Topic outline

  • 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.











    • 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 in

      the 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.



      • 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 additions




        Alkenes 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.









        • 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 compounds 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.

























          • UNIT 5: ALCOHOLS AND ETHERS

            Key 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. Definition
            Alcohols 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 atoms

            Alcohols 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. Nomenclature
            According 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 ol


            5.2. 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 ethers
            another 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 points

            The 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 alcohol
                                                            Highest boiling point                    lowest boiling point


            The 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 water

            The 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. Volatility

            Alcohols 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 water
            ii. Distilling with of benzene as a third component
            Note: Methanol can be prepared industrially by the reaction of carbon monoxide
            and 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 hydroxides



            5.6.5 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.