Amines are reactive in nature, mainly due to difference in electronegativity between Nitrogen and Hydrogen and, due to lone pair of electrons on Nitrogen. Amines behaves like nucleophiles and as good Lewis bases and there are various reactions to support this. Different degree of amines differ in their reactivity due to steric hindrance and electronic effects. Aromatic amines are less basic due to +R effect, hence have higher \(pK_b\) value than the aliphatic amines. Basic strength of amines also depend on stability of corresponding conjugate acid in aqueous medium.
Strongest Bronsted base out of the following will be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Toluidine
A Brønsted base accepts a proton with the lone pair on nitrogen; the more available that lone pair, the stronger the base.
Toluidine (\( \ce{CH3C6H4NH2} \)) has a free lone pair on an amino nitrogen, and the –CH3 group is electron releasing: it pushes electron density into the ring and towards nitrogen and increases the basic strength over aniline.
Why the other options are incorrect
Option (1) Pyrrole — Incorrect. Its nitrogen lone pair is part of the aromatic sextet; protonating it would destroy the aromaticity, so pyrrole is practically non-basic.
Option (2) Pyridine — Incorrect. Its lone pair sits in an sp2 orbital (33% s character), which holds the electrons closer to the nucleus, so it is less available than the lone pair of an alkyl-substituted aniline.
Option (4) 4-Nitrobenzamine — Incorrect. The –\( \ce{NO2} \) group is strongly electron withdrawing (−R, −I) and pulls the lone pair into the ring, so it is a very weak base.
NCERT (2026-27 edition):
Arylamines versus Ammonia – “… We know that greater the number of resonating structures, greater is the stability. Thus you can infer that aniline (five resonating structures) is more stable than anilinium ion. Hence, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia. 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.”