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 that tertiary amines?
(iii) Why are aliphatic amines stronger bases than aromatic amines?
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 that tertiary amines?
(iii) Why are aliphatic amines stronger bases than aromatic amines?
Part (i):
Amines are less acidic than alcohols of comparable molecular masses. The reason is explained below:
- Amines lose a proton to form an amide ion. Alcohols lose a proton to form alkoxide ions.
- Formation of an amide ion: R−NH2⟶R−NH−+H+
- Formation of an alkoxide ion: R−OH⟶R−O−+H+
- Since oxygen is more electronegative than nitrogen, So, the alkoxide ion is more stable as compared to an amide ion. Hence, alcohol is more acidic than amines
Part (ii):
In primary amines, Nitrogen atoms have two Hydrogen atoms which result in extensive intermolecular H - bonding. In tertiary amines, Nitrogen atoms do not have any Hydrogen atoms and hydrogen bonding is not possible.
Hence, primary amines have a higher boiling point than tertiary amines.
Part (iii):
Aliphatic amines are stronger bases than aromatic amines due to following reasons:
(a) The lone pair of electrons of the nitrogen atom of aromatic amines is involved in conjugation with the π−bond pairs of the ring as follows
(b) Anilinium ion obtained by accepting a proton is less stabilized by resonance. Due to the −R effect of the benzene ring, the electrons on the N- atom are less available in the case of aromatic amines. Therefore, the electrons on the N-atom in aromatic amines cannot be donated easily.
So, aniline is a weaker base than alkyl amines, in which +1 effect increases the electron density on the nitrogen atom.
Part (i):
Amines are less acidic than alcohols of comparable molecular masses. The reason is explained below:
- Amines lose a proton to form an amide ion. Alcohols lose a proton to form alkoxide ions.
- Formation of an amide ion: R−NH2⟶R−NH−+H+
- Formation of an alkoxide ion: R−OH⟶R−O−+H+
- Since oxygen is more electronegative than nitrogen, So, the alkoxide ion is more stable as compared to an amide ion. Hence, alcohol is more acidic than amines
Part (ii):
In primary amines, Nitrogen atoms have two Hydrogen atoms which result in extensive intermolecular H - bonding. In tertiary amines, Nitrogen atoms do not have any Hydrogen atoms and hydrogen bonding is not possible.
Hence, primary amines have a higher boiling point than tertiary amines.
Part (iii):
Aliphatic amines are stronger bases than aromatic amines due to following reasons:
(a) The lone pair of electrons of the nitrogen atom of aromatic amines is involved in conjugation with the π−bond pairs of the ring as follows
(b) Anilinium ion obtained by accepting a proton is less stabilized by resonance. Due to the −R effect of the benzene ring, the electrons on the N- atom are less available in the case of aromatic amines. Therefore, the electrons on the N-atom in aromatic amines cannot be donated easily.
So, aniline is a weaker base than alkyl amines, in which +1 effect increases the electron density on the nitrogen atom.
