OLEFIN-FORMING ELIMINATION INDUCED BY MERCAPTIDE IONS - VARIATION OF TRANSITION-STATE CHARACTER AS INDICATED BY SUBSTITUENT EFFECTS ON RATE

For react ions of a series of substituted benzyldimethylcarbinyl chlorides (5) with ethanethiolate ion in methanol solution, product ratios and rate constants have been measured. Dissected rate constants (k2) for EtS“-induced reaction to form substituted β, d-dimethylstyrenes (2) furnish a U-shaped Hammett plot vs. a. The change in p from negative with electron-releasing to positive with strongly electron-attracting substituents is easily rationalized in terms of the theory of the variable E2 transition state; the character of the transition state is considered to change as substituents change. The reversal in sign of p can alternatively be rationalized in terms of a change from the (E2 ijp to the E2 mechanism. The “E2C” mechanistic hypothesis provides little insight into these phenomena. Comparison of k2 values with two reagents shows that ethanethiolate is more reactive than methoxide ion except in the case of the p-nitro substrate. © 1979, American Chemical Society. All rights reserved.