Carbocation stability as predictor for electrophilic addition of HCl to chlorinated ethenes and propenes in the gas phase

Abstract

AbstractElectrophilic addition of halo acids to alkenes is commonly discussed in terms of the stability of the corresponding carbocation intermediate. In solution, this may be rationalized by reference to Hammond's postulate. The corresponding gas‐phase reaction takes place via a concerted mechanism without the formation of a cationic intermediate. Still, the series ethene, propene, and 2‐methylpropene show excellent correlation between proton affinities (PA) and gas‐phase activation energies for the addition of halo acids, with the activation energy decreasing linearly with increasing PA. Thus, PA presents itself as a potential predictor of reactivity also in the gas phase. To explore this possibility, we have performed high‐level calculations of cation stability (as reflected in PA) and activation energy for addition of HCl (Ea) for 12 chlorinated ethenes and propenes. Contrary to the hypothesis, for the present set of substrates, the correlation between PA and Ea is weak, and moreover, for subsets such as the series ethene, 1,2‐dichloroethene, and tetrachloroethene, Ea increases with increasing PA. An alternative approach of correlating Ea to carbon 1s ionization energies rather than PA shows moderately higher correlation overall and much better correlation within subsets of compounds that share the same number of chlorines attached to the carbon subject to electrophilic attack/core ionization.