Relationship of the selectivity effect of aromatic solvents with their electron-donor ability and organic reactant structure in free radical chlorination

The effect of several solvents on the selectivity of the free radical chlorination of 1,1-dichloroethane and 1-chloropropane is studied. The selective action of aromatic solvents on free radical chlorination is explained. This explanation implies that the process involves solvated chlorine atoms and their donor-acceptor complexes with aromatic molecules (ArH→Cl^•) as intermediates. Using the findings of this work and previous studies, the ratios of the rate constants for hydrogen-atom abstraction from different positions in chloroethane, 1,1-dichloroethane, 1-chloropropane, and 2-chloropropane by solvated chlorine atoms and ArH→-Cl^• complexes are determined. The differences in the activation energies of the competitive hydrogenatom abstractions from different positions in substrates by the ArH→Cl^• complexes and solvated chlorine radicals correlate with two HOMO energies of solvent and substrate molecules. The isokinetic relationship is found for all the systems under study (the isokinetic temperature, 523 K).