Evolution of Structure-Reactivity Correlations for the Hydrogen Abstraction Reaction by Hydroxyl Radical and Comparison with That by Chlorine Atom

A structure-reactivity correlation for the reaction (HO. + HCR3 -> HOH + CR3.) has been formulated: log k(298)(per H, cm(3)/molecule.s) = -0.000630 Delta H-r(2) -0.151 Delta H-r-1.056 Sigma F-1.053 Sigma R-21.26 (r(2) = 0.885; n = 70; mean unsigned deviation = 0.29 log units), where Delta H-r is the reaction enthalpy (kJ/mol) and F and R represent the dissection of Hammetts sigma(para) constant into its field/inductive and resonance effects, and compared for the analogous case for Cl. (ref 1). Although more exothermic, the dependence of HO. on Delta H-r is somewhat greater than Cl.. However the dependence on F and R is much less, suggestive of less charge separation in the transition state for the less electronegative HO. The range of k(OH) is significantly less than that of Cl., i.e., it is less dependent on substrate structure. Yet a crossover exists such that k(Cl) > k(HO) predominates for more reactive cases whereas k(Cl) < k(OH) characterizes the less reactive. The Arrhenius parameters reveal that this crossover results from a change in {E(Cl) - E(OH)} from negative to positive. In contrast, whereas the A factors for both increase significantly as reactivity increases, {A(Cl)/A(OH)} always exceeds unity.