| Résumé | It has been suggested (Clark, K. B.; Wayher, D. D. M. J. Am. Chem. Soc. 1991, 113, 9363-9365) that C-Br bond dissociation enthalpies (BDEs) in 4- YC 6H 4CH 2-Br decrease as Y becomes more electron-withdrawing because of increasing destabilization of the C(δ+)-Br(δ-) dipole and, furthermore, that the direction and magnitude of the effect of Y on 4-YC 6H 4Z-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- YC 6H 4CH 2-X, for X = H, Br, Cl, and F with Y = NH 2, HO, CH 3O, CH 3, H, CF 3, CN, NO 2 and BH 2, show that the effects of Y on CH 2-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 CH 2-X BDEs relative to Y = H. Clark and Wayner's intriguing hypothesis that the magnitude of the effects of Y on 4-YC 6H 4Z-X BDEs depends on the magnitude of the differences in the electronegativities of Z and X should be discarded. |
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