FURTHER-STUDIES ON THE PHOTOISOMERIZATION AND HYDROLYSIS OF CHLOROBIPHENYLS IN WATER - COMMON ION EFFECT IN THE PHOTOHYDROLYSIS OF 4-CHLOROBIPHENYL

The photochemistry of C-14-labeled 2-chlorobiphenyl (2-CIBP) and 3-chlorobiphenyl (3-CIBP) in water was studied by isotopic dilution. 2-CIBP yielded 2-hydroxybiphenyl (2-HOBP) exclusively. 3-CIBP, on the other hand, yielded 3-HOBP, 2-HOBP, and 4-HOBP and a small amount of biphenyl (2%). Thus, 2-CIBP reacts exclusively by a heterolytic process, whereas 3-CIBP reacts not only by a heterolytic pathway but also by competing isomerization (to 2-CIBP and 4-CIBP) and homolytic pathways. The photochemistry of 4-CIBP in 80:20 (v:v) H2O-CH3CN with added HCl or NaCl was also studied. These reactions were suppressed in the presence of HCl or NaCl. Further experiments demonstrated that the suppression was not due to any great extent to the ionic strength of the solution, spin orbit effects of heavy atoms, and proton quenching. These results are compatible with a mechanism originally proposed by Kropp in which the excited state of 4-CIBP, likely the triplet excited state, undergoes homolysis of the aryl carbon-chlorine bond to form a radical pair. Once formed the radical pair undergoes an electron transfer from the aryl radical to the chlorine atom to form a highly reactive aryl cation and chloride. Reaction of the aryl cation with added chloride then regenerates 4-CIBP. The overall reaction is equivalent to a common ion effect. The role of other mechanisms in the observed chemistry is also discussed.