Valence-Bond Isomers of Substituted Benzenes


  • Based on the opening lecture at the Kekulé Centenary Exhibition in Ghent (Belgium) on November 25, 1964. Cf. Meded. Vlaamse chem. Vereen. 26, 193 (1964); Chem. Engng. News 42, No. 49, 38 (1964).


Dewar benzene, Ladenburg benzene (prismane), and a combination of these bond systems (benzvalene) have variously been discussed as planar limiting structures of benzene, although they have no physical reality. If, however, we regard these systems as non-planar structures (as did their originators), they become chemical individuals, derivatives of which have recently been isolated. In addition to being a valance-bond isomer of benzene, benzvalene is also the first member of a still largely hypothetical series of molecules (the valenes), in which the skeleton atoms can change places with their neighbors to produce the valencebond isomers. The best-known representative of this class is bullvalene.