Photochemistry of 2′-halobenzanilides: Reaction mechanism

The UV photolysis of acetonitrile solutions of 2′-halobenzanilide were followed by UV-spectroscopy, HPLC and time resolved laser flash photolysis. The chemical yields of the major products produced from the irradiation with medium pressure Hg-lamp were measured. The chemical yields of the photosubstitution product ranged from 3 to 45% (the yield was 3% when the starting material was a solution of 2′-chlorobenzanilide in CH₃CN and was 45% when the starting material was a solution of 2′-bromobenzanilide in 10% 1 mol NaOH:CH₃CN). The quantum yield measurements of the photoproducts were carried out in N₂ and O₂ atmosphere in the presence and absence of OH ions. The results showed an enhancement in the yield of the photosubstituted product in the presence of OH ions and in switching from chloro- to bromo-substituent. The presence of O₂ lowered the yield of photosubstitution and photoreduction products. Time resolved studies showed two peaks, one appeared at 395 nm and the second centred around 315 nm. These two peaks were identical in both cases of 2′-halobenzanilides (halo = Cl or Br).