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A kinetic study of the reactions of the ethyl radical with the methyl esters of acrylic acid and its mono-methyl derivatives Kambanis, Stamatis M.

Abstract

The Arrhenius parameters have been measured for the addition of the ethyl radical to methyl acrylate and its mono-methyl derivatives, methyl methacrylate, methyl cis-crotonate and methyl trans-crotonate. The energy of activation for each of these derivatives is significantly greater than for methyl acrylate, probably because of the adverse polar effect introduced by the methyl group. The activation energy for the addition to methyl methacrylate appears to be lower than for either the cis- or trans- methyl crotonate. This can probably be attributed to the fact that the ∝-methyl group can more effectively stabilise the adduct radical resulting from the addition of the ethyl radical to the methyl methacrylate, than the β-methyl group can in the case of crotonates. The pre-exponential factor is significantly smaller for each of the methyl crotonate isomers than for methyl methacrylate; partial shielding of the β-carbon atom of methyl crotonate is thus expressed by a steric factor rather than by an energy term. In terms of the rate constant for the addition reaction over the range 70°C to 160°C, methyl trans-crotonate is significantly more reactive than methyl cis-crotonate, and each isomer is only about one tenth as reactive as the vinyl compounds, methyl acrylate and methyl methacrylate, in which the site of radical attack is comparatively unobstructed. The cis-trans isomerisation of methyl crotonate is photo-sensitised by diethyl ketone illuminated by 3130 Å radiation in the gaseous phase; a mechanism of triplet state energy transfer is proposed for the process below 200°C; above 200°C an alternative mechanism involving the reversible addition of the ethyl radical to the double bond becomes the predominant process of isomerisation.

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