Arrange the following amines in order of their increasing basic strength
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (C), (D), (A)
The basic strength of an amine depends on how freely the lone pair on nitrogen is available for a proton.
In aliphatic amines the alkyl groups push electron density onto nitrogen (+I effect), so they are much stronger bases than aryl amines.
In aryl amines the lone pair is delocalised into the benzene ring by resonance, so it is less available and the base is weak.
On the ring, an electron-releasing group such as \( \ce{-CH3} \) raises the basic strength, while an electron-withdrawing group such as \( \ce{-NO2} \) lowers it further.
The four amines:
(A) \( \ce{(CH3)2NH} \): dimethylamine, a secondary aliphatic amine (pKb 3.27).
(B) p-nitroaniline: aryl amine with an electron-withdrawing \( \ce{-NO2} \) group (pKb 13.0).
(C) aniline (pKb 9.38).
(D) p-toluidine: aryl amine with an electron-releasing \( \ce{-CH3} \) group (pKb 8.92).
A lower pKb means a stronger base.
Shortcut: p-nitroaniline (B) is the weakest and the aliphatic amine (A) the strongest base, and only Option (3) starts with (B) and ends with (A).