Which of the following groups when present at para position increases the basic strength of aniline?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → –NH2
Aniline is a base because the lone pair on its nitrogen can accept a proton. A group on the ring that releases electrons raises the electron density on this nitrogen, so the base becomes stronger; a group that withdraws electrons lowers it, so the base becomes weaker.
–NH2 at the para position releases electrons into the ring through the lone pair on its nitrogen, in the same way as –OCH3. So p-aminoaniline (benzene-1,4-diamine) is a stronger base than aniline.
Why the other options are incorrect
Option (1) –COOH — Incorrect. The carboxyl group is electron withdrawing, so it decreases the basic strength of aniline.
Option (3) –Br — Incorrect. A halogen is electron withdrawing (–X), so it decreases the basic strength.
Option (4) –NO2 — Incorrect. The nitro group is strongly electron withdrawing, so it decreases the basic strength.
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.”