Radicals formed by ultraviolet irradiation of substituted 4-chlorophenols

Abstract

Aqueous alkaline solutions (pH 8) of (I) 2-methyl-4-chlorophenol, (II) 2-cyclohexyl-4-chlorophenol, (III) 2-benzyl-4-chlorophenol, (IV) 2,2′-methylene-bis-(4-chlorophenol), (V) 2-methoxy-4-chlorophenol, have been irradiated in situ with u.v. light and studied by electron spin resonance spectroscopy with a view to investigating further the free radicals formed by photoallergens with this basic chemical structure. Radical species were observed in all cases within minutes of irradiation. The radical spectra were interpreted and assigned as arising from substituted semiquinone radical anions, showing the initial replacement of the 4-chloro group with OH to give the corresponding hydroquinones. In the presence of oxygen, the hydroquinones are oxidised to the corresponding semiquinone radicals. There was no direct evidence of the presence of initial carbon-centred aryl radicals formed by the loss of chlorine; radicals of this type were, however, demonstrated by spin-trapping experiments using 5,5-dimethyl-1-pyrroline N-oxide (DMPO).