Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

Abstract

It has been suggested (Clark, K. B.; Wayner, D. D. M. J. Am. Chem. Soc. 1991, 113, 9363−9365) that C−Br bond dissociation enthalpies (BDEs) in 4-YC6H4CH2-Br decrease as Y becomes more electron-withdrawing because of increasing destabilization of the Cδ+−Brδ- dipole and, furthermore, that the direction and magnitude of the effect of Y on 4-YC6H4Z-X BDEs could be correlated with the sign and magnitude, respectively, of the electronegativity difference between Z and X. The results of density functional theory (DFT) calculations using a locally dense basis set approach with the B3LYP functional and 6-311+G(2d,2p) as the primary basis set on 4-YC6H4CH2-X, for X = H, Br, Cl, and F with Y = NH2, HO, CH3O, CH3, H, CF3, CN, NO2 and BH2, show that the effects of Y on CH2-X BDEs are small (≤2.0 kcal/mol) for all four classes of compounds and are roughly equal for each Y for the three halides. Furthermore, almost all Y's reduce CH2-X BDEs relative to Y = H. Clark and Wayner's intriguing hypothesis that the magnitude of the effects of Y on 4-YC6H4Z-X BDEs depends on the magnitude of the differences in the electronegativities of Z and X should be discarded.