Aromatic compounds kekule forms

FIGURE 11.18. Resonance hybrids and bond lengths in aromatic compounds. For benzene, (a) Kekule forms and a common mode of depicting the aromatic character of benzene, (b) bond character and predicted bond lengths, and (c) experimental bond lengths and angles. 

The ozonization of aromatic compounds is more interesting and complex in that each bond in the aromatic system has a certain degree of double bond character and this is reflected in the amounts of ozonization products since this reaction is specific for do >le bonds. Benzene, for example, has six bonds of equal double bond character. This can be represented by its two Kekul resonance structures which we must realize do not denote independent, existing forms but they do approximate the relative amount of double bond character for each bond. 

The tautomer (1,3,5-hydroxy benzene) of this structure has two Kekule forms which are importance resonance structures. The compound will resemble benzene in being aromatic. 

A few days later, Wurtz presented Kekule s benzene theory to the Societe Chimique in Paris.Kekule began by pointing out that no one, "as far as I am aware," had attempted to apply the theory of atomicity of the elements to aromatic compounds. He stated that he had had a "fully formed idea" on this question since 1858, having published hints in that direction in his major paper of that year, but he had not regarded it as appropriate to unveil it publicly and in detail until now. The theory that follows is very incomplete, he warned, and may not ultimately be verified, but it may also be very useful in stimulating and guiding experimental work, to confirm or refute it, hence his desire to put it forward now. 

Heterocyclic compounds containing nitrogen, oxygen, or sulfur are by far the most common. Four important examples are given here in their Kekule forms. These faur compounds are all aromatic ... 

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. 

Kekule also proposed that carbon atoms could link together to form chains. When a compound undergoes a reaction in which the number of carbon atoms remains unaltered, he suggested that the carbon skeleton is unaffected and only the atoms joined to it are changed. Thus Kekule arrived at some of the fundamental ideas of aliphatic chemistry, but the nature of aromatic compounds remained unclear. The best suggestion that he could advance at this stage was that in aromatic compounds the carbon atoms are somehow arranged more densely. 

Benzene, and most aromatic compounds, usually undergo aromatic substitution reactions and not the addition reactions of ordinary alkenes. Phenanthiene (Structure 5), however, is different When treated with bromine, it simply adds the bromine to the 9,10 double bond to yield 9,10-dibromophenanthrene. This unusual reactivity of phen-anthrene can easily be understood from the valence bond viewpoint. In terms of valence bond pictures, we can desaibe the structure of the skeleton of this molecule to a first approximation with the complete set of five Kekule forms (resonance forms), as shown in Structure 5 ... 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be formed by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

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... 

The questions raised by Kekule s interpretation of the mechanism of this reaction led to a number of investigations which involved a further study of the oxidation of aromatic phenols and quinones. Thus, for example, Zincke in collaboration with Kuster48 and Rabinowitch 4 undertook a systematic investigation of the action of chlorine (potassium chlorate plus hydrochloric acid) on various derivatives of the three dihydroxybenzenes. The results of these experiments showed that these classes of benzene derivatives split up under conditions to give aliphatic compounds. The intermediate compounds which were formed from tetrahydrobenzene during these transfonnations, were identified as the hexachloro-o-diketone derivative (I) and the pentachloro-m-diketone derivative (II). 

Fluorocarbon compounds were radiolyzed to establish the existence and assess the role of valence tautomers in radiation chemistry. Dewar-octafluorotoluene and -decafluo-roxylene have been isolated and identified as products of the photolysis and radiolysis of their respective aromatic parent in condensed phase. Dewar hexafluorobenzene was irradiated to 10 Mrad doses and found to revert to its KekulS isomer with a G-value of 10, to polymerize with G 20, and, in addition, to give rise to a new isomer. The concept of an energy sink is introduced whereby part of the energy absorbed by an aromatic system is stored in the form of interconvertible valence isomers which react further to yield polymers or decay to the ground state. 

Formed from six carbon atoms and six hydrogen atoms, chemists puzzled over how to arrange a chain of six carbon atoms with six hydrogen atoms and still preserve the tetravalence of carbon. Kekule came up with the name aromatic for the generally pleasant smelling class of benzene-based compounds, and then he came up with the structure—again reportedly in a dream ... 

Indeed. By browsing the recent special issue of Chemical Reviews devoted to the topic of aromaticity, one may obtain an impression that not only is Kekule dead, but also the valence bond theory is dead. Moreover, disappointingly, one may also get the impression that Clar was never born Among several thousand references listed in that issue, excluding historical remarks, you can count on your fingers how many times Kekule and Clar are mentioned. In over 3200 references cited collectively in 18 articles on aromaticity, books by Clar were cited only eight times. Yet, it is difficult to look at the contributions to the discussion of the aromaticity of benzenoid hydrocarbons, which form the basis for discussion of aromaticity of non-benzenoid and heterocyclic compounds, without reference to Kekule and Clar. Are we forgetting the shoulders on which we stand ... 

---