Identify A, B, and C in the following reaction:

The correct answer is Option (2) → A: 2,4-dinitrochlorobenzene; B: 2,4-dinitrophenol; C: 2,4-diaminophenol

Three ideas decide the sequence: how the groups already on the ring direct a new nitro group; how nitro groups at the ortho and para positions make the chlorine easy to replace by –OH; and that tin and hydrochloric acid reduce every nitro group to an amino group.

In 1-chloro-4-nitrobenzene the chlorine directs to its ortho position (its para position is taken), and the nitro group directs to its meta position. Both point to the same carbon, C-2, so nitration gives 1-chloro-2,4-dinitrobenzene (A):

    \( \displaystyle \ce{ClC6H4NO2 + HNO3} \xrightarrow{\text{conc. }\ce{H2SO4}} \ce{ClC6H3(NO2)2 + H2O} \)

An aryl chloride normally resists nucleophilic substitution, but the two nitro groups ortho and para to the chlorine withdraw electrons and stabilise the negatively charged intermediate, so warm aqueous NaOH replaces Cl by –OH. Acidification frees the phenol, 2,4-dinitrophenol (B):

    \( \displaystyle \ce{ClC6H3(NO2)2 + 2NaOH -> NaOC6H3(NO2)2 + NaCl + H2O} \)

    \( \displaystyle \ce{NaOC6H3(NO2)2 + H+ -> HOC6H3(NO2)2 + Na+} \)

Sn/HCl is a strong reducing agent and reduces both nitro groups, giving 2,4-diaminophenol (C):

    \( \displaystyle \ce{HOC6H3(NO2)2 + 12[H]} \xrightarrow{\ce{Sn}/\ce{HCl}} \ce{HOC6H3(NH2)2 + 4H2O} \)

So A, B and C are 2,4-dinitrochlorobenzene, 2,4-dinitrophenol and 2,4-diaminophenol.

Shortcut: Options (2), (3) and (4) agree on A and B. Since Sn/HCl reduces every nitro group, C must have no nitro group left, which leaves only Option (2).

Why the other options are incorrect

• Option (1) A: 3,4-dinitrochlorobenzene; B: 3,4-dinitrophenol; C: 3,4-diaminophenol — Incorrect. The new nitro group would have to enter meta to chlorine and ortho to the first nitro group, against the directing effect of both groups.

• Option (3) A: 2,4-dinitrochlorobenzene; B: 2,4-dinitrophenol; C: 2-amino-4-nitrophenol — Incorrect. Reducing only one nitro group needs a mild, selective reagent such as ammonium sulphide; Sn/HCl reduces both.

• Option (4) A: 2,4-dinitrochlorobenzene; B: 2,4-dinitrophenol; C: 2-nitro-4-aminophenol — Incorrect. This is again a partial reduction, which Sn/HCl does not stop at.