.2 Nomenclature
Aldehydes
The systematic name of an aldehyde is obtained by replacing the terminal “e” from
the name of the parent hydrocarbon with “al.”In numbering the carbon chain of an
aldehyde, the carbonyl carbon is numbered one.
Ketones
The systematic name of a ketone is obtained by removing the terminal“e” from the
name of the parent hydrocarbon and adding “one.”The chain is numbered in the
direction that gives the carbonyl carbon the smallest number.Ketone contains
a carbon-oxygen double bond just like aldehyde, but for ketone carbonyl groupis
bonded to two alkyl groups.
6.2. Isomerism
6.2.1 Functional isomerism in aldehydes and ketones
Isomers are molecules that have the same molecular formula, but have a different
arrangement of the atoms in space. Functional group isomers are molecules that
have same molecular formula but contain different functional groups, and they
belong to different homologous series of compounds.
6.2.2. Position isomerism in ketones
Position isomerism is isomerism where carbon skeleton remains constant, but the
functional group takes different positions on carbon skeleton.
6.2.3. Chain isomerism in aldehydes and ketones
In chain isomerism the same number of carbons forms different skeletons.
Aldehydes with 4 or more carbon atoms and ketones with five or more carbon
atoms show chain isomerism.
6.3. Physical properties of aldehydes and ketones
6.3.1. Solubility in water aldehydes and ketones
The small molecules of aldehydes and ketones are soluble in water but solubility
decreases with increase of carbon chain. Methanal, ethanal and propanone - the
common small aldehydes and ketones are soluble in water at all proportions.
Even though aldehydes and ketones don’t form hydrogen bond with themselves,
they can form hydrogen bond with water molecules.
The slightly positive hydrogen atoms in a water molecule can be sufficiently
attracted to the lone pair on the oxygen atom of an aldehyde or ketone to form a
hydrogen bond.
Other intermolecular forces present between the molecules of aldehyde or ketone
and the water are dispersion forces and dipole-dipole attractions
Forming these attractions releases energy which helps to supply the energy
needed to separate the water molecules and aldehyde or ketone molecules from
each other before they can mix together.
Apart from the carbonyl group, hydrocarbon chains are non polar, they don’t
dissolve in water. By forcing hydrocarbon chain to mix with water molecules, they
break the relatively strong hydrogen bonds between water molecules without
replacing them by other attractions good like hydrogen bonds. This makes the
process energetically less profitable, and so solubility decrease.
6.3.2. Boiling points of aldehydes and the ketones
Methanal is a gas and has a boiling point of -21°C, and ethanal has a boiling point
of +21°C. The other aldehydes and ketones are liquids or solids, with boiling points
rising with rising of molecular mass hence rising of strength of Van der Waals force.
Comparing the physical properties of carbonyl compounds to those of alcohols
and alkanes
Physical properties of covalent compounds depend on intermolecular forces.
Compounds that have similar molecular mass but different intermolecular forces
have different physical properties.
Alcohols have higher boiling point than aldehydes and ketones of similar lengths. In
the alcohol, there is hydrogen bonding, but the molecules of aldehydes and ketones
don’t form hydrogen bonds.Aldehydes and ketones are polar molecules but alkanes
are non polar molecules.
6.4. Chemical properties of carbonyl compounds
6.4.1. Nucleophilic addition reactions
a. Polarity of carbonyl group
By comparing carbon-carbon double bond and carbon- oxygen double bond
the only difference between bonds C=C and C=O is distribution of electrons. The
distribution of electrons in the pi bond is heavily attracted towards the oxygen atom,
because oxygen atom is much more electronegative than carbon.
