Eliminations promoted by thiolate ions. Part IV. A Hammett equation study of the reactions of 1,1-diaryl-2,2,2-trichloro- and 1,1-diaryl-2,2-dichloro-ethane with benzenethiolate ions in ethanol

Abstract

The Hammett ρ value for the benzenethiolate-promoted dehydrochlorination of 1,1-diaryl-2,2,2-trichloroethane (DDT-type) compounds in ethanol is 2·11 at 65 °C. For S_N2 substitution of benzenethiolate for chloride in the 1,1-diaryl-2,2-dichloroethane (DDD) series, ρ= 0·41. Comparison of the former value with that for the conventional E2 reaction promoted by ethoxide in ethanol (2·34) suggests that the transition state for the benzenethiolate-promoted reaction is of the normal E2H variety, and is not an E2C-like transition state. The DDD S_N2 reaction series has been chosen as a model for possible E2C dehydrochlorination of DDT, and the disparity between the two ρ values indicates that the DDT transition state has considerably more β-carbanionic character than does the DDD transition state.

The fact that DDT is dehydrochlorinated by weak bases such as benzenethiolate and chloride ions more readily than is DDD is also taken as evidence in favour of the E2H mechanism. Potential-energy calculations, based on the assumption that both DDT and DDD eliminate via E2C transition states, give theoretical DDT: DDD rate ratios close to unity, which are widely different from the experimental values.

The assumption of additivity of substituent constants in diarylmethyl Hammett series is questioned and a novel method of obtaining linear plots is described.