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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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