Reconstruction of Organic Electron Theory. II. Cross-Conjugation.

Abstract

The main purpose of this series of research is to reconstruct theoretically the "Organic Electron Theory," which was established by Robinson and Ingold in the 1920s without any quantum-theoretical and mathematical basis but still used by organic chemists of the 21st century. The main strategy is graph-theoretical molecular orbital method (GTMO) using topological index Z and supplemented by the perturbation treatment by Coulson and Longuet-Higgins. In this paper the mean length of conjugation L is proposed to be defined for a conjugated hydrocarbon molecule with only one Kekulé structure. With this L the relative stability of isomers of conjugated hydrocarbons can be well predicted, and the extent and stability of heterosubsituted conjugated hydrocarbons can also be explained, which can be diagrammatically obtained by using the "curly arrow" but with no logical support. Thus verification and limitation of the conventional organic electron theory are gradually being accumulated and clarified.