Photoinduced reactions—L : Photolysis of halogenophenols in aqueous alkali and in aqueous cyanide

Abstract

On irradiation in aqueous alkali, several monohalogenophenols were found to react in several ways depending on the position of a substituent halogen. p-Substituted halogenophenols yielded 2,4′-dihydroxy-4-halogenodiphenyl (I). 2,4′-dihydroxydiphenyl (II), and phenol (III), in addition to the already known products, hydroquinone (V) and 4,4′-dihydroxydiphenyl (IV). The yield of V decreased in the order of Cl > Br > I, while those of III and IV increased in the order of Cl < Br < I. The effects of alkali concentration and of temperature on the reaction were examined with p-chlorophenol. Higher concentration of alkali and higher temperature accelerated the reaction and favoured the formation of III and V. m-Chlorophenol, on the other hand. gave resorcinol in high yield, which shows the electrophilic nature of the excited m-chlorophenol. o-Chlorophenol gave only a complex mixture of products including a large amount of a resinous material. Irradiation of p-halogenophenols in aqueous cyanide yielded p-cyanophenol in good yields, accompanied by a small amount of hydroquinone, while irradiation of m-chlorophenol gave resorcinol in high yield, accompanied by a small amount of m-cyanophenol. o-Chlorophenol again gave only intractable products. Reactions with a light at 2537 Å and with that over 2900 Å were substantially the same. although longer period of irradiation was required in the latter case. Some mechanistic discussions are included.