Abstract
Recently we presented structure-reactivity correlations for the gas-phase ambient-temperature rate constants for hydrogen abstraction from sp3-hybridized carbon by chlorine atom and hydroxyl radical (Cl•/HO• + HCR3 → HCl/HOH + •CR3); the reaction enthalpy effect was represented by the independent variable ΔrH and the “polar effect” by the independent variables F and R, the Hammett constants for field/inductive and resonance effects. Both these reactions are predominantly exothermic and have early transition states. Here we present a parallel treatment for Br• whose reaction is significantly endothermic with a correspondingly late transition state. In spite of lower expectations because the available data base is less extensive and much more scattered and because long temperature extrapolations are often required, the resulting least-squares fit (log k298,Br = ‒ 0.147 ΔrH ‒ 4.32 ΣF ‒ 4.28 ΣR ‒ 12.38 with r2 = 0.92) was modestly successful and useful for initial predictions. The coefficient of ΔrH was ≈4-fold greater, indicative of the change from an early to a late transition state; meanwhile the sizable coefficients of ΣF and ΣR indicate the persistence of the “polar effect”. Although the mean unsigned deviation of 0.79 log k298 units is rather large, it must be considered in the context of a total span of over 15 log units in the data set. The major outliers are briefly discussed.