Revisiting Markovnikov Addition to Alkenes via Molecular Electrostatic Potential

Molecular electrostatic potentials (MESP) surrounding the π-region of several substituted ethylenes (CH2CHR) have been characterized by locating the most negative-valued point (V min) 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 V min in these systems as compared to ethylene. The values of V min 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 V min location is almost equidistant and farther from the ethylenic carbon atoms. This and the less negative V min 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 (E int) between CH2CHR and HCl for the formation of the CH2CHR···HCl π-complex shows a good linear correlation with the corresponding V min value.