The theory of the stability of the benzene ring and related compounds

There are two fairly distinct problems involved in a treatment of the stability of the benzene ring. The first is to explain why of all the single-ring structures C n H n , that of benzene ( n = 6) is by far the most stable. The second is to examine the different models proposed by the chemists for the benzene ring in the light of quantum mechanics and to show in fact that they are all represented with varying probabilities in the complete model. Attempts have been made at both problems, but only the second has been worked out satisfactorily. In the present paper we attempt a more accurate solution than has hitherto been given of the first problem. Before commencing this it is necessary to give a description of previous work, since otherwise it is very difficult to see what is already certain and what remains to be done. In a series of papers Hückel has discussed at great length both of the problems mentioned above, not only for the simple benzene ring, but also for many of its substitution products. He evaluates the energy of the plane ring compound in two ways, one of which virtually amounts to the method of generalized electron pairs and the other to that of molecular orbitals. The chief weakness in his theory is that he considers only what he calls the p r electrons, viz., the n 2 p -electrons whose wave functions are odd for reflection in the plane of the ring. As we shall show, the three remaining bonding electrons on each carbon nucleus have a most important influence on the best value of n . Nevertheless, Hückel's work is quite satisfactory as regards the resonance between the p r electrons.