Kinetic Solvent Effects on Hydrogen Abstraction Reactions from Carbon by the Cumyloxyl Radical. The Importance of Solvent Hydrogen-Bond Interactions with the Substrate and the Abstracting Radical

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO*) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO*) have been studied in MeCN. The reaction of Cum. with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions strong HBD solvents influence the hydrogen abstraction reactivity been observed on going from benzene and PhCl to MeOH and TFE, lone pair and the solvent in the transition state. The similar k(H) values point toward differential hydrogen bond interactions of the latter The small reactivity ratios observed for the reactions of Cum. and from hydrocarbons of CumO*. With PA and indicative of hydrogen-bond observed for the reactions solvent with the substrate BnO. with PA and DMPA by alkoxyl radicals and indicates that DMPA a significant decrease in kH has interactions between the carbonyl of the aldehydes in MeOH and TFE and the radical in the transition state. (kH(BnO*)/k(H)(CumO*) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.