The mechanism of hydrogen atom abstraction from phenols involved in hydrogen bonding with ester carbonyl groups

Abstract

Rate constants have been determined in different solvents for hydrogen atom abstraction from alkyl-substituted phenols and deuteriated analogues by polyvinyl acetate radical (R˙). The following solvents were used: ethyl acetate, ethyl [²H₃]acetate, ethyl pivalate, ethyl trifluoroacetate, acetone, [²H₆]acetone, dimethyl sulphoxide. The reactivity of the phenols and the primary kinetic deuterium isotope effect (p.k.i.e.) demonstrate a strong solvent dependence exhibiting the highest values in ethyl trifluoroacetate (p.k.i.e. 57 ± 23 for 2,4,6-trimethylphenol at 50 °C). Incorporation of CH₃¹⁴COOCH₂CH₃ and (CH₃)₃C¹⁴COOCH₂CH₃ into polyvinyl acetate increases in the presence of phenols. These facts conclusively prove that hydrogen atom abstraction proceeds within the hydrogen bond to the radical formed by addition of R˙ to the ester carbonyl hydrogen bonded to phenol.