Pyrolysis of ethylpyridines. Relative stability of isomeric picolinyl radicals
Decomposition rates for β C—C bond homolysis in 2-, 3-, and 4-ethylpyridine were measured in a very low pressure pyrolysis (VLPP) system from 1099 to 1227 K, and relative stabilities of the three isomeric picolinyl radicals (C5H4NCH.2) were derived. Bond dissociation enthalpies at 298 K, D(C5H4NCH2—CH3), were found to be 316, 309 and 312 kJ mol–1 for 2-, 3-, and 4-ethylpyridine, relative to 310.2 kJ mol–1 for ethylbenzene, assuming that the A factor for all of these reactions is 1015.0 s–1. Differences in activation energies of the three ethylpyridine decompositions were the same as the differences in ΔH°f(298 K) of the corresponding methylpyridines; hence, the ΔH°f(298 K) values of all three picolinyl radicals are computed to be the same within ca. 2 kJ mol–1. These results were related to H-atom abstraction studies in solution.