Photoreaction of nitrobenzenes with hydrobromic acid

Nitrobenzene and three of its derivatives (3-CO2H, 3-OH, and 4-OH) react efficiently when irradiated (λ >340 nm) in concentrated hydrobromic acid typically to give high yields of 2,4,6-tribromoanilines. The quantum yield (Φ = 0.16 for nitrobenzene) is not changed appreciably by the electron withdrawing carboxy substituent, but is lowered by the electron donating hydroxy substituent. The reactivity suggests that electron transfer from bromide ion to the n,π* triplet is the primary process.