3. HOMOGENEOUS SENSITIZED SYSTEMS170 3·1 General background 170 3·1·1 Singlet oxygen as oxidant 171 3·2 Sensitized oxidation of chlorophenols by singlet oxygen 172 3·3 Homogeneous reactions of chlorophenols with metal complexes 176 4. PHOTOINDUCED DEGRADATION OF CHLOROPHENOLS 180 4·1 Free radical oxidation of chlorophenols 180 4·2 Free radical processes in semiconductor-photocatalysed degradation of chlorophenols 182 4·2·1 TiO 2 -mediated degradation of chlorophenols 183 4·3 Sensitization of chlorophenol degradation by supported catalysts 188 4·4 Radiation induced degradation of 2,4,6-trichlorophenol 191 5. REACTIONS OF CHLOROPHENOLS WITH TRANSIENT INTERMEDIATES 193 6. CONCLUSIONS 197 7. ACKNOWLEDGEMENTS 198 8. REFERENCES 198 ABSTRACTThe photodegradation of mono-, di-, tri-and pentachlorophenols in aqueous solution is surveyed from several viewpoints, namely kinetic and mechanistic, the nature of the reactive intermediates and final products, and the potential of photochemical means of treating water contaminated by chlorophenols. In direct photolysis, the roles of heterolytic and homolytic processes are considered, and the appearance of carbene, in addition to ionic and radical, intermediates noted. Sensitized photolysis deals with the roles of singlet oxygen and of a variety of metal complexes. The induced degradation of chlorophenols refers to the oxidation of chlorophenols by free radicals generated from photo-and radiolytic systems, particularly illuminated semiconductors such as titanium dioxide. The article finishes with an overview of the reactions of various types of reactive intermediates with chlorophenols. 1. dIonization potentials of 2-chlorophenol (8·66 eV) and 4-chlorophenol (8·94 eV) in organic solvents have been determined from charge transfer spectra with TCNE [51] e Values obtained for 4-chlorophenol for OH bond dissociation energies by this method, photoacoustic calorimetry [52], pulse radiolysis [53] and theoretical calculations [54] vary over the range 84·4-90·3 kcal mol -1 f Calculated in [50] g From [55] h n-butanol solution [56]

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