Abstract
Rate constants for the reaction (R’3C• + X2 → R’3CX + X•; X = F, Cl, Br, and I) are reviewed. Because of curved Arrhenius plots and negative EX values, empirical structure-reactivity correlations are sought for log kX,298 rather than EX. The well-known poor correlation with measures of reaction enthalpy is demonstrated. The best quantitative predictor for R’3C• is Σσp, the sum of the Hammett σp constants for the three substituents, R’. Electronegative substituents with lone pairs, such as halogen or oxygen, thus appear to destabilize the formation of a polarized pre-reaction complex and/or TS (δ+R---X---Xδ-) by σ-inductive/field electron withdrawal while simultaneously stabilizing them by π-resonance electron donation. The best quantitative predictor of the reactivity order of the halogens, I2 > Br2 >> Cl2 ≈ F2, is the polarizability of the halogen, α(X-X). For the data set of 60 rate constants which span 6.5 orders of magnitude, a modestly successful correlation of log kX,298 is achieved with only two parameters, Σσp and α(X-X), with a mean unsigned deviation of 0.59 log units. How much of this residual variance is the result of inaccuracies in the data compared with over-simplification of the correlation approach remains to be seen.