KINETIC SOLVENT ISOTOPE EFFECTS IN THE ADDITIONS OF BROMINE AND 4-CHLOROBENZENESULFENYL CHLORIDE TO ALKENES AND ALKYNES

The rates of bromination of selected alkenes and alkynes in methanol/methanol-d, acetic acid/acetic acid-d, and formic acid/formic acid-d have a nearly constant value of kH/kD = 1.23 ± 0.02. This kinetic solvent isotope effect is attributed to specific electrophilic solvation of the incipient bromide anion by hydrogen bonding in the rate-determining transition, state. The rates of bromination were measured in two solvents having the same values of the solvent parameter Y but different nucleophilicities in order to assess the importance of nucleophilic solvation. Significant nucleophilic solvent assistance is found for only alkylacetylenes. The kinetic solvent isotope effects of the addition of 4-chlorobenzenesulfenyl chloride to selected alkenes and alkynes in acetic acid/acetic acid-d vary from 1.00 to 1.28. These data are consistent with two mechanisms: One involves a tetravalent sulfur intermediate while the second is the sulfur analogue of the SN2 mechanism. © 1979, American Chemical Society. All rights reserved.