Lv. Jackson et Jc. Walton, Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: an EPR spectroscopic study of chain propagation, J CHEM S P2, (9), 2001, pp. 1758-1764
Citations number
35
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
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.