Question

Give plausible explanation for each of the following: 

(i) Why are amines less acidic than alcohols of comparable molecular masses? 

(ii) Why do primary amines have higher boiling point than tertiary amines? 

(iii) Why are aliphatic amines stronger bases than aromatic amines?

Answer 1

(i) Amines undergo deprotonation to form amide ions, while alcohols undergo deprotonation to form alkoxide ions. Because oxygen (O) is more electronegative than nitrogen (N), the negative charge is more readily accommodated in alkoxide ions (RO−) than in amide ions (R−NH−). Consequently, amines exhibit lower acidity compared to alcohols of similar molecular weights.

(ii) In primary amines, nitrogen atoms possess two hydrogen atoms, facilitating extensive intermolecular hydrogen bonding. Conversely, tertiary amines lack hydrogen atoms on nitrogen, rendering hydrogen bonding impossible. As a result, primary amines exhibit higher boiling points than tertiary amines.

(iii) Aliphatic amines display greater basicity than aromatic amines for several reasons: (a) Aromatic amines exhibit resonance, causing the lone pair of electrons on the nitrogen atom to be delocalized over the benzene ring, making it less available for protonation. (b) Aryl amine ions are less stable than alkyl amine ions, making protonation of aromatic amines less favorable.