Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: an EPR spectroscopic study of chain propagation

Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic ac ids containing linear, branched and cyclic alkyl substituents, as well as a llyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has bee n studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at tower temperatures , followed by spectra due to ejected carbon-centred radicals at higher temp eratures. Rate constants, for release of the carbon-centred radicals from t he cyclohexadienyl radicals, were determined from radical concentration mea surements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent an d with electron delocalisation in the ejected carbon-centred radical; 3,5- and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallyli c hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n- propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking ra dical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing COZR substituents.