Answer the question on the basis of passage given below:
There are three kinds of amines which are primary, secondary and tertiary amines. These amines may have alkyl group or aryl group or both alkyl and aryl groups. These amines can be distinguished from one another by various chemical tests.
A and B are, respectively:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → nitrobenzene and aniline
The sequence is the route from benzene derivatives to an azo dye: reduction of a nitro compound to an amine, diazotisation of the amine, and coupling of the diazonium salt with phenol.
The final product, p-hydroxyazobenzene, is formed by coupling benzenediazonium chloride (C) with phenol in alkaline medium:
\( \ce{C6H5N2+Cl- + C6H5OH} \xrightarrow{\ce{OH-}} \ce{C6H5-N=N-C6H4-OH + Cl- + H2O} \) (OH para to the azo group)
C is made from aniline (B) with \( \ce{NaNO2} \) + HCl at 273-277 K:
\( \ce{C6H5NH2 + NaNO2 + 2HCl} \xrightarrow{\ce{273-277 K}} \ce{C6H5N2+Cl- + NaCl + 2H2O} \)
B is obtained from A by catalytic hydrogenation with \( \ce{H2} \)/Pd in ethanol, so A is nitrobenzene:
\( \ce{C6H5NO2 + 3H2} \xrightarrow[\ce{ethanol}]{\ce{Pd}} \ce{C6H5NH2 + 2H2O} \)
So A and B are nitrobenzene and aniline.
Why the other options are incorrect
Option (2) aniline and nitrobenzene — Incorrect. The order is reversed; \( \ce{H2} \)/Pd reduces a nitro group, it does not turn aniline into nitrobenzene.
Option (3) aniline and benzenediazonium chloride — Incorrect. Benzenediazonium chloride is C, made from B; and \( \ce{H2} \)/Pd does not convert aniline into a diazonium salt.
Option (4) nitrobenzene and benzenediazonium chloride — Incorrect. Hydrogenation of nitrobenzene gives aniline, not the diazonium salt; the diazonium salt is C.
NCERT (2026-27 edition):
Preparation of Amines – “… Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium. Nitroalkanes can also be similarly reduced to the corresponding alkanamines.”
Chemical Reactions – “… Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction.”