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-YC 6 H 4 CH 2 -Br decrease as Y becomes more electronwithdrawing 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 6 H 4 Z-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 6 H 4 CH 2 -X, for X ) H, Br, Cl, and F with Y ) NH 2 , HO, CH 3 O, CH 3 , H, CF 3 , CN, NO 2 and BH 2 , show that the effects of Y on CH 2 -X BDEs are small (e2.0 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 6 H 4 Z-X BDEs depends on the magnitude of the differences in the electronegativities of Z and X should be discarded.For the past half-century, the influence, or lack of influence, of para-substituents, Y, on the Z-X bond dissociation enthalpies (BDEs) of compounds having the general formula 4-YC 6 H 4 Z-X has been a subject of intense experimental and theoretical interest. For phenols (Z ) O, X ) H), 3 anisoles (Z ) O, X ) CH 3 ), 4 and anilines (Z ) NH, X ) H), 5 experiment and theory agree that electron-donating Y groups decrease the Z-X BDEs and that electron-withdrawing Y groups increase the Z-X BDEs. These substituent-induced changes in BDEs are most substantial for phenols. The 4-YC 6 H 4 O-H (and 3-YC 6 H 4 O-H) BDEs can be correlated with the σ + substituent constant of Y, 3a,c and the resulting straight line has a slope F + ) 7.3 kcal/mol. 3a The values of ∆BDE (4-YC 6 H 4 O-H -C 6 H 5 O-H) are, for example, -5.6 and 4.7 kcal/mol for Y ) CH 3 O and CN, respectively. 3c Similar correlations can be made for anisoles, F + ) 2.9 kcal/mol, 4a and anilines, F + ) 3 kcal/mol. 5a For toluenes, 4-YC 6 H 4 CH 2 -H 6 (and the related 10-substituted 9-methylanthracenes 7 ), experiment and theory agree that the Ysubstituents have little or no effect on C-H BDEs, e.g., F + ) 0.5 kcal/mol. 6e The Y-substituents also have little or no effect on C-C BDEs in YC 6 H 4 CH 2 -CH 3 8 and 4-YC 6 H 4 C(CH 3 ) 2 -CH 3 . 9 In contrast to the foregoing, there is no agreement regarding the effect of substituents on benzyl bromide C-Br BDEs. Early studies on the unimolecular decomposition rates of benzyl bromides in the gas phase failed to reveal any substantial substituent effects on their C-Br BDEs. 10 Forty years later, in a ground-breaking study, Clark and Wayner 11 measured some 4-YC 6 H 4 CH 2 -Br BDEs in the liquid phase using the photoacoustic calorimetric (PAC) technique and found a rather large substituent effect. The most exciting part of their discovery was...

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