Jm. Tanko et al., SOLVENT PRESSURE EFFECTS IN FREE-RADICAL REACTIONS - A SELECTIVITY INVERSION IN FREE-RADICAL BROMINATIONS INDUCED BY SOLVENT, Journal of the American Chemical Society, 115(11), 1993, pp. 4520-4526
The results reported herein demonstrate a compelling link between sele
ctivity in free radical brominations and the internal pressure of solv
ent. In a study encompassing 18 solvents and/or solvent mixtures, the
rate constant ratio for S(H)2 ring opening of cyclopropylbenzene (k(C)
) vs hydrogen abstraction from toluene (k(H)) by bromine atom was foun
d to vary by nearly a factor of 20 as a function of solvent. The obser
ved rate constant ratio correlates with the cohesive energy density of
the solvent, which for the solvents utilized in this study is approxi
mately equal to internal pressure (k(C)/k(H) is found to increase with
increasing solvent pressure). A similar competition pitting cycloprop
ylbenzene against p-chlorotoluene shows an identical solvent effect. I
t is suggested that the variation in selectivity with solvent pressure
arises because the volume of activation associated with k(C) is more
negative than that for k(H) by ca. 20 cm3/mol.