Functional Group and Homologous Series

A functional group is defined as an atom or group of atoms linked with carbon which determines characteristic properties of the organic compounds. When organic compounds containing same functional group are arranged in series on the basis of their increasing molecular weight, having the difference of -CH2 (methylene group) in between two consecutive compounds, such series is called homologous series.

Functional group

Organic compound consists of two parts, a reactive (active) part, which is known as a functional group and a skeletal part consisting of carbon and hydrogen atoms, which is known as the alkyl group (Its general formula is C_nH_2n+1) and is denoted by -R).

Thus, a functional group is defined as an atom or group of atoms linked with carbon which determines characteristic properties of the organic compounds. The chemical properties of an organic compound solely depend on upon the nature of functional group present in the molecule. For instance-

The functional group of alcohol is -OH.

The functional group of aldehyde is -CHO.

The functional group of carboxylic acid is -COOH.

Some important functional group and their class are shown in the table below-

S.N	Functional Group	Name	Class of compounds
1.	-O-	Ether (Alkoxy)	Ethers
2.	-OH	Alcohol (hydroxy)	Alcohols
3.	-NH2	Amine (Amino)	Amines
4.	-X (X=Cl, Br, I)	Halide (halo)	Alkyl halides
5.	-CN or -C≡N	Cyanide (nitrile)	Cyanides or nitrile
6.	-NC or	Isocyanide	Isocyanide or carbylamines
7.	-CNO or -C≡N→O	Cyanate	Cyanates
8.	-NCO or -N=C=O	Isocyanate	Isocyanates
9.	-NO2or	Nitro	Nitro
10.	-NO or -N=O	Nitroso	Nitroso compounds
11.	-N=N-	Azo	Azo compounds
12.	-CHO or	Aldehyde (formyl)	Aldehydes
13.	-CO or	Keto	Ketones
14.	-COOH or	Carboxyl	Carboxylic acid
15.	-COOR or	Ester	Esters
16.	-COX or	Acyl halide	Acyl halides

Homologous series

When organic compounds containing same functional group are arranged in series on the basis of their increasing molecular weight, having the difference of -CH₂ (methylene group) in between two consecutive compounds, such series is called homologous series. The individual members of a homologous series are known as homologous and the process is known as homology.

Different members of a homologous series can be denoted by a common formula such as -

Alkane (C_nH_2n+2), Amino group (C_nH_2n+1.NH₂), alcohol (C_nH_2n+1.OH) etc.

Some of the examples of homologous series are as follows-

• Homologous series of alkanes (C_nH_2n+2)

Formula	IUPAC name
CH4	Methane
CH3-CH3	Ethane
CH3-CH2-CH3	Propane
CH3-CH2-CH2-CH3	Butane
CH3-CH2-CH2-CH2-CH3	Pentane
CH3-CH2-CH2-CH2-CH4-CH3	Hexane

• Homologous series of alcohols (C_nH_2n+1.OH)

Formula	IPUAC name
CH3-OH	Methanol
CH3-CH2-OH	Ethanol
CH3-CH2-CH2-OH	Propanol
CH3-CH2-CH2-CH2-OH	Butanol
CH3-CH2-CH2-CH2-CH2-OH	Pentanol
CH3-CH2-CH2-CH2-CH2-CH2-OH	Hexanol
CH3-CH2-CH2-CH2-CH2-CH2-CH2-OH	Heptanol
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-OH	Octanol

Characteristics of a homologous series

1) All the members of a homologous series contain same general formula and have similar structures,

2) Two successive members of a series are different in their molecular formula by a -CH₂ unit or 14 atomic mass (a.m.u).

3) Due to the presence of the same functional group, all the members of a homologous series are almost similar in chemical properties.

4) All the members of the same homologous series can be prepared by the similar general methods of preparation.

5) There is a regular gradation in the physical properties like melting point, density etc. among the members of the same homologous series.

6) The first member of a homologous series is somewhat different in the chemical properties of the subsequent members.

[IUPAC- International Union of Pure and Applied Chemistry]