Alkyl group , Functional group and Homologous series

 

This is the notes of Alkyl group , Functional group and Homologous series.

Alkyl Group 

A group or radical obtained by the removal of H-atom from an alkane is known as alkyl group or alkyl radical. It is generally represented as ‘R-’ . The name of alkyl groups are given by replacing primary suffix –ane by –yl in the name of parent alkane.

R-H(alkane)----  -H → R-(alkyl group)

E.g. CH4(methane) ----  -H → CH3– (methyl)

      CH3–CH3(ethane)----  -H → CH3 –CH2- (ethyl)

Functional group 

Organic compounds (except hydrocarbons) consists of two parts; a reactive part known as functional group and a skeleton of C and H atoms usually known as alkyl group. 

The properties of the organic compounds are largely controlled by the functional group. For e.g. The properties of ethyl alcohol are different from those of ethylamine although their alkyl radicals are same.

On the other hand, the properties of methyl alcohol and ethyl alcohol are similar due to same functional group in both though they have different alkyl radicals.

Functional Group is defined as an atom or a group of atoms that largely determines the chemical properties of the organic compounds. The compounds with the same functional group react in a similar way though they have different alkyl radicals. All such compounds with similar properties constitute a class or family.

Homologous Series

When the compounds belonging to a class are arranged in order of increasing molecular weights, a set of compounds is formed in which the members differ in composition from one another by –CH2 unit. Such a group of organic compounds is called Homologous Series.

The individual members of a homologous series are known as homologues and the phenomena is called homology.

For e.g. 

Alkanes (CnH2n+2 ): CH4 , C2H6 , C3H8 , C4H10, C5H12,… 

Alcohols (CnH2n+1OH): CH3 OH, C2H5OH, C3H7OH, C4H9OH,… 

Carboxylic acids (CnH2n+1 COOH): HCOOH, CH3 COOH, C2H5 COOH,….

Characteristics of homologous series: 

i. The members of a homologous series have same functional group. 

ii. All the members of a homologous series can be represented by same general formula and have similar structure . 

iii. The members of a homologous series have same general method of preparation. 

iv. The members of a homologous series have similar chemical properties.

v. The two successive members of a homologous series differ in their molecular formula by -CH2- unit, i.e., 14 amu. 

vi. There is regular gradation in the physical properties like m.pt., b.pt., density, etc. among the members of the same homologous series.

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