Extension of Structure–Reactivity Correlations for the Hydrogen Abstraction Reaction by Bromine Atom and Comparison to Chlorine Atom and Hydroxyl Radical

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 Δ_rH 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. Despite lower expectations because the available database is less extensive and much more scattered and because long temperature extrapolations are often required, the resulting least-squares fit (log k_298,Br = −0.147 Δ_rH – 4.32 ΣF – 4.28 ΣR – 12.38 with r² = 0.92) was modestly successful and useful for initial predictions. The coefficient of Δ_rH 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.