Insight into Markovnikov Reactions of Alkenes in Terms of Ab Initio and Molecular Face Theory

Electrophilic additions of hydrogen halides to alkenes in the gas phase are investigated with a high-level ab initio method, MP2/6-311+G(3df,2p). Based on this, the interesting features of these reactions along the IRC routes are characterized by the molecular face (MF) theory. For an alkene at the initial state, if the representative electron density (ED) encoded on the molecular face (MF) of the Markovnikov (M) carbon atom (the carbon with more hydrogen atoms) is larger than that of the anti-Markovnikov (AM) carbon atom (the carbon with fewer hydrogen atoms), the electrophilic addition reaction is predicted to proceed along the Markovnikov addition route; in the reverse situtation, the anti-Markovnikov addition route would be slightly preferred. It is then demonstrated that for a series of alkenes, the difference between activation energies of Markovnikov and anti-Markovnikov addition routes [DeltaE(#) ((M-AM))] has a good linear correlation with sign(K(ED))K(2) (ED), where K(ED) is characteristic of the electron density (ED) at the pi region in the initial state of the alkenes. Interestingly, there is a good linear correlation between our sign (K(ED))K(2) (ED) and the absolute values of difference in the core ionization energy between M and AM carbon atoms obtained by others (L. J. Saethre, T. D. Thomas, S. Svensson J. Chem. Soc. P2 1997, 2, 749.) in terms of the experimental study. In addition, the spatial dynamic changing features of the MF faces and interesting pictures of the electron transfer are clearly shown during the course of the electrophilic addition reactions. These results indicate that not only regioselectivity, but also activation energy and reactivity correlate with the pi charge distribution in the initial state of the alkenes for electrophilic addition reactions.