Comprehension:
Amines are derivatives of ammonia obtained by replacement of one, two or all the three hydrogen atoms by alkyl and / or aryl groups. Nitrogen atom of amines like ammonia is trivalent and carries an unshared pair of electrons. 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. Due to the presence of unshared pair of electrons, the angle C-N-E (where E is C or H) is less than tetrahedral angle 109.5°. Amines are classified as primary (1°), secondary (2°) and tertiary (3°) amines. All amines are basic in nature and basicity differs between primary amines, secondary amines and tertiary amines. In aryl amines, –NH2 group is directly attached to the benzene ring. C6H5NH2 is simplest example of arylamine. Lower aliphatic amines are soluble in water because they can form hydrogen bonds with water molecules.
To prepare a monosubstituted aniline derivative, the activating group –NH2 is to be controlled through acetylation with
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → acetic anhydride.
To obtain a monosubstituted product, the activating effect is reduced by acetylation with acetic anhydride (in the presence of pyridine), which converts –\( \ce{NH2} \) into the acetamido group, –\( \ce{NHCOCH3} \):
\( \displaystyle \ce{C6H5-NH2 + (CH3CO)2O -> C6H5-NH-CO-CH3 + CH3COOH} \)
In acetanilide the lone pair on nitrogen is shared with the neighbouring C=O group by resonance, so less of it is available to the ring, and the group is only moderately activating.
So the –\( \ce{NH2} \) group is controlled by acetylation with acetic anhydride.
Why the other options are incorrect
Option (1) acetic acid — Incorrect. With the basic amine it mainly forms a salt; it is not the reagent used for acetylation, which needs the more reactive anhydride or acid chloride.
Option (2) acetone — Incorrect. A ketone has no acyl group to transfer; with a primary amine it forms an imine (Schiff’s base), not an acetamide.
Option (4) ethanamine — Incorrect. It is itself an amine and cannot acetylate another amine.
NCERT (2026-27 edition):
Bromination – “… The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity. Substitution tends to occur at ortho- and para-positions. … This can be done by protecting the -NH2 group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.”
“… Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance. Therefore, activating effect of –NHCOCH3 group is less than that of amino group.”