A QUANTITATIVE STRUCTURE-ACTIVITY RELATIONSHIP FOR THE DIRECT PHOTOHYDROLYSIS OF META-SUBSTITUTED HALOBENZENE DERIVATIVES IN WATER

A quantitative structure-activity relationship (QSAR) is developed for the photolysis of meta-substituted halobenzene derivatives in water. With the exception of meta-chlorinated and -fluorinated benzonitriles and -fluorinated benzoic acids, upon irradiation of these compounds the carbon-halogen bond with the lowest bond strength is broken, whereupon the corresponding hydroxyl-derivative is being formed. In low concentrations the benzene-derivatives were irradiated at wavelengths ranging from 250 - 350 nm, to determine the pseudo-first order reaction rate constants. With the measured rate constants, the relative energy distribution of the light source used and the UV-spectra of the starting compounds, the quantum yields for photohydrolysis, PHI, were calculated relative to 3-chlorophenol. Only for the compounds undergoing photohydrolysis, the calculated quantum yields were 10log-transformed and correlated to generally available structural parameters for the ground state. The best correlation was obtained with the bond strength of the carbon-halogen bond to be broken and the summed inductive and steric constants of the substituents as parameters.