Animated Solution for Chemistry - Organic Chemistry: The increasing order of boiling points of the following compounds is

Intermolecular Forces in Phenols

When we talk about the boiling point of an organic compound, we are essentially discussing the energy required to overcome the intermolecular forces holding the molecules together in the liquid phase. The stronger these forces, the higher the boiling point. In the case of substituted phenols, the primary forces at play are hydrogen bonding and dipole-dipole interactions.

Let's break down the intermolecular forces for each of the four para-substituted phenols in our problem.

Compound I (p-Cresol): Here, we have a methyl group ($-C{H}_3$) attached to the phenol ring. The methyl group is non-polar and cannot participate in hydrogen bonding. Therefore, the only significant intermolecular force is the hydrogen bonding arising from the hydroxyl ($-OH$) group itself. This provides a baseline for our comparison, making it the compound with the weakest intermolecular association.

Compound IV (p-Methoxyphenol): In this molecule, the substituent is a methoxy group ($-OC{H}_3$). Unlike the methyl group, the oxygen atom in the methoxy group possesses lone pairs, allowing it to act as a hydrogen bond acceptor. Additionally, the $-OC{H}_3$ group introduces dipole-dipole interactions. These extra forces mean that p-methoxyphenol will have a higher boiling point than p-cresol.

Extensive H-Bonding

Compound III (p-Aminophenol): Things get interesting with the amino group ($-N{H}_2$). Both the $-OH$ and $-N{H}_2$ groups can act as both hydrogen bond donors and acceptors. This dual capability allows p-aminophenol molecules to form an extensive, multi-directional network of intermolecular hydrogen bonds. Naturally, this requires significantly more thermal energy to break, pushing its boiling point higher than that of p-methoxyphenol.

Compound II (p-Nitrophenol): The nitro group ($-N{O}_2$) is highly polar and acts as an exceptionally strong hydrogen bond acceptor. The intermolecular hydrogen bonding between the strongly acidic $-OH$ proton of one molecule and the highly electronegative oxygen of the $-N{O}_2$ group of another molecule is incredibly robust. This intense intermolecular association gives p-nitrophenol the highest boiling point among the four compounds.

By evaluating the strength and extent of the intermolecular forces, we can confidently arrange the compounds in increasing order of their boiling points:

This logical deduction leads us directly to the correct option, (b).