Question

Explain why propanol has a higher boiling point than that butane?

Answer 1

Reason for: High boiling point of Propanol than Butane:-

• Propanol (${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{OH}$) has a higher boiling point than that butane (${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$).
• The boiling point majorly depends upon 2 factors: Intermolecular bonding and Intermolecular forces

Intermolecular bonding:-

• The $-\mathrm{OH}$ group in propanol forms intermolecular hydrogen bonding.
• Thus it requires a lot of energy to break this hydrogen bond, as a result, the boiling point increases.
• But butane cannot form such bonds since it has no $-\mathrm{OH}$ group.
• Thus boiling point of butane is lower than propanol.

Intermolecular forces:-

• In the case of butane: The weak van der Waals force of attraction is the only force between the molecules.
• In the case of propanol: The formation of intermolecular hydrogen bonding and Van der Waals forces dominates.
• The effect of intermolecular forces and bonding are fully supported by the boiling point values recorded in theoretical data: The boiling point of propanol is $391\mathrm{K}$ and the boiling point of butane is $309\mathrm{K}$.

Hence, the hydroxyl group in propanol forms strong intermolecular forces of attraction in propanol which results in a high boiling point of propanol than butane.