Electron spin resonance studies of free-radical reactions in photolysed solutions of chloronitrobenzenes

Abstract

The e.s.r. signals detected during u.v. photolysis of chloronitrobenzenes in ethers are due to phenyl alkoxy nitroxide radicals and not to monohydronitrobenzene radicals as formerly supposed. The e.s.r. hyperfine splitting parameters of the radicals from eleven chlorine-substituted mononitrobenzenes are given. The alkoxy nitroxide radicals arise from the addition of solvent radicals, formed after hydrogen abstraction from the solvent by triplet-state nitrobenzene, to the parent nitrobenzene. The decay of alkoxy nitroxide radical derived from 2,3,5,6-tetrachloronitrobenzene in ethers involves a unimolecular non-ionic decomposition to yield the nitrosobenzene and an alkoxy-radical. β-Fission of the alkoxy-radical so formed can give an alkyl fragment which is then trapped by the nitrosobenzene to yield a characteristic alkyl nitroxide radical. Evidence is presented for the kinetic behaviour of alkyl aryl nitroxides in aqueous and aprotic systems which indicates that such radicals react with the monohydronitrobenzene radical during photolysis to yield a diamagnetic intermediate. In low-dielectric aprotic media this intermediate dissociates, to reform the original alkyl nitroxide radical after photolysis has ceased.