During chemical reactions nucleophiles will attack carbon of the carbonyl functional
group which bears apartial positive charge. While electrophile will attack oxygen of
the carbonyl functional group which bears a partial negative charge.
b. Reaction of HCN with aldehydes and ketones
Hydrogen cyanide adds to aldehydes or ketones to form cyanohydrins or
hydroxynitriles.The product has one more carbon atom than the reactant. For
example, ethanal reacts with HCN to form 2-hydroxypropanenitrile:
Because hydrogen cyanide is a toxic gas, the best way to carry out this reaction is to
generate hydrogen cyanide during the reaction by adding HCl to a mixture of the
aldehyde or ketone and excess sodium cyanide. Excess sodium cyanide is used in
order to make sure that some cyanide ion is available to act as a nucleophile. The
solution will contain hydrogen cyanide (from the reaction between the sodium or
potassium cyanide and the HCl)
The pH of the solution is maintained in range 4 - 5, because this gives the fastest
reaction. The reaction takes place at room temperature.
c. The mechanism of reaction between HCN and propanone
d. Application of the reaction
The product of the reaction above has two functional groups:
• The -OH group which behaves like ordinary alcohol and can be replaced by
other substituent like chlorine, which can in turn be replaced to give other
functional group, for example, an -NH2
group;
• The -CN group which can be hydrolysed into a carboxylic acid functional
group -COOH.
e.Reaction of NaHSO3 with aldehydes or ketones
The aldehyde or ketone is shaken with a saturated solution of sodium hydrogen
sulphite in water. Hydrogen sulphite with negative charge act as nucleophile,
where the product formed is separated as white crystals. Propanone react hydrogen
sulphite, as below:
Impure aldehyde and ketone can be purified by using this reaction. Impure
aldehyde or ketone is shaken with a saturated solution of sodium hydrogensulphite
to produce the crystals. Impurities don’t form crystals; these crystals formed are
filtered and washed to remove any impurities. Addition of dilute acid to filtered
crystals regenerates the original aldehyde. Dilute alkali also can be added instead
dilute acid.
6.4.2. Condensation reactions
a. Experimental reaction
The procedure of the preparation of Brady’s reagent and carbonyl compounds
changesslightly depending on the nature of the aldehyde or ketone, and the solvent
in which 2,4-dinitrophenylhydrazine is dissolved in. The Brady’s reagent for activities
(6.4.1) is a solution of the 2,4-dinitrophenylhydrazine in methanol and sulphuric acid.
Add a few drops of Brady’s reagent to either aldehyde or ketone. A bright orange or
yellow precipitate indicates the presence of the carbonyl group in an aldehyde or
ketone.
b. Structural formula of 2,4-dinitrophenylhydrazine.
The carbon of benzene attached to hydrazine is counted as number one.In
2,4-dinitrophenylhydrazine, there are two nitro groups, NO2,attached to the phenyl
group in the 2- and 4- positions.
c. The reaction of carbonyl compounds with 2,4-dinitrophenylhydrazine
Brady’s reagent is a solution of the 2,4-dinitrophenylhydrazine in methanol
and sulphuric acid. The overall reaction of carbonyl compounds with
2,4-dinitrophenylhydrazine is:
Where R and R’ represent alkyl groups or hydrogen(s); if both or only one is hydrogens
the starting carbonyl compound is an aldehyde. If both R and R’ are alkyl groups
the carbonyl compound is a ketone. The following molecule shows clearly how the
product is formed.
The product formed is named”2,4-dinitrophenylhydrazone”. The simple difference
consists in replacing suffix “-ine” by “-one”.
The reaction of 2,4-dinitrophenylhydrazine with ethanal produces ethanal
2,4-dinitrophenylhydrazone; The reaction of 2,4-dinitrophenylhydrazine with
butanal produces butanal 2,4-dinitrophenylhydrazone. This is an example of
condensation reaction.
