Homologous series

Introduction

A homologous series is a series of compounds exhibiting the same chemical properties as they belong to the same functional group.  Some of the similarities can be the length of a carbon chain( as in the case of straight-chained alkanes-paraffins) or the number of monomers in a homopolymer (as seen in amylose). A homologous series is a group of organic compounds with carbon atoms that differ from each other by one methylene(CH2) group. For example, even though methane, ethane, and propane have carbon atoms in common but they are differed by 1 or 2 or even more than 2 CH2 groups. For example, the formula of methane is CH4 and the formula of ethane is C2 H6. Each member of a homologous series is called a homolog. In a homologous series, the chemical properties are imparted by the functional group so all the members have similar chemical properties with different physical properties due to the difference in their molecular mass. The melting and boiling points increases as the molecular masses are increased.

 

Homologous series

General formula

Repeating unit

The functional group(s)

Straight-chain alkanes

CnH2n + 2 (n ≥ 1)

−CH2

H3C− ... −CH3

Straight-chain perfluoroalkanes

CnF2n + 2 (n ≥ 1)

−CF2

F3C− ... −CF3

Straight-chain alkyl

CnH2n + 1 (n ≥ 1)

−CH2

H3C− ... −CH2

Straight-chain 1-alkenes

CnH2n (n ≥ 2)

−CH2

H2C=C− ... −CH3

Cycloalkanes

CnH2n (n ≥ 2)

−CH2

Singly-bonded ring

Straight-chain 1-alkynes

CnH2n − 2 (n ≥ 2)

−CH2

HC≡C− ... −CH3

polyacetylenes

C2nH2n + 2 (n ≥ 2)

−CH=CH−

H3C− ... −CH3

Straight-chain primary alcohols

CnH2n + 1OH (n ≥ 1)

−CH2

H3C− ... −OH

Straight-chain primary monocarboxylic acids

CnH2n + 1COOH (n ≥ 0)

−CH2

H3C− ... −COOH

Straight-chain azanes

NnHn + 2 (n ≥ 1)

−NH−

H2N− ... −NH2



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