Chemistry

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

C_nH_2n+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 centre^1.

.

• 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.