Question

The boiling points of amines are lower than their comparable alcohols and carboxylic acids. Explain.

Hint:

Always compare hydrogen bonding capacity when reasoning about boiling points: stronger hydrogen bonding leads to higher boiling points. Oxygen forms stronger H-bonds than nitrogen, and carboxylic acids form dimers, making them especially high boiling.

Answer 1

Step 1: Intermolecular forces in amines.
Amines can form intermolecular hydrogen bonds because of the presence of a nitrogen atom with a lone pair. However, N–H bonds are less polar than O–H bonds, and nitrogen is less electronegative than oxygen. Thus, the hydrogen bonding in amines is weaker compared to alcohols and carboxylic acids.

Step 2: Intermolecular forces in alcohols.
Alcohols have O–H bonds, which are more polar and form stronger hydrogen bonds than N–H bonds. Hence, alcohols have higher boiling points than corresponding amines.

Step 3: Intermolecular forces in carboxylic acids.
Carboxylic acids form very strong dimeric hydrogen bonds in which two molecules are linked via two O–H…O bonds. This gives them the highest boiling points among the three classes of compounds. Conclusion:
Boiling point order for comparable molecular masses is: \[ \text{Carboxylic acids} > \text{Alcohols} > \text{Amines} \]