Cleavage of vinylic mercuric halides by aqueous acids

Abstract

AbstractIsotope 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‐SE2 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, kH/kD, is 2.55 ± 0.10. The competitive isotope effect, κH/κD, 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.