An analysis of the role of the C=C double bond on the fate of alkenylperoxy radicals

The major components of biodiesel fuels usually consist of one or more C=C double bonds in their molecular structure, which are known to influence strongly their low-temperature combustion characteristics. While alkylperoxy radicals (RO₂), the key intermediates in the low-temperature oxidation of alkanes, have been widely investigated, the role of the peroxy radicals with C=C double bond(s) is less understood. In this study, 2-heptene and 2,5-heptadiene were selected to represent the long-chain biodiesel molecules having one and two C=C double bond(s), respectively. The fate of the various peroxy radicals of these two molecules was investigated by using quantum chemistry methods at the CBS-QB3//B3LYP-6-311G(d,p) level and canonical/variational transition state theories. Results show that at low temperatures (500-900 K), the peroxy radicals of 2,5-heptadiene that has four vinylic and three allylic carbon atoms can be neglected; while for 2-heptene, the 2-heptenylperoxy radicals with O2 bonded with the alkylic carbons favor H-atom shift reactions, with their rates mostly influenced by the size of the cyclic transition states, less by the hydrogen abstraction sites, and least by whether there is a C=C double bond in the cyclic transition state.