STUDY OF THE AQUEOUS PHOTOCHEMISTRY OF 4-FLUOROPHENOL, 4-BROMOPHENOL AND 4-IODOPHENOL BY STEADY-STATE AND NANOSECOND LASER FLASH-PHOTOLYSIS

The mechanisms of the aqueous photoreactions of 4-fluorophenol, 4-brom ophenol and 4-iodophenol have been studied by steady state photolysis and by nanosecond laser flash photolysis. The same photoproducts are o btained as were found in the photolysis of aqueous 4-chlorophenol alth ough the photoreaction quantum yields vary considerably across the ser ies of compounds, As for 4-chlorophenol the carbene 4-oxocyclohexa-2,5 -dienylidene is formed from the halogenophenol by loss of HX and this species reacts efficiently with oxygen to form 1,4-benzoquinone O-oxid e which subsequently yields 1,4-benzoquinone. Consideration of the rea ction quantum yields together with the photophysical properties of the four halogenophenols suggests that the carbene is derived from the fi rst excited singlet state rather than a triplet excited state.