Which of the following is not a reason for carboxylic acids having higher boiling point than alcohols of comparable molecular masses?

Presence of more alkyl groups in carboxylic acids

The correct answer is option 4. Presence of more alkyl groups in carboxylic acids.

Let us delve into why carboxylic acids generally have higher boiling points compared to alcohols of comparable molecular masses, focusing on the factors mentioned:

Factors Contributing to Higher Boiling Points of Carboxylic Acids:

1. Presence of Electron-Withdrawing Carbonyl Group:

Carboxylic acids contain a carbonyl group \((C=O)\), which is highly electronegative due to the oxygen atom pulling electron density towards itself. This increases the polarity of the molecule overall. In contrast, alcohols have a hydroxyl group \((OH)\), which is less electronegative than the carbonyl group. The increased polarity in carboxylic acids leads to stronger intermolecular interactions (like dipole-dipole interactions), which require more energy to overcome, thus raising the boiling point.

2. Higher Polarity of OH Bond in Carboxylic Acids:

In carboxylic acids, the \(OH\) bond is more polar compared to alcohols. This increased polarity enhances the formation of hydrogen bonds between carboxylic acid molecules. Hydrogen bonding is a strong intermolecular force that requires a significant amount of energy to break, contributing further to the higher boiling point of carboxylic acids.

3. Formation of Cyclic Dimers Through Hydrogen Bonding:

Carboxylic acids can form cyclic dimers in the liquid state through intermolecular hydrogen bonding. This involves two carboxylic acid molecules forming hydrogen bonds between the \(COOH\) groups, resulting in a stable dimer structure. The formation of these cyclic dimers increases the cohesion between molecules in the liquid phase, leading to a higher boiling point.

4. Comparison with Alcohols:

Alcohols of comparable molecular masses typically have lower boiling points compared to carboxylic acids due to the absence of the electron-withdrawing carbonyl group and the lower polarity of the \(OH\) bond. While alcohols can also participate in hydrogen bonding, the interactions are generally weaker compared to carboxylic acids due to the differences in polarity and structure.

Conclusion: In summary, the higher boiling points of carboxylic acids compared to alcohols of similar molecular masses are primarily due to the presence of the electron-withdrawing carbonyl group, the higher polarity of the OH bond, and the ability to form stable cyclic dimers through hydrogen bonding. These factors collectively contribute to stronger intermolecular interactions in carboxylic acids, requiring more energy to separate molecules in the liquid phase, thus resulting in higher boiling points.