Further studies on the photoisomerization and hydrolysis of chlorobiphenyls in water. Common-ion effect in the photohydrolysis of 4-chlorobiphenyl

Abstract

The photochemistry of 14C-labeled 2-chlorobiphenyl (2-ClBP) and 3-chlorobiphenyl (3-ClBP) in water was studied by isotopic dilution. 2-C1BP yielded 2-hydroxybiphenyl (ZHOBP) exclusively. 3-ClBP, 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-ClBP reacts not only by a heterolytic pathway but ala0 by competing isomerization (to 2-ClBP and 4-ClBP) and homolytic pathways. The photochemistry of 4-C1BP in 8020 (v:v) H20-CH,CN with added HCl or NaCl was also studied. 'These reactions were suppressed in the presence of HC1 or NaC1. 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-ClBP, 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-ClBP. The overall reaction is equivalent to a common ion effect. The role of other mechanisms in the observed chemistry is also discussed.

Homolytic vs Heterolytic Paths in the Photochemistry of Haloanilines

“…A variation of this path is cage electron transfer within the primarily formed radical pair leading to the ion pair in an indirect way (path a′). 6 In other cases, the reaction proceeds from a (hetero)excimer (path b) 4b,5,7 or from an exciplex (e.g., with an aliphatic amine 8 or with an aniline, 9 path c) rather than directly from the excited state.…”

“…The decrease of the triplet energy in naphthalenes and in higher arenes makes cleavage unlikely. A variation of this path is cage electron transfer within the primarily formed radical pair leading to the ion pair in an indirect way (path a‘) …”