A homologous series is a group of organic compounds that have:

Correct option is D

Correct Answer: D) the same functional group but differ by a –CH₂ group

Explanation:

A homologous series is a group of organic compounds that have the same functional group and exhibit similar chemical properties, but each successive member differs by a –CH₂ (methylene) group. This means that each compound in the series has a constant increment in molecular mass of 14 atomic mass units (AMU).

For example, the alkane homologous series follows the general formula CₙH₂ₙ₊₂:

• Methane (CH₄)
• Ethane (C₂H₆) (differs from methane by –CH₂)
• Propane (C₃H₈) (differs from ethane by –CH₂)
• Butane (C₄H₁₀) (differs from propane by –CH₂)

Information Booster:

Homologous series have a gradual change in physical properties (e.g., boiling point, melting point increase with molecular mass).
All members contain the same functional group, leading to similar chemical properties.
Examples of homologous series:

• Alkanes (CₙH₂ₙ₊₂) → Saturated hydrocarbons
• Alkenes (CₙH₂ₙ) → Contain a double bond
• Alcohols (CₙH₂ₙ₊₁OH) → Contain the –OH functional group
• Carboxylic acids (CₙH₂ₙ₊₁COOH) → Contain the –COOH functional group
  
• Homologous series follow a general formula, making their properties predictable.

Analysis of Other Options:

• Same molecular formula (Incorrect) – Compounds in a homologous series have a similar formula but differ by –CH₂.
• No relation to each other (Incorrect) – Members of a homologous series are closely related in chemical behavior.
• Same structural formula (Incorrect) – They have similar but not identical structures, differing by –CH₂.