Effect of solvents on the selectivity of free-radical chlorination of functionalized ethanes, propanes, and tetrachloroethene

Free radical reactions of hydrogen abstraction from methane, ethane, propane, and isobutane by radicals X (X = Cl-*,Cl- Br-*, I-*, and CH3*) are used as an example to study the applicability of the molecular mechanics (MM) method and the modified intermediate neglect of differential overlap method (PM3) to describing the trends in activation energies. The activation energies of only some, but not all, reaction series are adequately described by the MM (if special parameterization is used) and PM3 methods. All of the four reaction series follow the same correlation within the framework of the parabolic model of a transition state. The correlation of activation energies with the energies of highest occupied molecular orbitals (HOMO) of substrates and solvents is explained. For the correct description of trends in the HOMO energies in the series of studied molecules, the PM3 method was parameterized. The MM and PM3 methods are used to reveal the effect of a solvent on the activation energies of the regioselective and substrate-selective free-radical chlorination of substituted ethanes and propanes. The calculations of the structure and strength of complexes formed by solvent molecules, chloroethane, its radical, and the chlorine radical made it possible to propose a mechanism for the solvent effect on regioselectivity. This mechanism consists in the stabilization of the transition state formed by the U.-abstraction of a carbon atom from the C-Cl bond in the aromatic solvent. Different trends exist for aromatic and nonaromatic solvents for changes in the activation energies of the competitive chlorination of tetrachloroethene and 1,2-dichloroethane depending on the solvent concentration. These trends can be reduced to one trend if one considers the HOMO energies of solvents and the substrate and takes into account reactant concentrations.