PHOTOCHEMICAL BEHAVIOR OF HALOPHENOLS IN AQUEOUS-SOLUTION

The mechanism of photolysis by direct absorption of chloro-, bromo- an d 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 fir st 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 yie ld into cyclopentadiene carboxylic acids. By nanosecond laser-flash ph otolysis the first transient detected is a ketene which is converted i nto fulvene-6, 6 diol and then into the cyclopentadiene carboxylic aci ds. 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. Acco rding to the experimental conditions, this transient reacts with oxyge n producing a benzoquinone-O-oxide and subsequently p-benzoquinone, wi th water leading to hydroquinone or with another molecule of halopheno l producing a halogenodihydroxybiphenyl. It was also observed that 4-c hlororesorcinol behaves as both 4-ClPhOH and 2-ClPhOH. Chlorohydroquin one does not react as monohalophenols, the main photoproducts are hydr oquinone and chlorobenzoquinone. This reaction is consistent with a ra dical mechanism. The transformation of halophenols can be sensitized b y phenol and in most cases by hydroquinone. The half-life of the tripl et state of hydroquinone was evaluated at 0.9 mu s and the intersystem crossing yield at 0.39. The sensitization of 3-FPhOH was observed wit h 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.