Question

Identify the reason for the highest boiling point of R-COOH as compared to alkanes, ethers, alcohols and aldehydes.

• A. Formation of carboxylate ion
• B. Van der waals force of attraction
• C. Formation of intramolecular hydrogen bonding
• D. Formation of intermolecular hydrogen bonding

Hint:

Carboxylic acid molecules join in pairs through two hydrogen bonds between molecules.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
The boiling point depends on how strongly molecules attract each other. Alkanes have only weak van der Waals forces, ethers and aldehydes have dipole attraction, and alcohols have hydrogen bonding.

Step 2: Why acids are highest
Carboxylic acids have both an O-H and a C=O group. Two molecules join through two hydrogen bonds to make a dimer. The hydrogen bonds are between separate molecules, so they are intermolecular. This strong association needs a lot of heat to break.

Step 3: Check the options
(A) Carboxylate ion formation is an ionisation and not the reason. (B) Van der Waals forces are present in all, so they cannot explain the highest value. (C) Intramolecular hydrogen bonding is within one molecule and would lower the boiling point. (D) Intermolecular hydrogen bonding is correct.

Final Answer:
Intermolecular hydrogen bonding, which forms dimers, gives carboxylic acids the highest boiling points, option (D). \[ \boxed{\text{Intermolecular hydrogen bonding}} \]