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A kinetic study of the addition of the ethyl radical to conjugated dienes and related compounds Brown, Alistair Chalmers Ramsay

Abstract

A kinetic survey of the reactions of the ethyl radical with conjugated cyclic defines and related compounds has revealed a great variety of reactivity with respect both to addition and metathesis. An attempt to correlate reactivity with molecular structure has been presented both in the language of polymerization kinetics and of molecular orbital theory. In particular, the correlation of the energy of activation for the addition reaction with free valence and with localization energy has been pursued with some measure of success. The results of this survey indicate that characteristic values exist for the weighted A factors of both the addition and metathetical reactions. If the measured A factor is divided by the number of equivalent most-reactive sites in the molecule, values of the quotient lie within a narrow range. A characteristic value of this quotient appears to be common to molecules of diverse structural types. The results of this survey closely parallel the results of other investigators in related fields. In particular, rate constants for the addition of the ethyl radical in the gas phase are linearly related to rate constants for the addition of the methyl and the poly-styryl radical in solution. The few deviations observed in the comparison with the polyvinyl acetate and the polyacrylonitrile radicals may be explained in terms of polar effects by reference to the general equations for radical reactivity proposed by Bamford, Jenkins and Johnston and by Schwan and Price. The significance of the ratio of rate constants for addition and metathesis of a given compound with the ethyl radical has been considered as an index for reactivity in homopolymerization. The values of the ratio accord well with the published data on the degree of polymerization of these compounds. Finally the kinetic measurements have provided further information on certain special topics including the reactions of the cyclohexadienyl radical, the electron affinity of cyclooctatetraene and the structure of cycloheptatriene.

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