Question

Carboxylic acid has a higher boiling point than aldehyde, ketone and even alcohols of comparable molecular mass. It is due to their _______________.
A) More association of carboxylic acid via Van der Waals force of attraction
B) Formation of intramolecular H-bonding
C) Formation of intermolecular H-bonding
D) Formation of carboxylate ions.

Answer 1

Dimer is formed by the combination of two molecules. Due to the formation of dimer the boiling point increases due to increase in surface area.

Complete step by step solution:
Boiling point refers to the point where liquid and vapor state exists in equilibrium with each other. It is a fixed point at constant temperature.
Boiling point depends upon various factors such as molecular mass, size of chain, bonding present in the molecule etc. In the question we have been given that molecular masses of carboxylic acid, aldehyde, ketone and alcohols are comparable, so they must have the same boiling point but this is not so aldehyde and ketone does not have hydrogen bonding and hence they have lowest boiling points. In alcohols hydrogen bonding exists and hence has a greater boiling point than aldehyde and ketone. In Carboxylic acid the extent of hydrogen bonding is very high and it forms dimer in association with another carboxylic acid and hence now a single unit of carboxylic acid containing two molecules of carboxylic acid. So the overall molecular mass for a single molecule increases and hence the boiling point increases. Hence due to association of one carboxylic acid with another it has the highest molecular mass.

Hence, the correct option is A.

Note:
Hydrogen bonding is only shown by alcohols and carboxylic acid and not aldehyde and ketone, because in aldehyde and ketone oxygen is ${\text{s}}{{\text{p}}^2}$ hybridized with carbon and ${\text{s}}{{\text{p}}^2}$ hybridized oxygen does not undergo hydrogen bonding. Only a ${\text{s}}{{\text{p}}^2}$ hybridized carbon such as in carboxylic acid and alcohol undergoes hydrogen bonding.