Intramolecular electron transfer and dehalogenation of anion radicals. 4. Haloacetophenones and related compounds

Halogen-substituted acetophenones and several other carbonyls and carboxyl derivatives were reduced with e/sub aq//sup -/in irradiated aqueous solutions to produce the anion radicals. In certain cases, reduction by (CH/sub 3/)/sub 2/CO/sup -/ was also used. The anion radicals undergo intramolecular electron transfer and dehalogenation to yield inorganic halide ions. The rates of these reactions varied over a wide range (10/sup 1/-10/sup 7/s/sup -1/) and were strongly dependent on the nature of the halogen and the other substituent and their relative positions in the molecule. The results obtained here are discussed along with previous measurements on cyano and nitro derivatives. The rate of dehalogenation is found to be dependent on the electron affinity of the other substituent on the ring. Hammett's substituent constants do not correlate with the observed rates. However, spin density distribution in the anion radicals, as derived from ESR parameters, yields a good qualitative correlation. Increased spin density on the ring carbon bearing the halogen is accompanied by an increase in the rate of C-X bond scission. The spin density represents in this case the negative charge density, since protonation of the anion radicals in minor changes in spin densities but causes a dramatic decrease of the ratemore » of dehalogenation. 3 figures, 2 tables.« less