In the following series of chemical reactions, identify the final product D.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2-bromotoluene
This sequence uses a nitrogen group as a temporary director. The group steers the bromine into place and is then removed completely, by reducing it to –\( \ce{NH2} \), diazotising it, and replacing the diazonium group with hydrogen using \( \ce{H3PO2} \).
Bromination (A): In 4-nitrotoluene, –\( \ce{CH3} \) is an activating ortho/para director, and its para position is already taken. –\( \ce{NO2} \) is a meta director. Both groups point to the same carbon, ortho to –\( \ce{CH3} \) and meta to –\( \ce{NO2} \). So A is 2-bromo-4-nitrotoluene:
\( \displaystyle \ce{CH3C6H4NO2 + Br2 -> CH3C6H3(Br)NO2 + HBr} \)
Reduction (B): Sn/HCl reduces –\( \ce{NO2} \) to –\( \ce{NH2} \), giving 3-bromo-4-methylaniline:
\( \displaystyle \ce{CH3C6H3(Br)NO2 + 6[H]} \xrightarrow{\ce{Sn/HCl}} \ce{CH3C6H3(Br)NH2 + 2H2O} \)
Diazotisation (C): \( \ce{NaNO2} \)/HCl at 273–278 K converts –\( \ce{NH2} \) into the diazonium group, –\( \ce{N2^{+}Cl^{-}} \). Writing Ar for the 3-bromo-4-methylphenyl group:
\( \displaystyle \ce{ArNH2 + NaNO2 + 2HCl} \xrightarrow{\text{273-278 K}} \ce{ArN2^{+}Cl^{-} + NaCl + 2H2O} \)
Deamination (D): Hypophosphorous acid replaces the diazonium group by hydrogen:
\( \displaystyle \ce{ArN2^{+}Cl^{-} + H3PO2 + H2O -> ArH + N2 ^ + H3PO3 + HCl} \)
The nitrogen group has now gone, leaving –\( \ce{CH3} \) and –Br on neighbouring carbons. So D is 2-bromotoluene.
Why the other options are incorrect
Option (1) toluene — Incorrect. Only the diazonium group is replaced by hydrogen; the bromine atom stays on the ring.
Option (2) 3-bromotoluene — Incorrect. The position meta to –\( \ce{CH3} \) is ortho to –\( \ce{NO2} \), and neither group directs bromine there.
Option (3) 4-bromotoluene — Incorrect. The para position was held by the nitrogen group, which is replaced by hydrogen, not by bromine. Replacing it by Br would need CuBr/HBr (Sandmeyer reaction).
NCERT (2026-27 edition):
Replacement by H
