Application of the Hammett equation to non-aromatic unsaturated systems. VIII Ionization potentials

Ionization potentials of substituted ethylenes, trans-1-substituted propenes, and several sets of substituted carbonyl derivatives have been correlated with the extended Hammett equation. The electrical effect in the case of the ethylenes and propenes is characterized by a value of ε ≈ 2.0, whereas the carbonyl derivatives show values of ε in the range 0 to 0.4. This difference in the composition of the electrical effect is ascribed to loss of a π electron from the ethylenes and propenes and of an n electron from the carbonyl derivatives. For purposes of comparison, several sets of ionization potentials of substituted benzenes were also correlated with the extended Hammett equation. Values of ε ranged from 1 to 3; thus the substituted benzenes are comparable in their behavior to the ethylenes and propenes.