Resonance structure of benzene

Although the resonance structures of benzene show it as a cyclo-hexatriene, because of its fully delocalized n system and the closed shell nature of this n system, benzene does not undergo addition reactions like ordinary unsaturated compounds. The destruction of the n electron system during addition reactions would make the products less stable than the starting benzene molecule. However, benzene does undergo substitution reactions in which the fully delocalized closed n electron system remains intact. For example, benzene may be reacted with a halogen in the presence of a Lewis acid (a compound capable of accepting an electron pair) to form a molecule of halobenzene. 

Thus in the free radical several valency bond structures arc possible in addition to the normal resonance structures of benzene which are still present in /, and resonance amongst all these states will lead to an increase in the stability of the free radical. Consequently, for the dissociation of dibenzyl less energy is required than for the dissociation of ethane because... 

We can now state that each carbon-to-carbon linkage in benzene contains a sigma bond and a partial pi bond. The bond order between any two adjacent carbon atoms is therefore between 1 and 2. Thus molecular orbital theory offers an alternative to the resonance approach, which is based on valence bond theory. (The resonance structures of benzene are shown on p. 349.)... 

These two structures are resonance structures of benzene. Benzene is also commonly represented as shown below ... 

Neither structure I nor structure II alone provides a good description of the true bonding of benzene. Structures I and II are called resonance structures. A doubleheaded arrow is used to indicate resonance structures. Each resonance structure is a legitimate Lewis structure. The best description of the structure of the molecule is taken to be the average of the resonance structures, sometimes called a resonance hybrid. For the resonance structures of benzene, above, the C-C bond order is 1.5. That is, the properties of the bond are about halfway between that of a C-C single bond and that of a C-C double bond. 

The symbol on the left shows the pi bond delocalized over the entire molecule. The symbol on the right shows only one of the two resonance structures of benzene it is an incomplete representatiom... 

Figure 7.12 I The two resonance structures of benzene show there are two ways to distribute three double bonds among the six carbon atoms. Benzene does not behave chemically like a compound with three double bonds, however, because the true structure is an average between these two resonance structures.

Aromatic behavior may be understood in terms of both geometrical and electronic structure based aspects Although alternate single and double bonds are depicted in the usual individual Kekule resonance structures of benzene (Fig. 2), due to aromaticity all the bonds in benzene are of equal length which is in between that of single and double bonds. 

Figure 1.28 The major resonance structures of benzene (hydrogen atoms omitted for clarity). A circle inscribed in a hexagon is often used to representaii the resonance forms of benzene.

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