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.

In formamide, reliable data may be obtained for abstraction of hydrogen from the formyl position. The reactivity of the formyl hydrogen is similar to that in related compounds HCOX where X is H, CH₃, or OCH₃. There is evidence for the existence of secondary attack by a disproportionation reaction that has no parallel in acetamide, viz., CH₃+ HCONH₂→ CH₄+·CONH₂ CH₃+·CONH₂→ CH₄+ HNCO. The complementary route to HNCO CH₃+ HCONH₂→ CH₄+ HCONH· CH₃+ HCONH·→ CH₄+ HNCO, is unimportant. The secondary reaction prevents a direct estimate of the primary reactivity of the amide group in formamide.