Valence bond theory Kekule structure, benzene

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond theory is a resonance hybrid of the two Kekule or canonical forms 4 and 9, conventionally shown as in Figure 1.2, and so each carbon-carbon bond apparently has a bond order of 1.5. 

Resonance between imaginary structures having localised bond (valence bond theory) or delocalization of it orbitals (molecular orbital theory) have both been found to explain the bonding state of benzene. The benzene molecule may be represented either as a hybrid of Kekule structures (valence bond theory) or as a regular carbon hexagon having an inscribed circle or dotted circle that symbolizes the three delocalized n orbitals. 

More than anyone else it has been Linus Pauling (b. 1901) who has been responsible for the development and application of the valence bond theory. In the early 1930s he deduced from quantum mechanics the tetrahedrally directed valencies of carbon, and he introduced the concept of the hybridisation of atomic orbitals. He introduced the idea of resonance as the quantum-mechanical counterpart of mesomerism. The wavefiinction for the molecule must contain terms for all possible structures, and the molecule is said to resonate between them. In 1933 Pauling described the benzene molecule as a resonance hybrid between the two Kekule structures and the three possible Dewar structures (Figure 11.22). 

The resonance theory is very useful in accounting for, and in many cases predicting, the behavior of substances with tt bonds. However, it is not omnipotent. One example where it fails is cyclobutadiene, for which we can write two equivalent valence-bond structures corresponding to the Kekule structures for benzene ... 

A recent summary of the history and dynamics of the theoretical models of benzene39 cites a view that even though the current molecular orbital (MO) view of benzene seems complete and ultimate while the valence bond (VB) view seems obsolete, the recent findings about zr-distortivity in benzene indicate that the benzene story is likely to take additional twists and turns that will revive the VB viewpoint (see footnote 96 in ref 39). What the present review will show is that the notion of delocalized zr-systems in Scheme 1 is an outcome of both VB and MO theories, and the chemical manifestations are reproduced at all levels. The use of VB theory leads, however, to a more natural appreciation of the zr-distortivity, while the manifestations of this ground state s zr-distortivity in the excited state of delocalized species provides for the first time a physical probe of a Kekule structure .3... 

In earlier chapters, and above, I have shown how Kekule used his sausage formulas and models with heuristic confidence, even while disclaiming a realistic interpretation of them. He was convinced that valence was a constant rather than a variable quantity for any element he rejected out of hand the notion that two affinities of a single atom could satisfy one another he provided an appealing model for theories of double bonds, triple bonds, and unsaturated affinities ("gaps"), as well as a way easily to distinguish "atoms of the radical" versus "atoms of the type" he was led to conclusions regarding certain structures (e.g., of pyrotartaric acid and of isopropyl alcohol) that other structuralists immediately jumped on as errors and he discussed different theoretical options for the benzene nucleus. All of these are examples of positions that can be deduced or otherwise derived from contemplation of... 

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