Methyl radical reactions with ethylamine and deuterated ethylamines

Abstract

The rates of hydrogen-abstraction by methyl radicals from the three different sites in ethylamine have been measured by studying three isotopically different ethylamines: CH₃CH₂NH₂, CH₃CH₂ND₂ and CD₃CH₂NH₂. Photolysis of azomethane over the temperature range 110–180°C has been used as the source of methyl radicals, and velocity constants have been obtained for five elementary reactions.

Abstraction takes place from all three sites. The methylene group is the most reactive site in the molecule, and the methyl group the least. At 150°C, 67 % of the hydrogen comes from CH₂, 29 % from NH₂ and about 4 % from CH₃. These results contrast with previous studies on other amines, in which the amine group has always been the most reactive. The reactivity of the amine group in ethylamine is similar to that in methylamine in possessing a somewhat lower A-factor than normal. The primary isotope effect arising on substitution of ND₂ for NH₂ is in accord with expectations based on zero-point-energy differences. Arrhenius parameters (based on a value of 10^13.34cm³ mole^–1 sec^–1 for the dimerization of methyl radicals) have been determined as follows: [graphics omitted]

For attack on the methyl group of ethylamine at 150°C, a value for k_CH3 of 10^5.8cm³ mole^–1 sec^–1 has been estimated from the corresponding value for k_CD3 of 10^5.1cm³ mole^–1 sec^–1: attack is too slight for separate values of A and E to be determined precisely.