During the chemical reaction, the change takes place only on nitrogen (-NH2) of
hydrazine in 2,4-dinitrophenylhydrazine. If the -NH2 group is attached to other
groups a similar reaction as that of 2,4-dinitrophenylhydrazine will take place:
6.4.3. Oxidation reactions using KMnO4/H+ and K2Cr2O7/H+
a. Difference in reactivity of ketones and aldehydes with K2Cr2O7
By considering the structural formulae of aldehydes and ketones, the difference is
only the presence of a hydrogen atom attached to the carbonyl functional group in
the aldehyde whereas ketones have a alkyl group instead.
During chemical reaction aldehydes react with oxidizing agent; hydrogen on
carbonyl functional group is replaced by oxygen, look on figure below. The presence
of hydrogen atom makes aldehydes very easy to oxidize, in other words aldehydes
are strong reducing agents.
For ketone, absence of hydrogen on carbonyl functional group makes ketones to
resist oxidation. But very strong oxidising agents like potassium permanganate
solution oxidize ketones - and they do it in a destructive way, by breaking carboncarbon bonds.
Aldehyde oxidation can take place in acidic or alkaline solutions. Under acidic
solutions, the aldehyde is oxidized to a carboxylic acid. Under alkaline solutions, acid
formed react with base to form a salt of carboxylic acid.
b. Oxidation of aldehyde by K2Cr2O7/H+ solution
Add few drops of the aldehyde or ketone to a solution of potassium dichromate
(VI) acidified with dilute sulphuric acid. If the color doesn’t change in the cold, the
mixture is warmed gently in a beaker containing hot water.
6.4.4. Oxidation reactions using Tollens’ reagent
a. Difference in reactivity of Ketones and Aldehydes with Tollens’ reagent
Aldehydes can also be oxidized into carboxylic ions in basic medium.Tollens’ reagent
is a solution of diamminesilver (I) ion, [Ag(NH3)2]+ and OH-.In order to identify if a
substance is aldehyde or ketone, add few drops of Tollens reagent to test tubes
containing aldehyde or ketone and warm gently in a hot water bath for a few minutes. The formations of sliver mirror or grey precipitate is an indication of the presence of aldehyde.
6.4.5. Oxidation reactions using Fehling ;or Benedict; solution
a. Difference in reactivity of Ketones and Aldehydes with Fehling or Benedict
solution.
Fehling’s solution and Benedict’s solution react with aldehyde in the same way; both
solutions contain Cu2+ and OH- . In Fehling’s solution Cu2+ is complexed with tartrate
ligand butin Benedict’s solution Cu2+ is complexed with citrate ligand.
Don’t worry about ligands, important reagents are Cu2+ and OH- , ligands tartrate and
citrate are used to prevent formation of precipitate copper (II) hydroxide or copper
(II) carbonate.
A few drops of Fehling’s solution or Benedict’s solution is added to the aldehyde or
ketone and the mixture is warmed gently in a hot water bath for a few minutes.
6.4.6. Iodoform reaction with aldehydes and ketones
Activity 6.4.6
Materials:
a. Reagents for iodoform reaction
There are two different mixtures that can be used to do iodoform test, these
mixture are:
• Iodine and sodium hydroxide solution
• Potassium iodide and sodium chlorate (I) solutions
Don’t worry about Potassium iodide and sodium chlorate(I) solutions, Potassium
iodide and sodium chlorate(I) react to form final solution containI2 and OH-. Both mixtures contain the same reagents.
Each of these mixtures contains important reagent I2 and OHwhich react with
aldehyde or ketone. When I2 and OH- is added to a carbonyl compound containing the group CH3 CO (blue in the cycle) as shown below, pale yellow precipitate (triiodomethane) is formed.
a. Description of iodoform test
For iodine and sodium hydroxide solution
Iodine solution, I3- , is added to aldehyde or ketone, followed by just enough sodium
hydroxide solution to remove the colour of the iodine. If pale yellow precipitate
doesn’t form in the cold, it may be necessary to warm the mixture very gently. The positive result is pale yellow precipitate of CHI3
For potassium iodide and sodium chlorate (I) solutions
Potassium iodide solution is added to a small amount of aldehyde or ketone, followed
by sodium chlorate (I) solution. If pale yellow precipitate doesn’t form in the cold,
warm the mixture very gently. The positive result is pale yellow precipitate of CHI3.
