Comparison of the Nucleophilicities of Alkynes and Alkenes. Quantitative Determination of the Nucleophilicities of Alkynes toward Carbenium Ions

Relative reactivities of alkenes and alkynes toward diarylmethyl cations have been determined by direct rate measurements and by competition experiments in dichloromethane. At −70°C alkynes are found to be one to two orders of magnitude less reactive than analogously substituted alkenes (e.g. phenylacetylene/styrene), but the reactivity difference reduces strongly as the temperature is raised. The stereochemistry of the vinyl chlorides produced by addition of benzhydryl chlorides to alkynes is characterized.