Iodine atom-catalysed isomerisation of cis,trans,trans- and trans,cis,-trans-1,6-diphenylhexa-1,3,5-trienes and of trans,cis,trans,trans-1,8-dphenylocta-1,3,5,7-tetraene

Abstract

Rate constants have been measured for iodine atom-catalysed isomerisations of cis,trans,trans-(III) and trans,cis-,trans-diphenylhexatriene (IV), and of trans,cis,trans,trans-diphenyloctatetraene (V). The results have been compared with those of cis,trans-diphenylbutadiene (II) and simple olefins. Addition of an iodine atom is rate determining in all three cases, but at lower temperatures (<–15°) the isomerisation of the cis,trans,trans-hexatriene gradually becomes a diffusion controlled reaction. Differences in the rate constants are due mainly to differences in the activation energy of the addition reaction. It appeared that extension of conjugation in the starting olefin, which is accompained by a similar change in the intermediate radical of the isomerisation [e.g. (II)→(III) or (IV)→(V)], leads to a 60-fold increase in the rate constant. Introduction of an additional double bond which does not change the type of intermediate [(II)→(IV) or (III)→(V)] lowers the isomerisation rate ca. 40-fold.