Benzene Kekule forms

The two Kekule structures for benzene have the same arrangement of atoms but differ m the placement of electrons Thus they are resonance forms and neither one by Itself correctly describes the bonding m the actual molecule As a hybrid of the two Kekule structures benzene is often represented by a hexagon containing an inscribed circle... 

Each of the following may be represented by at least one alternative resonance structure in which all the six membered nngs correspond to Kekule forms of benzene Write such a resonance form for each... 

They also defined a state of metaionie in which no structural formula for the so-called tautomers corresponds to the "real" structure of the reacting molecule. The Kekule forms of benzene are examples of metaionie. 74... 

However, in recent years this basis has been somewhat undermined due to a critical reappraisal of experimental data on the benzene structure which, surprisingly, showed that a rigorous experimental proof of the generally accepted D6h structure of benzene is actually nonexistent It turned out that the X-ray structural data for benzene are compatible not only with the crystallographically ordered Dbh structure but also with the disordered Dih model associated with superposition of Kekule-type benzene molecules rotated by 60° with respect to each other about the threefold axis, both static and dynamic types of disorder being conceivable [87AG(E)782]. It has been shown by very simple calculations that if the difference between the C—C and C=C bond lengths in the D3h form is... 

The method that commonly is used is to draw a set of structures, each of which represents a reasonable way in which the electrons (usually in p orbitals) could be paired. If more than one such structure can be written, the actual molecule, ion, or radical will have properties corresponding to some hybrid of these structures. A double-headed arrow <—> is written between the structures that we consider to contribute to the hybrid. For example, the two Kekule forms are two possible electron-pairing schemes or valence-bond structures that could contribute to the resonance hybrid of benzene ... 

The members of a set of structures, as the two Kekule structures for benzene, have no individual reality. They are hypothetical structures representing different electron-pairing schemes. We are not to think of benzene as a 50 50 mixture of equilibrating Kekule forms. 

Another classic example of resonance is the benzene molecule. The localized resonance forms are termed Kekule forms (after Friedrich August Kekule, who first deduced the structure of benzene) and have alternating single and double bonds between carbon atoms. The actual benzene molecule is a resonance hybrid of the contributing resonance forms as the bond lengths are equal (single and double bonds have different lengths). 

This six-membered ring is not benzenoid (does not correspond to Kekule form of benzene). 

The classical formulae of the two hydrocarbons are given in the same figure below the more modern formulae. It is seen that the classical formulae of butadiene put a single bond between the two center carbon atoms the streamer picture, however, shows that the middle bond has some double bond character also. It is seen that the x-bond picture of benzene can correspond to either of the two Kekule forms but is better than either alone because it does not convey the erroneous impression that three of the six C—C bonds are single bonds. 

Benzene is symmetrical and the circle in the middle best represents this. However, it is impossible to draw mechanisms on that representation so we shall usually use the Kekule form with three double bonds. This does... 

It should, therefore, be remembered that, in the representation of the electron configurations of, for example, the two Kekule forms of benzene by classical structural formulae, the bond lengths and bond angles must be considered equal, thus in this case with all C—C distances and angles equal to those in benzene. 

Mills and Nixon based their interpretation on the Kekule time averaged oscillating model of benzene. This picture is obsolete, since it was conclusively shown subsequently that benzene had a single-well potential. The idea of rapidly equilibrating Kekule forms requires double-well potential, which has been abandoned in the meantime. Therefore, the MN-effect does not exist and this anachronistic structural proposal can be safely discarded [16]. 

In the hands of Pauling this treatment has been successful in correlating many facts in terms of a number of possible structural i ormulae, and it has been widely developed by Wheland and many others. Sometimes the valence bond structures betv een which resonance is considered to take place are natural and plausible, such as the two Kekule forms of benzene on other occasions it is tiecessary to postulate less plausible structures of liigh energy such as the ionic forms of carbon dioxide see Long i ). The... 

FIGURE 11.18. Resonance hybrids and bond lengths in aromatic compounds. For benzene, (a) Kekule forms and a common mode of depicting the aromatic character of benzene, (b) bond character and predicted bond lengths, and (c) experimental bond lengths and angles. 

Note The representation of benzene with a circle to represent the n system is fine for questions of nomenclature, properties, isomers, and reactions. For questions of mechanism or reactivity, however, the representation with three alternating double bonds (the Kekule picture) is more informative. For clarity and consistency, this Solutions Manual will use the Kekule form exclusively. 

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