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

摘要

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 center dot/HO center dot + HCR3 -> HC1/HOH + center dot CR3); the reaction enthalpy effect was represented by the independent variable Delta H-r 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 center dot 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 Delta H-r - 4.32 Sigma F- 4.28 Sigma R - 12.38 with r(2) = 0.92) was modestly successful and useful for initial predictions. The coefficient of Delta H-r was similar to 41-fold greater, indicative of the change from an early to a late transition state; meanwhile the sizable coefficients of Sigma F and ER indicate the persistence of the "polar effect". Although the mean unsigned deviation of 0.79 log k(298) 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.