Q3(a). Toluene to m-bromotoluene: an amino group as a temporary director

Introduction. CH₃ is an ortho/para director, so direct bromination of toluene gives 2- and 4-bromotoluene, never the 3-isomer. The bromine must be placed by a stronger ortho/para director that sits para to CH₃; that group (-NHCOCH₃) is afterwards removed by diazotisation and reduction (deamination).

Scheme: Steps 1–3: nitration, reduction, acetylation.

Scheme: Steps 4–5: bromination directed by NHAc, then removal of the acetyl group.

Scheme: Steps 6–7: diazotisation and replacement of N₂⁺ by H.

1. Nitration of toluene gives 2- and 4-nitrotoluene; the para isomer is separated (fractional crystallisation/distillation).
2. Reduction (Sn/HCl or H₂/Ni) gives 4-methylaniline (p-toluidine).
3. Acetylation with acetic anhydride gives 4-methylacetanilide. -NHCOCH₃ is still a strong ortho/para director but is less activating than -NH₂, so bromination stops at monosubstitution.
4. Bromination: the para position of NHAc is taken by CH₃, so Br enters ortho to NHAc, which is meta to CH₃. Product: 2-bromo-4-methylacetanilide.
5. Hydrolysis (acid or base) releases the amine: 2-bromo-4-methylaniline.
6. Diazotisation with NaNO₂/HCl at 0–5°C gives the arenediazonium chloride.
7. Hypophosphorous acid (H₃PO₂) replaces -N₂⁺ by H (radical chain reduction), leaving 3-bromotoluene (m-bromotoluene).

Conclusion. Toluene → 4-nitrotoluene → p-toluidine → p-methylacetanilide → 2-bromo-4-methylacetanilide → 2-bromo-4-methylaniline → diazonium salt → 3-bromotoluene. The amino group places Br and is then removed.

Application. Temporary amino or acetamido directors removed by H₃PO₂ give meta-disubstituted benzenes that cannot be made directly, such as 1,3,5-tribromobenzene from aniline.