Cleavage of vinylic mercuric halides by aqueous acids

Isotope effects, general acid catalysis, and relative reactivities show that proton transfer to one of the unsaturated carbon atoms is rate determining for the acidolysis of unsaturated alkylmercuric halides. For compounds, R₁R₂C?CHHgX, substitution of CH₃ for H at R₁ or R₂ leads to an acceleration of a factor of ～ 30. This relatively small acceleration, the relative facility of the reactions, and the magnitude of the Br^? catalytic terms, suggests an olefin–mercuric halide complex as the product of the rate-determining step, rather than a simple carbonium ion. The Brøonsted catalysis law is obeyed with a variety of carboxylic acids, giving an ∝ of 0.69 ± 0.04, but acids of other structures give substantially deviant catalytic coefficients, in a pattern similar to that generated by other A-S_E2 reactions. The acetic acid catalytic coefficient is larger by a factor of 10² than that predicted if it were due to specific hydronium ion–general base catalysis instead of true general acid catalysis. The overall solvent isotope effect, k_H/k_D, is 2.55 ± 0.10. The competitive isotope effect, κ_H/κ_D, is 6.84 ± 0.06. Taken with a model in which the proton is transferred directly from the H₃O⁺ unit of the aquated proton to the substrate, these are sufficient to successfully predict the rate at all intermediate isotopic compositions.