Effect of Acetylene Structure on the Rates and Products of Addition of 4-Chlorobenzenesulfenyl Chloride

The rates and products of addition of 4-chlorobenzenesulfenyl chloride to acetylene and 12 alkyl-substituted derivatives have been determined in 1,1,2,2-tetrachloroethane at 25 °C. The effect of the alkyl groups on the rate of addition to 10 of the 13 alkynes can be correlated by means of the Taft equation: log k₂ = −4.47Σσ* + 1.64. The three compounds that do not lie on the line contain tert-butyl substituents. Polar effects of the substituents on the rates are dominant and steric effects are important only for tert-butyl groups. The product regiochemistry, however, is determined by the steric bulk of the substituent. The results are consistent with a mechanism involving bridged thiirenium ion-like rate and product determining transition states.