Resonance of Valency Structures Benzene

Following a similar argument for the molecule of benzene, it is impossible to prefer one of the two valency forms to the other since the electron of the first carbon atom may equally well be exchanged with that of the second carbon atom as with that of the sixth. Consequently both forms exist in the molecule and when these are superimposed there arises the transitional structure which possesses a lower energy th n either of the original states and is therefore more stable. Unfortunately it is impossible to represent this transitional structure with the use of the conventional valency nomenclature and it was to overcome this difficulty that the idea of the resonance of valency structures was introduced by Slater and Pauling. The which-tly 

It is important to stress the difference between resonance and tauto-merism. The concept of resonance has been introduced to describe the delocalization of electrons in all the different forms of the molecule as represented by valence bond structures, among which resonance is said to occur, the configuration of the nuclei is the same. In tautomerism, on the other hand, the atoms are arranged in different ways and tautomeric changes, in which one form is changed into the other, occur as the result of chemical reactions. The tautomeric forms are in fact different chemical entities and in theory and frequently in practice, each can be isolated. 

For benzene we have considered so far only the Kekule structures in which the bonds are formed between neighbouring atoms such forms represent the greatest possible interaction of the electron clouds, since the distance between neighbouring carbon atoms is only i 4 A, and will thus contribute more than other possible structures to the resultant state of the molecule. But it is necessary to consider all possible valency structures among which there is resonance and thus for benzene the three Dewar forms must be considered  

It may be shown by quantum mechanics that the number of independent or canonical structures amongst which resonance can occur is given by, 

The five structures are the two Kekul and the three Dewar forms. It is possible, however, to construct other structures of benzene e.g. by joining atoms in the meta position with a tt bond or by making all three it bonds of the Dewar type  

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