Homologous Series: Definition,Properties and Characteristics

homologous series is a group of compounds that share the same basic chemical makeup, but differ in the number of iterations of a certain aspect of their structure. also An homologous series is a group of compounds with similar structural features that can be represented by a general formula whose members differ by one -CH2– unit.
An important characteristic of a homologous series is a repeating unit. For example, the alkane group contains the CH2 repeating unit. This means that the compounds are identical with the exception of the number of CH2 units in the compound.
The chemical properties of the members of an homologous series are due to the presence or absence of functional groups. As all members of the same series have the same functional groups, then the chemical properties are similar. They are not identical because chemical properties are also modified by other factors, such as the position of the functional group, or branching in the carbon chain.
The physical properties of an homologous series show a trend as the series is ascended. This is due to the increased relative molecular mass, as each member of the series differs from the previous member by one -CH2– unit.
Physical properties include:

  1.  density
  2.  melting point
  3.  boiling point
  4.  diffusion rate
  5.  solubility
  6.  refractive index
  7.  conductivity


1. Boiling Point

The boiling point of the compounds in a homologous series increases as more units are added. This occurs because the surface area of the compound increases when the length of the compound does.

Characteristics of Homologous Series

  1. Members of the series can be represented by a general formula. Ethanol, propanol and butanal have the same general formula, CnH2n+1OH
  2. Successive members differ from each other by –CH2. The difference between propanol and ethanol is CH2 that has a relative molecular mass of 14.
  3. Physical properties change regularly with increasing number of carbon atoms. The boiling points of alcohols increase from ethanol, propanol to butanol.
  4. Members have similar chemical properties because they have same functional group. Ethanol, propanol and butanol undergo oxidatoin produce carboxylic acids.
  5. Members of the homologous series can be prepared using the same method.  Ethanol, propanol & butanol can be prepared by hydration of alkene.


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