Question

Which among the following has highest boiling point?

• A. Butyric acid
• B. Valeric acid
• C. Acetic acid
• D. Formic acid

Hint:

Within any homologous series (alkanes, alcohols, acids), boiling point ALWAYS increases as the molar mass (number of carbons) increases! Branching lowers it, but here they are all straight chains.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks us to identify the carboxylic acid with the highest boiling point among a given list of straight-chain aliphatic carboxylic acids.

Step 2: Detailed Explanation:
Boiling points of liquids are fundamentally determined by the strength of the intermolecular forces holding the molecules together.
For compounds belonging to the exact same homologous series (here, all are carboxylic acids possessing identical, strong intermolecular hydrogen bonding properties via dimerization), the primary determining factor becomes the overall size and mass of the non-polar alkyl chain.
As the carbon chain length increases:
- The overall molecular mass increases.
- The molecular surface area massively expands.
- This directly results in stronger, more extensive van der Waals dispersion forces between adjacent alkyl chains.
- Because there are significantly stronger dispersion forces holding the larger molecules together, it requires more thermal energy to separate them and push them into the gas phase. Hence, the boiling point increases predictably with molecular mass.
Let's identify the number of carbon atoms in each acid provided:
(d) Formic acid (Methanoic acid): 1 carbon atom.
(c) Acetic acid (Ethanoic acid): 2 carbon atoms.
(a) Butyric acid (Butanoic acid): 4 carbon atoms.
(b) Valeric acid (Pentanoic acid): 5 carbon atoms.
Since Valeric acid has the longest carbon chain and the highest molecular mass, it possesses the strongest van der Waals forces and therefore the highest boiling point.

Step 3: Final Answer:
Valeric acid has the highest boiling point, matching option (b).