Methylene II. Gas phase reactions with ethyl chloride

Abstract

In this work methylene was prepared by the photolysis of ketene, and the experiments include observations of the effects of changing the wavelength of the photolysing light and of introducing foreign gases. Results are consistent with a free-radical mechanism in which CH 2 abstracts a chlorine or a hydrogen atom from C 2 H 5 Cl: CH 2 +CH 3 CH 2 Cl→ k cl ĊH 2 Cl+CH 3 ĊH 2 , CH 2 +CH 3 CH 2 Cl→ k h1 ĊH 3 +ĊH 3 CH 3 Cl, } CH 2 +CH 3 CH 2 Cl→ k H2 ĊH 3 +CH 3 ċHCl. } ( k H ) All the fourteen products of the radical recombinations have been identified. Disproportionation of radicals and decomposition of excited molecules formed by recombinations yield additional products. Methylene insertion does not appear to play a significant role. When the incident light contains wavelengths in the region 2450 to 4000Å we find that k Cl / k H =1·62, k H1 / k H2 =0·098. If shorter wavelengths are excluded, or if nitrogen is added, lower values of k Cl / k H are obtained. On the other hand, in the presence of carbon monoxide the value of k Cl / k H may be greatly increased. It is suggested that these findings are attributable to differences in reactivity between singlet and triplet methylene. At longer wavelengths, or when nitrogen is present, the relative concentration of the singlet is reduced, but in the presence of carbon monoxide the triplet is removed preferentially (De Graff & Kistiakowsky 1967). Singlet methylene appears to be highly discriminating in its reactions, abstracting chlorine preferentially, while the triplet discriminates in favour of hydrogen abstraction. A kinetic analysis based on these ideas and consistent with the experimental observations shows that k S Cl / k S H >16·3, k T Cl / k T H <0·14. The selectivities shown by the two species of methylene are thought to be a result of differences in electronic structure rather than energy content.