### Theory:

A homologous series is a group or class of organic compounds that have the same general formula and similar chemical properties but differ by a $$-CH_2$$ group.

Consider the members of the alkanes listed in the below table. The following are their condensed structural formulas:

 Name Molecular formula Methane $$CH_4$$ Ethane $$CH_3CH_3$$ Propane $$CH_3CH_2CH3$$ Butane $$CH_3(CH_2)_2CH3$$ Pentane $$CH_3(CH_2)_3CH3$$

If you look at the preceding series, each successive member has one more methylene group than the preceding member, which is why they are called homologs.

Characteristics of homologous series:

1. Each member of the series differs from the previous or succeeding member by one methylene group ($$–CH_2$$) and thus by $$14 amu$$ in molecular mass.

For example the homologous series of alcohol,

 Name Molecular formula Methanol $$CH_3OH$$ Ethanol $$CH_3CH_2OH$$ Propanol $$CH_3(CH_2)_2OH$$ Butanol $$CH_3(CH_2)_3OH$$ Pentanol $$CH_3(CH_2)_4OH$$

2. A homologous series members share the same elements and functional group.

For example the homologous series of carboxylic acid,
 Name Molecular formula Methanoic acid $$HCOOH$$ Ethanoic acid $$CH_3COOH$$ Propanoic acid $$CH_3CH_2COOH$$ Butanoic acid $$CH_3(CH_2)_2COOH$$ Pentanoic acid $$CH_3(CH_2)_3COOH$$

3. A general molecular formula is used to represent them.
Example:
For example, alkanes ($$C_nH_{2n + 2}$$), alkenes ($$C_nH_{2n}$$) and alkynes ($$C_nH_{2n – 2}$$).
4. Members of each homologous series exhibit a regular gradation in physical properties as their molecular mass increases.
Example:
For example, alkanes group compounds have weak Van Der Waals force.
5. Members of a homologous series have similar chemical properties.
Example:
For example, all alkanes on reaction with oxygen produce carbon dioxide, water with energy liberation.
6. A common method can be used to prepare all of the members in a homologous series.
Example:
For example, aldehydes can be prepared by oxidising primary alcohol.
$$RCH_2OH\xrightarrow{K_2Cr_2O_7} R-CHO$$