Q.1 (viii) The correct order for basic strength of amines and ammonia is _____.
A. NH3 > R–NH2 > R2NH > R3N
B. NH3 > R3N > R2NH > R–NH2
C. NH3 < R–NH2 > R3N > R2NH
D. NH3 < R–NH2 < R2NH > R3N
Best answer
Answer: D. NH3 < R–NH2 < R2NH > R3N
Answer: (d) NH3 < R–NH2 < R2NH > R3N
Alkyl groups are electron-releasing (+I effect), so aliphatic amines are stronger bases than ammonia, and the secondary amine is more basic than the primary amine. In aqueous solution the tertiary amine is weaker than the secondary amine because its conjugate acid is poorly stabilised by hydrogen bonding with water and the bulky alkyl groups hinder solvation. So the secondary amine is the strongest base, as option (d) shows (e.g. for methyl amines in water: (CH3)2NH > CH3NH2 > (CH3)3N > NH3). Options (a), (b) and (c) place ammonia or the secondary amine wrongly.
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