THE REACTIVITY OF THE CYCLIC POLYENES TOWARDS FREE RADICALS: II. METATHESIS WITH THE ETHYL RADICAL

Abstract

Arrhenius parameters have been measured for metathesis between the ethyl radical and certain cyclic polyenes, chosen to reveal the extent to which particular structural features in a cyclic molecule may influence its reactivity towards metathesis. The highest reactivity was observed in cyclohexadiene-1,4 and is associated with the greatest increase in the delocalization of the π-electron system, which is only slightly offset by a minor change in the conformation of the carbon ring. Intermediate reactivity was displayed by the conjugated molecules cyclohexadiene-1,3 and cycloheptatriene, whereas cyclooctatetraene, which possesses no methylenic hydrogen atoms, was much less reactive. Very low reactivity was found in bicyclo[2.2.1]heptadiene-2,5; this molecule possesses the carbon ring structure of cyclohexadiene-1,4 symmetrically bridged by a methylene group, and the constraint imposed by this bridge upon the geometry of the ring appears to limit the reactivity of the molecule decisively.The ratio of the rate constants for disproportionation and combination between the cyclo-hexadienyl radical and the ethyl radical is kd/kc = 0.38 ± 0.03. The nature of the interaction between these two radicals is discussed, and the significance of this ratio is considered.