632
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
NH 2
typical aliphatic
amine
NH 2
NH 2
NH 2
NH 2
resonance stabilization possible in the
base, but not the conjugate acid
NH 2
electron-withdrawing
inductive effect
H 2 N
CH 3
electron-donating effect
stabilizes canjugate acid
NH 2
electron-withdrawing
inductive effect
HN
HN
resonance effect from
both aromatic rings
sp 2
sp 3
As we move to N-methylaniline, we see only a modest change in pK a . This is undoubtedly due to the
electron-donating effect of the methyl group, and this would be expected to stabilized the conjugate acid,
increasing observed basicity. There is a modest increase in basicity, but it is apparent that the resonance
effect, as in aniline, is also paramount here, and this compound is also a weak base. However, diphenylamine
(N-phenylaniline) is an extremely weak base; this can be ascribed to the resonance effect allowing electron
delocalization into two rings.
On the other hand, benzylamine (pK a 9.3) is a much stronger base than aniline, somewhat weaker than
cyclohexylamine. Benzylamine is merely an aliphatic amine, with an electron-withdrawing aromatic ring
separated by one carbon from the nitrogen. There is no chance of a resonance effect as in aniline, and the
inductive effect is smaller than if the phenyl were directly bonded to nitrogen. The net result is a small
lowering of charge on nitrogen with destabilization of the conjugate acid, and hence a lower basicity than
cyclohexylamine.
(b) The reference point here is the acidity of cyclohexanecarboxylic acid (pK a 4.9), a typical aliphatic
acid. Benzoic acid is a stronger acid (the smaller the pK a , the stronger the acid), and this can be
ascribed to the hybridization-derived electron-withdrawing inductive effect of the phenyl ring. This helps
to delocalize the charge on the conjugate acid and is a stabilizing factor. Inductive effects in the opposite
direction destabilize the conjugate base, and we see a weakening of acidic strength. This is why 4methylbenzoic acid is not as strong an acid as benzoic acid, though the inductive effect of the methyl is
small.
CO 2 H
sp 2
inductive effect
stabilizes
conjugate base
CO 2 H
sp 3
CO 2 H
CH 3
inductive effect
destabilizes
conjugate base
HO
O
O
HO
O
O
resonance destabilizes
the conjugate base
HOW TO APPROACH EXAMINATION QUESTIONS: SELECTED PROBLEMS AND ANSWERS
NH 2
typical aliphatic
amine
NH 2
NH 2
NH 2
NH 2
resonance stabilization possible in the
base, but not the conjugate acid
NH 2
electron-withdrawing
inductive effect
H 2 N
CH 3
electron-donating effect
stabilizes canjugate acid
NH 2
electron-withdrawing
inductive effect
HN
HN
resonance effect from
both aromatic rings
sp 2
sp 3
As we move to N-methylaniline, we see only a modest change in pK a . This is undoubtedly due to the
electron-donating effect of the methyl group, and this would be expected to stabilized the conjugate acid,
increasing observed basicity. There is a modest increase in basicity, but it is apparent that the resonance
effect, as in aniline, is also paramount here, and this compound is also a weak base. However, diphenylamine
(N-phenylaniline) is an extremely weak base; this can be ascribed to the resonance effect allowing electron
delocalization into two rings.
On the other hand, benzylamine (pK a 9.3) is a much stronger base than aniline, somewhat weaker than
cyclohexylamine. Benzylamine is merely an aliphatic amine, with an electron-withdrawing aromatic ring
separated by one carbon from the nitrogen. There is no chance of a resonance effect as in aniline, and the
inductive effect is smaller than if the phenyl were directly bonded to nitrogen. The net result is a small
lowering of charge on nitrogen with destabilization of the conjugate acid, and hence a lower basicity than
cyclohexylamine.
(b) The reference point here is the acidity of cyclohexanecarboxylic acid (pK a 4.9), a typical aliphatic
acid. Benzoic acid is a stronger acid (the smaller the pK a , the stronger the acid), and this can be
ascribed to the hybridization-derived electron-withdrawing inductive effect of the phenyl ring. This helps
to delocalize the charge on the conjugate acid and is a stabilizing factor. Inductive effects in the opposite
direction destabilize the conjugate base, and we see a weakening of acidic strength. This is why 4methylbenzoic acid is not as strong an acid as benzoic acid, though the inductive effect of the methyl is
small.
CO 2 H
sp 2
inductive effect
stabilizes
conjugate base
CO 2 H
sp 3
CO 2 H
CH 3
inductive effect
destabilizes
conjugate base
HO
O
O
HO
O
O
resonance destabilizes
the conjugate base