Question

Which of the following compounds would have the highest boiling point? Select one: a. alcohols b. amines C. alkenes d. acetals

Answer 1

Alcohols are generally have high boiling point because of the strong hydrogen bonding with more electronegative oxygen. Where as Amines generally have lower boiling points than alcohols of comparable molar mass because amines have weaker hydrogen bonds than alcohols.

For example let us consider methanol (32 g/mol) and methylamine (32 g/mol).

Methanol, boiling point = 65 °C

Methylamine, boiling point = -6 °C

The strong intermolecular hydrogen bond is the reason for higher boiling point of methanol. The H-bond in methylamine is weak since it is between hydrogen and less electronegative nitrogen.

In alkenes, the only intermolecular force is the van der Waals force of interaction which is much weaker than hydrogen bonding. Hence alkenes have lower boiling point than alcohols and amines.

In the case of acetals, since hydrogen bonds cannot be formed between the molecules, the boiling point of this compound less compared to alcohol. The simplest acetal, dimethoxymethane, also called methylal with molar mass 76g/mol is having boiling point of 42°C.

Hence Alcohols have highest boiling point. (option A)