Benzene Kekule based structure

In 1866 only a few years after publishing his ideas concerning what we now rec ognize as the structural theory of organic chemistry August Kekule applied it to the structure of benzene He based his reasoning on three premises... 

In 1866, August Kekule used the principles of structural theory to postulate a structure for the benzene molecule. Kekule based his postulation on the following premises ... 

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 3.9 The weights of Kekule valence structures of smaller benzenoid hydrocarbons based on the count of benzene rings with three C=C bonds. 

Before each of the classes is fully described, let us explain why we have /outclasses and not two classes (aromatic and anti-aromatic), or why three classes (aromatic, non-aromatic, and anti-aromatic) will not suffice. There are no problems with the dichotomy aromatic—anti-aromatic based on the presence of only 4n + 2 or only An conjugated circuits in the set of Kekule valence structures of a compound, respectively. The problem is with the non-aromatic class, which would include structures having both An+ 2 and An conjugated circuits. Some structures in this class may show a greater similarity with benzene, and on the other hand they may show some similarity to anti-aromatic compounds. The problem is that the class of such neither aromatic nor anti-aromatic structures, that have both 4n -I- 2 and An conjugated circuits, is so large and so broad that it becomes of little use. 

One explanation for the structure and stability of benzene and other arenes is based on resonance according to which benzene is regarded as a hybrid of the two Kekule structures... 

We have so far emphasized the nature of the wave function. We now examine the energies of some different arrangements ofthe bases. In Table 15.2 we show energies for five levels of calculation, Kekule-only, Kekule plus Dewar, SCF, SCVB, and full TV structures, where energies are given as the excess energy due to the tv system over that from the core. Cooper eta/. [61] gave the SCVB treatment of 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 Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

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