Reaction of iodoform test
The reagents of iodoform test are I2 and OHsolution. The reaction takes place into
two main steps:
• Three hydroxides, OH- , remove three hydrogens from methyl group and the
place of hydrogen is taken by iodide.
6.5. Preparation methods of aldehydes and ketones
6.5.1. Oxidation of alcohols
a. alcohol by K2\Cr2O7/H+
Potassium dichromate (VI) acidified with dilute sulphuric acid is used as oxidizing agent during the preparation of aldehyde or ketone. Primary alcohol is oxidized to aldehyde, oxygen atom from the oxidising agent removes two hydrogens; one from the -OH group of the alcohol and the other hydrogen comes from the carbon that is attached to hydroxide functional group .
b.Technique of stopping oxidation of aldehyde
The aldehyde produced by oxidation of alcohol could make further oxidation to a
carboxylic acid if the acidified potassium dichromate (VI) is still present in solution
where reaction takes place. In order to prevent this further oxidation of aldehyde to
carboxylic the following technique are used.
• Use an excess of the alcohol than potassium dichromate (VI). Potassium
dichromate (VI) is limiting reactant hence there isn’t enough oxidising agent
present to carry out the second stage of oxidizing the aldehyde formed to a
carboxylic acid.
• Distil off the aldehyde as soon as it forms. Removing the aldehyde as soon as it
is formed this means that aldehyde is removed from solution where oxidizing
agent is, to prevent further oxidation. Ethanol produces ethanal as shown by
the following reaction.
c. Oxidation of alkene by KMnO4/H+
6.5.2. Preparation of ketone by distillation of calcium acetate
Procedure: Transfer 15g of calcium acetate in 50ml round bottom flask fixed on
a stand, and place it on a heating mantle fitted with a condenser and a receiver
flask. Adjust the temperature until the condensation starts. Use the aluminium
foil to insulate the flask. Heat the flask and collect the acetone in receiver flask. The
obtained product is a crude acetone and needs to be purified.Set up a distillation
apparatus and distil the crude product to obtain pure acetone (56Oc). Do not forget
to use stirrer bar which must be placed in the round bottom flask containing the
acetone.
6.6. Uses of aldehydes and ketones
Aldehydes and ketones have many uses for example in industries such as
pharmaceutical industry and in medicine.
a. Formaldehyde:
Formaldehyde is a gas at room temperature but is sold as a 37 percent solution in water.
Formaldehyde is used as preservative and germicide, fungicide, and insecticide
for plants and vegetables. Formaldehyde is mainly used in production of certain
polymers like Bakelite (Figure 6.1). Bakelite and formaldehyde is used as
monomers in production of Bakelite
b. Acetone as solvent:
Acetone is soluble in water at all proportions and also dissolves in many organic
compounds. Boiling point of acetone is low, 56 °C, which makes it easier to be removed by
evaporation. Acetone is an industrial solvent that is used in products such as paints,
varnishes, resins, coatings, and nail polish removers.
c. Aldehydes and ketones
Organic molecules that contain ketones or aldehydes functional group are found
in different foods such as irish potatoes, yellow bananas.
Aldehydes and ketones perform essential functions in humans and other living organisms. For examples sugars, starch, and cellulose, which are formed from simple molecules that have aldehyde or ketone functional group
d. Aldehydes and ketones in human’s body
Aldehydes and ketones functional group are found in humans hormones like progesterone,
testosterone.
Activities: 0 -
UNIT 7: CARBOXYLIC ACIDS AND ACYL CHLORIDES
Key unit competency:
The learner should be able to compare the chemical nature of the carboxylic acids
and acid halides to their reactivity.
