G. G. Wubbels, E. J. Snyder, E. Coughlin
Apr 1, 1988
Citations
0
Influential Citations
17
Citations
Journal
Journal of the American Chemical Society
Abstract
Irradiation of 4-bromonitrobenzene at 313 nm in aqueous solutions containing hydronium ion and relatively high concentrations of chloride ion cleanly but inefficiently forms 4-chloronitrobenzene. 2-Bromonitrobenzene is unreactive in photosubstitution by chloride ion. The photosubstitution is exclusively an acid-catalyzed reaction, and it is second order in chloride ion concentration. One chloride ion is consumed in the reaction, and the other functions as a catalyst. HCl-catalyzed photoreduction of 4-bromonitrobenzene is a major and highly efficient concomitant reaction when 2-propanol is present at 0.01 to 0.3 M; the major photoreduction products are 4-bromonitrosobenzene and 4-bromo-2-chloroaniline. The extrapolated limiting quantum yields (at infinite reagent concentration) are 0.5 for photosubstitution and 0.5-0.7 for photoreduction. The second-order dependence on chloride ion is found to require a quadratic expression rather than a simple squared term in the quantum yield expression. The parameters of the quadratic expression are found by a novel procedure based on iterative calculation to convergence to find the best fit to the plotted data points. The photosubstitution mechanism is proposed to involve electron transfer from chloride ion to the /sup 3/n,..pi..* state forming an exciplex; acid catalysis is attributed to protonative capture of the exciplex.