Functional Group and Homologous Series
Subject: Chemistry
Overview
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 and 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 CnH2n+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 (CnH2n+2), Amino group (CnH2n+1.NH2), alcohol (CnH2n+1.OH) etc.
Some of the examples of homologous series are as follows-
- Homologous series of alkanes (CnH2n+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 (CnH2n+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]
Reference
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).
Things to remember
- 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 CnH2n+1) and is denoted by -R).
- A functional group is defined as an atom or group of atoms linked with carbon which determines characteristic properties of the organic compounds.
- Some important functional group and their class.
- 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.
- Some of the examples of homologous series
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