Learning objectives
• Explain the physical properties and uses of carboxylic acids and acyl chlorides
• Describe the inductive effect on the acidity of carboxylic acid
• Explain the reactions of carboxylic acids and acyl chlorides
• Apply the IUPAC rules to name different carboxylic acids acyl chlorides
• Write the structural formula and isomers of carboxylic acids
• Distinguish between carboxylic acids from other organic compounds using
appropriate chemical test
• Prepare carboxylic acids from oxidation of aldehydes or primary alcohols
• Compare the physical properties of carboxylic acids to those of alcohols
• Outline the mechanisms of esterification and those of reaction of acyl
chlorides with ammonia , amines and alcohols
• Develop a culture of working as a team group activities and self-confidence
in presentation
• Appreciate the uses of carboxylic acids as the intermediate compounds in
industrial processes such as aspirin, vinegar and perfumes
Carboxylic acid is classified in the family of organic compounds due to the presence
of carboxyl group (-COOH) in their chemical formula. The general formula for
carboxylic acids is R-COOH where R- refers to the alkyl group of the molecule.
7.1.1. Nomenclature
Carboxylic acids are named by following the general rules of naming organic
compounds, where the suffix ‘oic acid’ is added to the stem name of the longest
carbon chain that contains the acid functional group. The side branches are also
positioned by starting from the carbon with carboxylic functional group.
The carboxylic group takes priority to other functional group when numbering
carbons in thecase of substituted chain.
• Optical isomers
Optical isomers have the same molecular formula and the same structural formula,
but they are different in the spatial arrangement of atoms and their optical properties.
An organic compound shows optical isomerism, when there is chiral carbon (a
carbon atom attached to four diverse groups) in its structure. A chiral carbon is also
known as asymmetric carbon.
Just as the right hand and left hand are mirror images of another but not superimposable, optical isomers, also known as enantiomers, are different from each other and can have different properties. For example, muscles produce D-lactic acid when they contract, and a high amount of this compound in muscles causes muscular pain and cramps.
These molecules are optical isomers, because they have opposite optical activities.They can be distinguished by a plane-polarized light where one enantiomer rotates the light to the right while the other rotates it to the left.
Enantiomers are often identified as D- or L- prefixes because of the direction in which
they rotate the plane polarized light as shown in figures 7.2 and 7.3. Enantiomers
that rotate plane polarized light in clockwise direction are known as dextrorotatory
(right-handed) molecules and enantiomers that rotate plane polarized light in
anticlockwise direction are known as levorotatory (left-handed) molecules.
A solution containing equal amounts of enantiomers, 50% levorotatory and 50%
dextrorotatory is known as a racemic mixture that will not rotate polarized light,
because the rotations of the two enantiomers cancel each other out.
7.2. Physical properties of carboxylic acids
a. Physical state
Many carboxylic acids are colorless liquids with disagreeable odors. Aliphatic
carboxylic acids with 5 to 10 carbon atoms are all liquids with a “goaty” odors (odor
of cheese). These acids are also produced by the action of skin bacteria on human
sebum (skin oils), which accounts for the odor of poorly ventilated storerooms.
The acids with more than 10 carbon atoms are wax-like solids, and their odor
diminishes with increasing molar mass and resultant decreasing volatility.
Anhydrous acetic acid freezes at (17oC) slightly below ordinary room temperature,
reason why it is called glacial acetic acid (Figure 7.4). But a mixture of acetic acid
with water solidifies at much lower temperature.
b. Melting and boiling point
Carboxylic acids show a high degree of association through hydrogen bonding.
Because of this, they have high melting and boiling points compared to other
organic compounds of the same mass or number of carbon atoms.
Carboxylic acids have high melting and boiling points because their hydrogen
bonds enhance the possibility of bringing two acid molecules together by forming
a kind of dimer.
7.3. Acidity of carboxylic acids
Solutions of carboxylic acid turn blue litmus paper red; they do not change the
color of red litmus paper; therefore, they are acids as other mineral acids such as
HCl (aq).
