Hydrogen abstraction from amides. Isotope effects, reactivity of different sites, and secondary effects in the reaction of methyl radicals with formamide and acetamide

Abstract

The gas-phase attack by methyl radicals on acetamide, acetamide-d3, formamide, and formamide-N-d2 has been studied, and hydrogen and deuterium abstraction rates have been measured. Methyl and methyl-d3 radicals were produced by photolysis of acetone and acetone-d6. Measurements on acetamide were made in the temperature range 122–259°C; the maximum temperature regularly employed with the formamides was 202°C. Rate constants for the reactions below have been derived, giving the following Arrhenius parameters: [graphic omitted] Throughout the temperature range studied, the rates of abstraction of hydrogen from the different sites in acetamide are approximately equal; e.g., at 150°C abstraction from the nitrogen atom accounts for 52% of the attack. The individual reactivities are compared with those of methyl and amino groups in other molecular environments, and the reactivities of the methyl and methyl-d3 groups in acetamide and acetamide-d3 are discussed in terms of theories of kinetic isotope effects. Comparison of the present work with published data for methyl radical attack on N-dimethyl acetamide enables the reactivity of the N-methyl groups in that compound to be assessed.