Pyrolysis of ethylpyridines. Relative stability of isomeric picolinyl radicals

Abstract

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 10^15.0 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.