Successive Photosubstitution of Hexachlorobenzene with Cyanide Ion

We report a novel nucleophilic polysubstitution reaction of hexachlorobenzene (HCB) with cyanide ion in acetonitrile/water. Successive photocyanations of HCB occur with high quantum yield (φdiss → 0.18) without the need for an electron acceptor, to give as products pentacyanophenol, 4-chloro-2,3,5,6-tetracyanophenol, and a dichlorotricyanophenol. The phenol functional group is introduced by competing hydrolysis of the polycyanochlorinated benzenes. Sensitization and quenching experiments indicate a triplet reactive excited state. Variation of [CN-] at constant [HCB] follows the expected relationship φdiss −1 ∝ [CN-]-1, but variation of [HCB] at constant [CN-] shows that the reaction becomes less efficient with increasing [HCB], consistent with the formation of an unproductive excimer