Decomposition of hexachloroacetone in dimethyl sulphoxide solution: a source of trichloromethanide anions

Abstract

Hexachloroacetone (HCA) decomposes in dimethyl sulphoxide solution at room temperature. In the presence of an excess of 1,3,5-trinitrobenzene (TNB) the progress of the reaction can be monitored by the appearance of the visible absorption characteristic of the formation of the trichloromethyl Meisenheimer adduct of TNB in nearly quantitative yield (2 mol of adduct per mol of HCA). The reaction is slower than the decomposition of trichloroacetic acid under the same conditions, the difference being attributable to a large negative entropy of activation. The reaction rate is markedly increased by the addition of water and slightly depressed by addition of acid. The kinetics are consistent with a ratelimiting breakdown of the gem-diol hydrate of HCA into trichloroacetic acid and trichloromethanide anions, followed by the more rapid decomposition of trichloroacetic acid.