Reaction force analysis of solvent effects in the addition of HCl to propene.

We have investigated computationally, via reaction force analyses, the addition of HCl to propene, both Markovnikov and anti-Markovnikov, in the gas phase and in chloroform solution. The calculations were carried out at the CCSD(T)/aug-pVTZ//B3LYP/aug-cc-pVDZ level. A particular interest was in the magnitudes of the two components of the activation energies that are defined by the minimum of the reaction force for each process. The total activation energies for Markovnikov and anti-Markovnikov addition are found to be, respectively, 39.7 and 45.9 kcal/mol in the gas phase and 27.1 and 34.9 kcal/mol in chloroform solution. In solution, the first portion of the reaction (prior to the reaction force minimum) involves substantial stretching of the H-Cl bond, which makes that contribution to the total activation energy greater than in the gas phase. However the second part of the activation is much less energy demanding in solution for both the Markovnikov and anti-Markovnikov additions. The overall preference for Markovnikov addition is due to the electrostatic potential of propene favoring the initial approach of the HCl hydrogen to the terminal carbon.