Pressure Effects on Azocumene Decomposition Rates, Efficiencies of Radical Production, and Semibenzene Dimers

Article abstract of DOI:10.1021/jo01311a016

Rates of thermal decomposition of azocumene (1a) and p,p'-dimethylazocumene (1b) in cumene or chlorobenzene are decreased by pressure with decomposition activation volumes of +5 cm³/mol (1a,cumene), +4.3 cm³/mol (1a,chlorobenzene) and +4.6 cm³/mol (1b,cumene).Efficiencies (f) of radical production, with di-tert-butyl nitroxide (DBNO) or thiophenol as scavengers, decrease with pressure; however, the pressure dependence of f with DBNO is anomalous.This is explained by the formation of semibenzene dimers as cage products, which subsequently decompose to give cumyl radicals.The corresponding α-ortho semibenzene dimers of cumyl radicals are formed from both 1a and 1b.The α-para semibenzene dimer from decomposition of 1a in chlorobenzene appears to rearrange to give p-cumylcumene.No such rearrangements appears to occur from the α-ortho dimers nor is it likely that any α-para dimer is formed from 1b decomposition.An apparent activation volume for thermal decomposition of the α-ortho semibenzene dimer in cyclohexane at 20 deg C is ca. +6 cm³/mol.Pressure favors disproportionation over coupling for geminate cumyl radicals.