Photochemical behaviour of halophenols in aqueous solution

Abstract

The mechanism of photolysis by direct absorption of chloro-, bromo-and fluorophenols (XPhOH with X=Cl, Br or F) is much more dependent on the position of the halogen than on its nature. In all cases, the first step is a heterolytic C-X scission with release of the halogenated acid HX. 3-XPhOH is almost specifically converted into resorcinol. The anionic form of 2-XPhOH is transformed with a rather high quantum yield into cyclopentadiene carboxylic acids. By nanosecond laser-flash photolysis the first transient detected is a ketene which is converted into fulvene-6, 6 diol and then into the cyclopentadiene carboxylic acids. The phototransformation is about 10 times less efficient from the neutral form and not so specific. The irradiation of 4-XPhOH leads to the formation of a carbene which is the first detected transient. According to the experimental conditions, this transient reacts with oxygen producing a benzoquinone-O-oxide and subsequentlyp-benzoquinone, with water leading to hydroquinone or with another molecule of halophenol producing a halogenodihydroxybiphenyl. It was also observed that 4-chlororesorcinol behaves as both 4-ClPhOH and 2-ClPhOH. Chlorohydroquinone does not react as monohalophenols, the main photoproducts are hydroquinone and chlorobenzoquinone. This reaction is consistent with a radical mechanism. The transformation of halophenols can be sensitized by phenol and in most cases by hydroquinone. The half-life of the triplet state of hydroquinone was evaluated at 0.9µs and the intersystem crossing yield at 0.39. The sensitization of 3-FPhOH was observed with phenol but not with hydroquinone leading to the conclusion that the energy level of its triplet state lies in the range 310–350 kJ mol⁻¹. It is lower than 310 kJ for the other halophenols studied here.