Question

Give reason of the following
Carboxylic acids have higher boiling point than alcohols of comparable molecular masses. 
 

Hint:

Carboxylic acid boiling points are even higher than those of alcohols because of the formation of "cyclic dimers".

Answer 1

Carboxylic acids have higher boiling points than alcohols of comparable molecular masses due to the following reasons:
1. Hydrogen Bonding:
Both carboxylic acids and alcohols are capable of forming hydrogen bonds due to the presence of the hydroxyl group (-OH). However, carboxylic acids have a unique ability to form strong hydrogen bonds because they contain both a hydroxyl group and a carbonyl group (C=O). This allows carboxylic acids to form a dimer structure in which two molecules are connected by hydrogen bonds between the -OH group of one molecule and the C=O group of another. This dimerization creates stronger intermolecular forces compared to alcohols, which can only form hydrogen bonds through their -OH group. The result is a higher boiling point for carboxylic acids, as it requires more energy to break these stronger intermolecular forces.
2. Intermolecular Forces:
Carboxylic acids exhibit stronger intermolecular forces due to the formation of these dimers. The presence of both the hydrogen bonding through the -OH group and the dipole-dipole interaction between the carbonyl group and the hydroxyl group makes carboxylic acids more cohesive. In contrast, alcohols only exhibit hydrogen bonding through the -OH group, which is weaker in comparison. Therefore, carboxylic acids have significantly higher boiling points, as the stronger intermolecular forces result in a higher amount of energy required to vaporize the molecules.
3. Polarity:
Carboxylic acids are more polar than alcohols because the carbonyl group (C=O) adds to the overall dipole moment of the molecule. This increased polarity enhances the intermolecular forces, further raising the boiling point compared to alcohols, which only have the -OH group as their polar site. The increased polarity of carboxylic acids contributes to stronger attractions between the molecules, requiring more heat to overcome these attractions.
Therefore, carboxylic acids have higher boiling points than alcohols of comparable molecular masses due to their ability to form stronger hydrogen bonds, the dimerization of molecules, and the increased polarity of the carboxyl group, all of which contribute to stronger intermolecular forces.