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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
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