MP Board · Class 10 · Science · Carbon and its CompoundsWhat is homologous series? Write any four characteristics of a homologous series. Draw the structures of two successive members of alcohol homologous series and write their molecular formulas.

Definition of Homologous Series

• A homologous series is a family of organic compounds having similar chemical properties and general formulas, in which each successive member differs from the previous one by a $-\text{CH}_2$ group (or a molecular mass difference of 14 u).

Characteristics of a Homologous Series

1. General Formula: All members of a homologous series can be represented by the same general formula. For example, alkanes are represented by $\text{C}n\text{H}{2n+2}$.
2. $-\text{CH}_2$ and Mass Difference: Any two successive members differ in their molecular formula by a $-\text{CH}_2$ unit and in molecular mass by $14 \text{ u}$ ($12$ for carbon + $2$ for hydrogen).
3. Chemical Properties: The chemical properties of all members in a series are very similar because they possess the same functional group. For example, all alcohols react with sodium to evolve hydrogen gas.
4. Gradation in Physical Properties: As the molecular mass increases down the series, physical properties such as melting point, boiling point, and solubility show a gradual change (usually an increase in melting and boiling points due to increased intermolecular forces).

Successive Members of Alcohol Homologous Series

• First Member (Methanol):
  • Molecular Formula: $\text{CH}_3\text{OH}$
  • Structure:

       H
       |\nH - C - O - H
       |
       H
    

• Second Member (Ethanol):
  • Molecular Formula: $\text{C}_2\text{H}_5\text{OH}$
  • Structure:

       H   H
       |   |\nH - C - C - O - H
       |   |
       H   H