Comprehension:
Physical and Chemical properties of Amines
Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridised. Each of the three sp3 hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base as well as act as a nucleophile. Lower aliphatic amines are soluble in water because they form hydrogen bond with water molecules and also form strong intermolecular hydrogen bonding. Solvation, +I and +R groups affect the electron density over nitrogen atom. Their pKb value lies in the range of 3 to 4.22. The most widely used method for preparing amines involves the reduction of nitro group, nitriles, amides, etc. using different reagents like H2/Pd, Sn+HCl, H2/Ni, LiAlH4+H2O, Br2+NaOH etc. Primary amines can be easily detected by the carbylamine test and Hinsberg's reagent. Diazonium salt can be synthesized from amines easily for synthesis of variety of new compounds.
Which of the following compounds is least basic ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → p-Nitroaniline
The basic strength of an amine depends on how available the lone pair on nitrogen is for accepting a proton. The lower the pKb, the stronger the base.
Aliphatic amines: alkyl groups push electrons towards nitrogen (+I effect), so they are stronger bases than ammonia.
Aromatic amines: the lone pair is delocalised into the benzene ring by resonance, so it is less available, and they are weaker bases than ammonia.
Substituents on the ring: an electron donating group (–\( \ce{CH3} \)) increases basicity; an electron withdrawing group (–\( \ce{NO2} \)) pulls the lone pair further into the ring by its −R and −I effects and decreases basicity strongly, especially from the para position.
Ethylamine (pKb 3.29) > p-toluidine (8.92) > aniline (9.38) > p-nitroaniline (13.0)
So the least basic compound is p-nitroaniline.
Why the other options are incorrect
Option (1) Aniline — Incorrect. Its lone pair is delocalised into the ring, but there is no electron withdrawing group to reduce its basicity further.
Option (2) p-Toluidine — Incorrect. The –CH3 group donates electrons, so it is slightly more basic than aniline.
Option (4) Ethylamine — Incorrect. An aliphatic amine; the +I effect of the ethyl group makes it the strongest base of the four.
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.”