Organic or carboxylic acids are weak acids in opposition to some mineral acids such
as hydrochloric acids which are strong acids.According to Arrhenius’ theory of acids
and bases, strong acids dissociate completely in water to give hydrogen ion, H+(aq) or
H3O+, whereas weak acids dissociate partially. The hydrogen ion released combines with a water molecule to form H3O+ a hydrate positive ion called hydronium H3O+:
The carboxylate ion formed by ionization of the acid is more stable than the acid
because it has many resonance structures.
Ethanoic acid is a weaker acid than methanoic because its methyl group has a
positive inductive effect; that is to mean that it pushes electrons towards the O-H
bond hence make hydrogen ion stable and not easily leaving.
The greater the number of such groups, the greater the effected and therefore
the weaker will be the acid.For example, 2,2-dimethylpropanoic is weaker than
2-methylpropanoic acid which is in turn weaker than propanoic.
The same rule applies to the increase in the length of the alkyl group chain.Butanoic
acid is a weaker acid than propanoic acid which shows that the acidity strength
decreases as the alkyl chain increases.
On the other hand, when an electron withdrawing group (a group with a negative
inductive effect) is present, the opposite effect is observed. For example,
chloroethanoic acid is a stronger acid than ethanoic acid. This is because chlorine
being electronegative, will withdraw electron towards itself thus reducing the
electron density around the O-H bond thus weakening it. It causes O-H bond to
easily break, and the concentration of hydrogen ions will be high in the solution.
The more the number of groups with negative inductive effect, the greater is the
effect and hence the more acidic will be the solution. Trifluoroacetic acid is more
acidic than trichloroacetic, dichloroacetic, chloroacetic and acetic acid because
fluorine is more electronegative than chlorine and hydrogen. It will strongly
withdraw electron towards itself, hence makes easier for the proton to leave.It must
also be noted that the further away the electronegative element, the less the effect.
For example, 3-chlorobutanoic acid is therefore a weaker acid than 2-chlorobutanoic
acid.
7.4. Preparation of carboxylic acids
Carboxylic acids are common and vital functional group; found in amino acids,
fatty acids etc. and provide the starting raw material for acid derivatives such as
acyl chlorides, amides, esters and acid anhydrides. There are several methods
of preparation of carboxylic acids where the most common are discussed in this
section.
7.4.1. From primary alcohols and aldehydes
Different carboxylic acids can be prepared by oxidation of either primary alcohols
or aldehydes. In the process, the mixture of alcohol is heated under reflux with an
oxidizing agent such acidified potassium permanganate or potassium dichromate.
Primary alcohols are first oxidized to aldehydes then further oxidation of aldehydes
produces carboxylic acid.
7.4.2. Hydrolysis of acid nitriles and amides with acid or alkali
When nitriles are hydrolyzed by water in acidic medium and the mixture is submitted
to heat, the reaction yields carboxylic acids.
7.4.3. From dicarboxylic acid
Monocarboxylic acids can be prepared by heatingcarboxylic acids which have two
carboxylic functional groups attached to the same carbon atom.
Note that the reaction is used to reduce length of the carbon chain. The mono
carboxylic acid prepared has one carbon atom less than the starting dicarboxylic
acid.
7.4.4. From organomagnesium compounds (Carboxylation reaction)
Grignard reagents react with carbon dioxide gas, and when the intermediate compound formed is hydrolyzed it finally forms carboxylic acid.
Mechanism:
7.4.5. From alkenes (Oxidation of alkenes)
Carboxylic acids are also obtained by heating alkenes with concentrated acidified
potassium permanganate. The reaction unfortunately forms a mixture of compounds
that mustbe later separated.
Note that the hydrolysis of carboxylic acid derivatives such as amide, esters, acyl
chloride and acid anhydrides also produce the corresponding acids.
