Revisiting Markovnikov Addition to Alkenes via Molecular Electrostatic Potential

Abstract

Molecular electrostatic potentials (MESP) surrounding the π-region of several substituted ethylenes (CH2CHR) have been characterized by locating the most negative-valued point (Vmin) in that region. The substituents have been classified as electron donating and withdrawing on the basis of the increase or decrease in the negative character of Vmin in these systems as compared to ethylene. The values of Vmin show a good linear correlation with the Hammett σp constants, suggesting that the substituent electronic effects in substituted ethylenes and substituted benzenes are basically similar. With electron-donating substituents, the position of MESP minimum is closer to the unsubstituted carbon facilitating the π-complex formation of it with HCl at this site. Such a regiospecific π-complex formation is found to favor the formation of Markovnikov-type transition state for the addition of HCl to CH2CHR. For the electron-withdrawing substituents, the Vmin location is almost equidistant and farther from the ethylenic carbon atoms. This and the less negative Vmin values account for the less regiospecific CH2CHR···HCl π-complexes as well as the transition states for the HCl addition to CH2CHR when R is an electron-withdrawing group. The interaction energy (Eint) between CH2CHR and HCl for the formation of the CH2CHR···HCl π-complex shows a good linear correlation with the corresponding Vmin value.