Kekule forms aromatic rings

The Kekule forms where the double bonds are delocalized around the aromatic ring are found also in the anilinium ion. 

This is because in these structures, known as Kekule forms, IT electrons are shared, not only by the two carbon atoms of each bond, but between atoms adjacent to bonds as well. This results in the delocalization of the it electrons around the ring (aromaticity). We can visualize the tt orbital system of an aromatic ring as a circuit of electrons. All adjacent pairs of carbon atoms have the same characteristic, with an equivalence of 1 5 bonds between each pair. 

There are only 12 tt electrons spread over this ring system. In addition, Kekule forms cannot be written for the central benzoid ring without formal bonds between atoms of the neighboring rings. Because of the long distance separating the atoms of the formal bond, it will not be stable. Thus this hydrocarbon will not be sufficiently stabilized by resonance and is not expected to be aromatic. 

The nondelocalized Kekule form depends on the ring system. Thus = 35 for five-membered and for fused five- and six-membered rings, and 33.3 for six-membered rings. Since benzene is a fully delocalized system, Khas the value zero and for benzene is 100. Similarly, the aromaticity index (/ ) for thiophene is 66 while those for pyrrole and furan are 59 and 43, respectively. Quantitatively, these values agree with that predicted from their chemical behavior. 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

Cyclic compounds other than benzene can, however, possess aromatic or benzenoid properties. This arises when they are planar and possess some double bonds which enable their formulae to be expressed in alternative Kekule-type structures. In such compounds an overlap of p orbitals occurs between adjacent atoms, and this allows the n electrons to become delocalised and form a continuous ring of electron density (Figure 6.21). An aromatic compound of this kind will sustain a magnetically induced ring current, and the bond lengths are all equivalent, lying between single and double bond values. Benzene with 6 n electrons was the first of these aromatic compounds to be encountered and seriously studied by chemists. 

Borazine is a colourless liquid (mp 215 K, bp 328 K) with an aromatic odour and physical properties that resemble those of benzene. The B N distances in the planar B3N3 ring are equal (144 pm) and close to those in the layered form of BN (Table 13.2). This is consistent with a degree of delocalization of the N lone pairs arotmd the ring as represented in 13.28. Structure 13.27 gives one resonance form of borazine, analogous to a Kekule structure for benzene. Despite the... 

---