7.4.6. Laboratory preparation of acetic acid
7.5. Reactions of carboxylic acids
The reaction of acids with carbonates is the basis for the chemical test of carboxylic
acid functional group and it can be used to distinguish carboxylic acids from other
functional groups in qualitative analysis.
Carbon dioxide produced is also tested by lime water and it turns lime water milky
(Figure 7.8)
It is noted that the oxygen atom in the ester formed comes from the alcohol and the
one in water is from the acid. In the mechanism of esterification, the acid loses -OH
group while the alcohol loses H-atom.
7.5.3. Reduction of carboxylic acids
Carboxylic acids are reduced to primary alcohols on treatment with reducing agent
such as LiAlH4 in dry ether or by use of hydrogen in the presence of Ni catalyst. The
reduction does not form aldehyde as an intermediate product, like in oxidation of
primary alcohols.
7.6. Uses of carboxylic acids
Food industry and nutrition
• Food additives: Sorbic acid, benzoic acid, etc.
• Main ingredient of common vinegar (acetic acid).
• Elaboration of cheese and other milk products (lactic acid).
Pharmaceutical industry
• Antipyretic and analgesic (acetylsalicylic acid or aspirin).
• Active in the process of synthesis of aromas, in some drugs (butyric or butanoic acid).
• Antimycotic and fungicide (Caprylic acid and benzoic acid combined with salicylic acid).
• Active for the manufacture of medicines based on vitamin C (ascorbic acid).
• Manufacture of some laxatives (Hydroxybutanedioic acid).
Other industries
• Manufacture of varnishes, resins and transparent adhesives (acrylic acid).
• Manufacture of paints and varnishes (Linoleic acid).
• Manufacture of soaps, detergents, shampoos, cosmetics and metal cleaning products (Oleic acid).
• Manufacture of toothpaste (Salicylic acid).
• Production of dyes and tanned leather (Methanoic acid).
• Manufacture of rubber (Acetic acid).
• Preparation of paraffin candles (Stearic acid)
7.7. Acyl chlorides and nomenclature
Acyl halides are compounds with the general formula where the–OH group of
carboxylic acid has been substituted by a halogen atom. The acyl remaining
structure is represented as:
Their isomers can be chain isomerism, positional isomerism and functional isomerism
with chloro aldehydes and ketones, alcohols with double bond C=C and chlorine as
a substituent, cyclic ethers with chlorine.
Acid chlorides have not many applications in our everyday life, but industrially they
are used in synthesis of perfumes and nylons, which are polymers of high importance
in textile industry. They can also be used in pharmaceutical industries to synthesize
drugs with aromatic ester or amide functional groups like aspirin or paracetamol.
7.7.1. Physical properties
Appearance
Acyl chlorides are colourless fuming liquids. Their characteristic strong smell is
caused by hydrogen chloride gas that is produced when they get in contact with
moisture (see figure 7.7). For example, the strong smell of ethanoyl chloride is a
mixture of vinegar odour and the acrid smell of hydrogen chloride gas.
b. Solubility
Acyl chlorides are slightly soluble in water due to their small dipole that can interact
with the polarity of water molecule. They cannot be said to be soluble in water
because they readily react with water. It is impossible to have a simple aqueous
solution of acyl chlorides, rather we have the products of their reaction with water.
c. Boiling and melting points
Acyl chloride molecules interact by Van der Waals forces whose strength increases
with the increase in molecular masses of the compounds.
The boiling and melting points of acyl chlorides increases as their molecular masses
rise. They have lower boiling and melting points than alcohols and carboxylic acids
of the same number of carbon atoms, because they lack hydrogen bonds.
7.7.2. Reactions of acyl chlorides
The chemistry of acyl chlorides is dominated by nucleophilic substitution, where
a stronger nucleophile replaces chlorine atom of acyl chloride. They undergo
nucleophilic substitution reactions more easily than alkyl halides and carboxylic
acids because the nucleophile targets the carbon which is deficient in electrons and
-Cl is better leaving group than -OH group.
