Dewar theory

Apparently, it became clear from the previous chapters that the Dewar theory is essentially topological although it does not use explicitly the terminology and methods of the graph theory. 

The experimentally observed activation energy for the decomposition of an aromatic molecule is linearly related to its delocalization energy as calculated from Dewars theory. 

This way of tabulating the various points in the Dewar theory is somewhat cumbersome. In predicting the preferred transition states for monocyclic pericyclic reactions the only information that is really required is the number of participant electrons. Since we are concerned here only with even electron systems, whether they be charged or electrically neutral, the total number of electrons is therefore 4 or 4n + 2. A concise summary of the Dewar theory is then given by ... 

When it is realized that a Hiickel transition state involves zero or an even number of inversions and that an anti-Hiickel transition state has an odd number of inversions, then the direct and complete correspondence of the Dewar theory and general Woodward-Hoffmann rule (p. 100) is clearly apparent. 

M. J. S. Dewar, The Molecular Orbited Theory of Organic Chemistry, McGraw-Hill, New York, 1969,... 

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

The most notable studies are those of Ingold, on the orienting and activating properties of substituents in the benzene nucleus, and of Dewar on the reactivities of an extensive series of polynuclear aromatic and related compounds ( 5.3.2). The former work was seminal in the foundation of the qualitative electronic theory of the relationship between structure and reactivity, and the latter is the most celebrated example of the more quantitative approaches to the same relationship ( 7.2.3). Both of the series of investigations employed the competitive method, and were not concerned with the kinetics of reaction. 

Dewar, M.J.S. The Electronic Theory of Organic Chemistry Oxford University Press London, 1949 p 52. 

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

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