Q2(a). p-Bromonitrobenzene: Sandmeyer and von Richter routes

Introduction. 1-Bromo-4-nitrobenzene is taken down two routes. On the left the nitro group is reduced and the amine is replaced by a nitrile (Sandmeyer). On the right cyanide reacts in the von Richter reaction, which puts a COOH group ortho to the place the nitro group left (cine substitution); the acid is then made into an amide and degraded to an amine (Hofmann). The two routes end in isomers of different orientation.

Left route: M and N

Scheme: M = 4-bromoaniline; N = 4-bromobenzonitrile

M. Sn/HCl reduces -NO₂ to -NH₂ (via nitroso and hydroxylamine; six electrons): M = 4-bromoaniline. The C–Br bond is untouched.

N. Nitrous acid at 0–5°C converts the amine into the arenediazonium ion ArN₂⁺ (diazotisation); CuCN replaces N₂ by CN through an aryl radical and Cu(I)/Cu(II) electron transfer (Sandmeyer reaction): N = 4-bromobenzonitrile. The low temperature keeps the diazonium salt from losing N₂ to water.

Right route: O, P and Q

Scheme: O = 3-bromobenzoic acid; P = 3-bromobenzamide; Q = 3-bromoaniline

O. The nitro group activates the ring towards nucleophiles, but cyanide does not displace Br. It adds at the carbon ortho to NO₂ (meta to Br). The nitrile carbon is then attacked by an oxygen of the neighbouring nitro group; through a cyclic intermediate the nitrogen atoms of the two groups end up bonded together and leave as N₂, and the former nitrile carbon becomes COOH. The nitro group is lost and the acid sits on the carbon next to where it was: O = 3-bromobenzoic acid. The release of N₂ (shown with 15N labels) proves that the nitrogen of the nitrile and of the nitro group combine.

P. SOCl₂ gives the acid chloride (-SO₂, -HCl), and ammonia gives P = 3-bromobenzamide, ArCONH₂.

Q. In the Hofmann rearrangement Br₂/NaOH forms the N-bromoamide; its anion loses Br⁻ while the aryl group moves from carbon to nitrogen (concerted 1,2-shift, with retention at the migrating carbon), giving the isocyanate ArN=C=O. Water hydrolyses it to the carbamic acid, which loses CO₂: Q = 3-bromoaniline, one carbon shorter than P.

Conclusion. M = 4-bromoaniline, N = 4-bromobenzonitrile, O = 3-bromobenzoic acid, P = 3-bromobenzamide and Q = 3-bromoaniline. The Sandmeyer route keeps the para pattern; the von Richter route gives the meta isomers, because the new group enters ortho to the departing nitro group.

Application. The Sandmeyer reaction is the standard way to put CN, Cl or Br into positions that direct substitution cannot reach; the Hofmann rearrangement is used industrially to make anthranilic acid from phthalimide (dyes, saccharin).