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 (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 σ_p 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.