Methyl and trifluoromethyl radical attack on methyl formate: isotope effects and reactivities at different sites

Abstract

Isotopic labelling has been employed to determine the position and extent of hydrogen abstraction from the formyl and methoxyl sites in methyl formate by methyl and trifluoromethyl radicals. Attack is principally at the formyl group, but at 182° a significant proportion of abstraction occurs from the methoxyl; 15 % for attack by CH₃ and 25 % for attack by CF₃. Values for rate constants (at 182°) and for Arrhenius parameters are as follows: [graphic omitted] Isotope effects at the formyl group are similar for both methyl and trifluoromethyl abstractions. At 182°, the velocity constant for H-abstraction is 6.2 times that for D-abstraction when attack is by methyl radicals; the isotopic factor is 8 when attack is by trifluoromethyl radicals. These effects are due to activation energy differences since quotients of A-factors do not differ from unity. Zero-point-energy differences, however, are less by ca. 0.5 kcal mole^–1 than the differences in activation energy, and proton tunnelling may be important.