Internal pressure's role in the selectivity of the bromine radical

The results of our studies demonstrate a link between selectivity in free radical brominations and the internal pressure of solvent. The study encompassed l8 solvents and/or mixtures over which the rate constant ratio for SH2 ring opening of cyclopropylbenzene (kC) vs hydrogen abstraction from toluene (kH) by bromine atom was found to vary by nearly a factor of twenty as a function of solvent. The observed rate constant ratios correlate with the cohesive energy density of the solvent, which is approximately equal to the internal pressure (kC/kH is found to increase with increasing solvent pressure) for the solvents used in this study. A similar competition pitting cyclopropylbenzene against p-chlorotoluene shows the same solvent effect. It is suggested that the observed variation in selectivity "with solvent pressure occurs because the volume of activation with kC is more negative than for kH by about 20 cm3/mol.