Question

Why primary amines have higher boiling points than tertiary amines?

Hint:

More N-H bonds = More hydrogen bonding = Higher boiling point. Order of boiling points: $1^\circ \gt 2^\circ \gt 3^\circ$.

Answer 1

Step 1: Hydrogen bonding in primary amines.
Primary amines ($R-NH_2$) have two N-H bonds. Nitrogen is electronegative, making these bonds polar. Primary amine molecules can form intermolecular hydrogen bonds ($N-H \cdots N$) with adjacent molecules.
Step 2: No hydrogen bonding in tertiary amines.
Tertiary amines ($R_3N$) have no N-H bonds at all. Without N-H bonds, intermolecular hydrogen bonding between tertiary amine molecules is not possible. They interact only through weaker van der Waals forces.
Step 3: Effect on boiling point.
Breaking intermolecular hydrogen bonds requires significant thermal energy. The more extensive the hydrogen bonding network, the more energy is required to vaporise the substance and the higher the boiling point.
Step 4: Conclusion.
Primary amines, having strong and extensive intermolecular hydrogen bonds, require more energy to vaporise than tertiary amines. Therefore, primary amines have higher boiling points than tertiary amines of similar molecular mass.