The common reactions of acyl chlorides include reactions with water, alcohols and
ammonia and amines. These reactants have a very electronegative element that
has a free lone pair of electrons to act as a nucleophile.
Reaction with alcohols
They react with alcohol to produce esters with high yields than esterification of an
alcohol and carboxylic acid, since Cl-atom in acyl chloride is a better leaving group
than O-H for the case of carboxylic acid. The difference in electronegativity is the
main reason for this observation.
Reaction with ammonia and amine
Acyl chlorides react with ammonia and amines to yield amides. Ammonia, primary
amines and secondary amines form primary amides, secondary amides and tertiary
amides respectively.
Activities: 0 -
UNIT 8: ESTERS, ACID ANHYDRIDES, AMIDES AND ITRILES.
Key unit competency:
To be able to relate the functional groups of esters, acid anhydrides, amides and
nitriles to their reactivity, preparation methods and uses.
8.1. Structure and nomenclature of esters
8.1.1. Structure of esters
In unit 7, the reactions of carboxylic acids were discussed. The reactions of carboxylic acids produce the derivatives of acids such as esters, acid halides, acid anhydrides and amides.
The general molecular formula of esters is Cn H2nO2 and their general structural
formula is: RCOOR’ or
Esters are known for their distinctive odor and they are commonly responsible for
the characteristic of food (fruits) aroma, flowers and fragrances. Esters are found in
nature but they can be also synthesized. Both natural and synthetic esters are used
in perfumes and as flavoring agents.
8.1.2. Nomenclature of esters
The nomenclature of esters follows some steps. When naming esters the alkyl
group R’ is named followed by the name of RCOO- group.
The group name of the alkyl or aryl portion is written first and is followed by the
name of the acid portion. In both common and International Union of Pure and
Applied Chemistry (IUPAC) nomenclature, the -ic ending of the corresponding acid
is replaced by the suffix –ate. Some examples of names of esters are given in Table
8.1.
8.1.3. Physical properties and uses of Esters
B. Comparing boiling points of alcohols, carboxylic acids and esters
Materials and Chemicals
Propan-1-ol, propanoic acid and methyl ethanoate, test tubes, test tube holders
(lacks), heaters, and thermometers.
Procedure
1. Put 10 mL of each substance in a labeled test tube.
2. Boil carefully substances are volatile and flammable
3. Use a thermometer to measure the boiling point of each substance.
4. Record the results and compare them. Suggest a reason for the difference
in boiling points of the three substances.
Conclusion: Esters have lower boiling points than alcohols and carboxylic
acids because they lack hydrogen bonds. A compound having hydrogen bonds
has a high boiling point because, to break that bond requires higher energy.
Other physical properties of esters
i. Lower esters have sweet fruity smells
ii. Melting and boiling points of esters increase as the molecular mass
increases.
iii. Small esters are fairly soluble in water but the solubility decreases as the
length of the chain increases
8.1.4. Uses of Esters
Esters find various uses:
i. They are used as organic solvent
ii. Due to their aroma, they are used as constituent of fragrance, essential oils,
food flavoring and cosmetics.
iii. They are used to manufacture soaps, detergents and glycerol.
iv. They are used to provide energy in the body
v. Polyesters are used to produce plastics etc.
8.2. Preparation and chemical properties of esters
8.2.1. Preparation of Esters
The preparation of esters involves different types of reaction such as esterification,
reaction of an acid chloride with an alcohol and the reaction of acid anhydrides
with alcohols.
1.Esterification reaction
In units five and seven, it is mentioned that esters can be produced by a reaction
between alcohols and carboxylic acids in strong acidic medium acting as a catalyst.
The acid is commonly a concentrated sulphuric acid, under reflux (Figure 8.3). The
reaction is generally called “Esterification” (a condensation reaction which involves
the addition of the alcohol and acid molecules followed by an elimination of a
water molecule).