Theoretical study of the addition of hydrogen halides to olefins: A comparison between (HCl)₂ and (HF)₂ additions to ethylene

The addition of HCl and (HCl)2 to ethylene is theoretically studied by means of ab initio SCF techniques using the 3‐21G basis set. A statistical thermodynamics analysis is also carried out. When a comparison is made with the HF and (HF)2 additions to ethylene, the general fact of the catalytic action of the second HX molecule in the (HX)2 complex is revealed. The behavior of HF, however, is very different from that of HCl, since in the former case the driving force in the termolecular transition state is the bonding between fluorine and carbon, while in the latter case the driving force is the bonding between carbon and hydrogen, as generally accepted for electrophilic reactions. The use of density plots of the transition states on the plane of the cycle allows us to conclude that, while both HF and HCl termolecular transition states are hexacyclic, the bimolecular transition state of the HCl addition is bicentric and not tetracentric as previously reported. Copyright © 1987 John Wiley & Sons, Inc.