The correct increasing order of basic strength for the following compounds is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → II < I < III
The basic strength of an aromatic amine depends on how easily the lone pair on nitrogen is available to accept a proton. In aniline this lone pair is partly drawn into the ring by resonance. A group at the para position changes the electron density on nitrogen:
II (p-nitroaniline), weakest — –NO2 is an electron withdrawing group; it pulls the lone pair further away from nitrogen and decreases the basic strength below that of aniline.
I (aniline), middle — no substituent; the reference compound.
III (p-toluidine), strongest — –CH3 is an electron releasing group; it pushes electron density towards nitrogen and increases the basic strength above that of aniline.
NCERT (2026-27 edition):
9.6 Chemical Reactions – “… In case of substituted aniline, it is observed that electron releasing groups like –OCH3, –CH3 increase basic strength whereas electron withdrawing groups like –NO2, –SO3H, –COOH, –X decrease it.”