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

Abstract

A structure-reactivity correlation for the reaction (HO(•) + HCR3 → HOH + CR3(•)) has been formulated: log k298(per H, cm(3)/molecule·s) = -0.000630ΔrH(2) - 0.151ΔrH - 1.056∑F - 1.053∑R - 21.26 (r(2) = 0.885; n = 70; mean unsigned deviation = 0.29 log units), where ΔrH is the reaction enthalpy (kJ/mol) and F and R represent the dissection of Hammett's σpara 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 ΔrH 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.

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

“…Following the lead of eqs and , , we finally show in Figure and eq a plot in which the Evans–Polanyi and Hammett independent variables are combined. The considerable improvement in the correlation indicators is apparent and thus demonstrates the utility of this multivariable approach to correlating radical abstraction reactivity for cases with late (α′ = 0.84) as well as early transition states.…”

“…The database of experimental gas-phase rate constants k Br is less extensive and less internally consistent than those for k Cl or k OH , nor do we have the advantage of a compilation of evaluated and recommended values as we had for k Cl and k OH . In addition, because of the lower reactivity of Br•, Arrhenius parameters have typically been obtained at higher temperature and therefore require longer, and questionable, extrapolations to 298 K. A compilation is given in Table . − All formulas have the carbon from which hydrogen abstraction occurs at the beginning followed by the substituents attached, that is, CH 3 X, CH 2 XY, or CHXYZ.…”

“…These previous correlations are shown as eqs 1 and 2 as the slightly truncated forms in which a minor term in Δ r H 2 has been dropped. 2 Such hydrogen atom transfer has long been known to be facilitated by substituents R that increase the radical stability and hence the exothermicity…”

Highly Oxygenated Molecules from Atmospheric Autoxidation of Hydrocarbons: A Prominent Challenge for Chemical Kinetics Studies

“…Steric restrictions can further slow down the rate of H‐shifts . The strength of the C─H bonds, for example, as calculated from structure activity relationships, depends on the branching and the structure of the carbon backbone and on the types of adjacent functional groups .…”

Comment on “Radicalicity: A scale to compare reactivities of radicals” (Chem. Phys. Lett. 618 (2015) 99–101)

“…While it is indeed true that the activation energy of a radical reaction typically decreases as its exothermicity increases, as expressed historically in the Evans-Polanyi relationship [4], it has also been recognized for over half a century that the relationship is usually not strictly linear because the rate constants are also often dependent on polar effects and steric effects [5][6][7] as well as thermochemical effects. Our recent studies [8][9][10] Therefore we suggest that the term "radicalicity", presented [2] as a measure of radical reactivity, not be adopted because it is in reality simply another expression for thermochemistry for which there is already adequate terminology (BDE).…”