Identify A, D
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A = 2-bromo-4-nitrotoluene, D = 2-bromobenzoic acid
The sequence starts from 4-nitrotoluene and uses the –\( \ce{NH2} \) group (made from –\( \ce{NO2} \)) only as a temporary group that is removed through a diazonium salt. First, bromination: –\( \ce{CH3} \) directs ortho/para and –\( \ce{NO2} \) directs meta, so both send Br to the carbon ortho to CH3. A is 2-bromo-4-nitrotoluene:
\( \displaystyle \ce{CH3C6H4NO2 + Br2} \xrightarrow{\ce{FeBr3}} \ce{CH3C6H3(Br)NO2 + HBr} \)
Next, Sn/HCl reduces the nitro group to –\( \ce{NH2} \) (B, 3-bromo-4-methylaniline):
\( \displaystyle \ce{CH3C6H3(Br)NO2 + 6[H]} \xrightarrow{\ce{Sn/HCl}} \ce{CH3C6H3(Br)NH2 + 2H2O} \)
Then nitrous acid at 273–278 K diazotises the amine (C):
\( \displaystyle \ce{CH3C6H3(Br)NH2 + NaNO2 + 2HCl} \xrightarrow{\text{273-278 K}} \ce{CH3C6H3(Br)N2^+Cl^- + NaCl + 2H2O} \)
Hypophosphorous acid replaces the diazonium group by H, giving 2-bromotoluene:
\( \displaystyle \ce{CH3C6H3(Br)N2^+Cl^- + H3PO2 + H2O -> CH3C6H4Br + N2 + H3PO3 + HCl} \)
Finally, acidified \( \ce{KMnO4} \) oxidises the side-chain –\( \ce{CH3} \) to –COOH, so D is 2-bromobenzoic acid:
\( \displaystyle \ce{CH3C6H4Br + 3[O]} \xrightarrow{\ce{KMnO4/H+}} \ce{HOOCC6H4Br + H2O} \)
Option (4) shows this pair: the toluene with Br ortho to \( \ce{CH3} \) and \( \ce{NO2} \) para to it, and benzoic acid with Br ortho to COOH.
Why the other options are incorrect
Option (1), 3-bromo-4-nitrotoluene and 2-bromobenzaldehyde — Incorrect. A has Br ortho to \( \ce{NO2} \) (meta to \( \ce{CH3} \)), a position both groups disfavour, and D is drawn as 2-bromobenzaldehyde; \( \ce{KMnO4} \) oxidises the side chain right up to –COOH.
Option (2), 2-bromo-4-nitrotoluene and a nitro-substituted bromobenzoic acid — Incorrect. A is right, but D still carries the \( \ce{NO2} \) group; the nitro group was removed in the Sn/HCl, diazotisation and \( \ce{H3PO2} \) steps.
Option (3), a bromobenzoic acid and 2-bromo-3-nitrotoluene — Incorrect. The structures are in the wrong order (the benzoic acid is given as A), and the toluene drawn is 2-bromo-3-nitrotoluene, with \( \ce{NO2} \) next to Br instead of para to \( \ce{CH3} \).
NCERT (2026-27 edition):
Replacement by H – “Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.”