Kekule forms

Write structural formulas for toluene (CeHsCHj) and for benzoic acid (C6H5CO2H) (a) as resonance hybrids of two Kekule forms and (b) with the Robinson symbol... 

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... 

Hint if you draw the alternative Kekule form, you can push electrons around the ring... 

Kekule form does the thiophen ring contain two double bonds. To refer to the compound as 2, 3 -dihydrothieno(3, 2 l,2)indolizine is obviously unsatisfactory. Thus although for relatively simple molecules the Stelzner system is adequate, it would require modification to cover the more complex cases. Since the IUPAC system is capable of straightforward application to a much wider range of compounds, there seems... 

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). 

Kekule forms of benzoic acid Robinson symbol... 

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

By rewriting the benzenoid ring in its alternative Kekule form, both rings become benzenoid. 

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]. 

The niost efficient stabilizaticn of the intermediate carbocaNon produced by ejectrophilic attack on naphthafene comes from those resonance forms which retain one fully benzenoici ring. Ir the case of 1-substitution, two such structures can be drawn. 8 and 9 (plus their Kehule forrnsj. For 2-substitution there is oniy one structure. 10 and its Kekule form). The intermediate from attack at the i -position is the more stable and therefore the 1 -substituted product is favoured. 

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... 

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