Question Details

The molecular formula of second homologue in the homologous series of monocarboxylic acid is

Options

CH₃COOH

CH₃CH₂COOH

CH₃CH₂CH₂ – COOH

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Correct Answer :

Option A

CH₃COOH

Homologous series of any functional groups differs in 1 CH₂ group, therefore; 1^st Homologue ⇒ HCOOH 2^nd Homologue ⇒ CH₃COOH

Solution :

The correct answer is CH₃COOH.

1. Definition of Homologous Series:
A homologous series is a group of organic compounds that share the same functional group and similar chemical properties. Successive members in a homologous series differ from each other by a methylene group (-CH₂-) and have a difference of 14 u in their molecular masses.

2. Monocarboxylic Acids:
Monocarboxylic acids are organic acids containing a single carboxylic acid group (-COOH). The general molecular formula for the homologous series of monocarboxylic acids can be represented as:
$C_{n} H_{2 n + 1} C O O H$ (where $n \geq 0$ )
or:
$C_{n} H_{2 n} O_{2}$ (where $n \geq 1$ )

3. Identifying the Homologues:
• First Homologue: The simplest monocarboxylic acid is methanoic acid (formic acid), which contains one carbon atom. Its molecular formula is HCOOH.
• Second Homologue: The second member of this series is ethanoic acid (acetic acid), which contains two carbon atoms. Its molecular formula is CH₃COOH.
• Third Homologue: The third member is propanoic acid, which contains three carbon atoms. Its molecular formula is CH₃CH₂COOH.

4. Analyzing the Options and Attached Image:
• The attached image shows the branched structure:
$C H_{3} - C H (C H_{3}) - C O O H$
This corresponds to 2-methylpropanoic acid (isobutyric acid), which has 4 carbon atoms and is an isomer of the fourth homologue (butanoic acid).
• Thus, the molecular formula of the second homologue is CH₃COOH.