In a homologous series of alcohols, the similarity in chemical behaviour is primarily due to:

Correct Option

The correct option is Presence of the same functional group.

Explanation

A homologous series is a sequence of organic compounds that share the same general formula and functional group. While the carbon chain length increases by a -CH₂- unit between successive members, the functional group remains constant, governing the chemical nature of the series.

Option Analysis

• Presence of the same functional group is Correct: The chemical properties of an organic compound are determined primarily by its functional group (e.g., the hydroxyl group -OH in alcohols). Since all members of a homologous series possess the same functional group, they exhibit similar chemical behavior and undergo similar reactions.
• Equal molecular masses is Incorrect: In a homologous series, the molecular mass is not equal; it increases by 14 atomic mass units (u) for each successive member due to the addition of a -CH₂- group.
• Identical number of carbon atoms is Incorrect: The number of carbon atoms is not identical; it increases by one carbon atom as one moves to the next member of the series (e.g., Methanol has 1 carbon, Ethanol has 2).
• Identical boiling points is Incorrect: Physical properties, such as boiling points and melting points, are not identical. They show a regular gradation (usually increasing) as the molecular mass and van der Waals forces increase down the series.

Key Takeaway: In any homologous series, the functional group dictates the chemical properties (similarity), while the increasing molecular mass dictates the physical properties (gradation).