Rehybridization and the Chemical Reaction: POAV and 3D-HMO Analysis.

The present article is to appear in a special issue of Tetrahedron in honor of Professor M. J. S. Dewar. In this study chemical reactions are examined from a new standpoint - rehybridization. With the possible exception of oxidation- reduction processes, all chemical reactions involve rehybridization. Bond-making and bond-breaking processes are nothing more than changes in hybridization. In the present study two catogories of pericyclic reactions are examined with the pi- orbital axis vector (POAV) analysis and the three-dimensional (3D) HMO theory. The first makes use of a series of related molecular structures derived from X-ray crystallography which provide information on the reaction pathway between the valence tautomers 1,6-methanol[10]annulene (1a) and the bisnorcaradiene (1b). The second category involves structures calculated by Dewar and coworkers along the minimum energy reaction pathway of the Cope rearrangement of 1,5-hexadiene (2). The analysis provides a clear delineation of the structural and electronic changes which accompany chemical reaction. The sense of the pi- orbital axis vectors at the bridgehead atoms is shown to be the determining feature in dividing the potential surface of 1 between bridged-[10]annulene (1a) and bisnorcaradiene (1b).