Evolution of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Chlorine Atom

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Abstract
Empirical structure–reactivity correlations are developed for log k298, the gas-phase rate constants for the reaction (Cl• + HCR3 → ClH + CR3•). It has long been recognized that correlation with ΔrH is weak. The poor performance of the linear Evans–Polanyi formulation is illustrated and was little improved by adding a quadratic term, for example, by making its slope smoothly dependent on ΔrH [η ≡ (ΔrH – ΔrHmin)/(ΔrHmax – ΔrHmin)]. The “polar effect” (δ−Cl---H---CR3δ+)⧧ has also been long discussed, but there is no formalization of this dependence based on widely available independent variable(s). Using the sum of Hammett constants for the R substituents also gave at best modest correlations, either for σpara or for its dissection into F (field/inductive) and R (resonance) effects. Much greater success was achieved by combining these approaches with the preferred independent variable set being either [(ΔrH)2, ΔrH, ΣF, and ΣR] or [η, ΔrH, ΣF, and ΣR]. For 64 rate constants that span 7 orders of magnitude, these correlation formulations give r2 > 0.87 and a mean unsigned deviation of k units, with even better performance if primary, secondary, and tertiary reaction centers are treated separately.

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