KekulS

Structurally benzene is the simplest stable compound having aromatic character, but a satisfactory graphical representation of its formula proved to be a perplexing problem for chemists. Kekule is usually credited with description of two resonating structures which. 

The basic pattern common to all aromatics is the benzene ring as illustrated in Kekule s formula ... 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

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 t, 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 fomred 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. 

In the case of 1,3-butadiene, RAMSES combines the two double bonds to form a single, delocalized r-electron system containing four electrons over all four atoms (Figure 2-50a). The same concept is applied to benzene. As shown in Figure 2-50b, the three double bonds of the Kekule representation form one electron system with six atoms and six electrons. 

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

If has long been known that the enthalpy of hydrogenalion of benzene (49.8 kcal moU Conant and Kistiakowsky, 1937) is not the same as three times the enthalpy of hydrogenation of cyclohexene (3 x 28.6 kcal moU ). Evidently, the double bonds that w e write in the Kekule structure of benzene... 

Run benzene using HMO. Write out the full bond order matr ix, enter ing zero for any element off the tridiagonal. What is the bond order of benzene Is there any Kekule-type alternation in this model ... 

From Kekule s Four-Valent Carbon to Five- and Higher-Coordinate Hypercarbon Chemistry... 

One of the cornerstones of the chemistry of carbon compounds (organic chemistry) is Kekule s concept, proposed in 1858, of the tetra-valence of carbon. It was independently proposed in the same year by Couper who, however, got little recognition (vide infra). Kekule realized that carbon can bind at the same time to not more than four other atoms or groups. It can, however, at the same time use one or more of its valences to form bonds to another carbon atom. In this way carbon can form chains or rings, as well as multiple-bonded compounds. 

Kekule claimed that the concept came to him during a late night ride on a London omnibus in 1854. 

To what degree Kekule s reeollection was factual we don t know, but Couper and Butlerov independently had developed similar, more well-defined concepts of valenee bonding, whieh may have not been entirely unknown to Kekule. 

Kekule s fame and his extensive contributions to chemistry as a leading German professor of his time certainly overshadowed Loschmidt. 

I certainly do not want to minimize Kekule s major contributions to chemistry and their significance, but clearly there were—as is generally the case—other contributors who played a significant role and should be remembered. 

Hughes, E. D., (1959). Theoretical Organic Chemistry. International Union of Pure and Applied Chemistry Section of Organic Chemistry. (The Kekule Symposium.) London Butterworths. 

From the concept of isomerism we can trace the origins of the structural theory—the idea that a precise arrangement of atoms uniquely defines a substance Ammonium cyanate and urea are different compounds because they have different structures To some degree the structural theory was an idea whose time had come Three scientists stand out however for independently proposing the elements of the structural theory August Kekule Archibald S Couper and Alexander M Butlerov... 

Shortly thereafter but independently of Kekule Archibald S Couper a Scot working m the laboratory of Charles Adolphe Wurtz at the Ecole de Medicine m Pans and Alexan der Butlerov a Russian chemist at the University of Kazan proposed similar theories... 

A 1968 German stamp com bines a drawing of the struc ture of benzene with a portrait of Kekule... 

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... 

Kekule advanced the venturesome notion that the six carbon atoms of benzene were joined together m a ring Four bonds to each carbon could be accommodated by a sys tern of alternating single and double bonds with one hydrogen on each carbon... 

In 1861 Johann Josef Loschmidt who was later to become a professor at the University of Vienna pri vately published a book con taming a structural formula for benzene similar to the one Kekule would propose five years later Loschmidt s book reached few readers and his ideas were not well known... 

A flaw m Kekule s structure for benzene was soon discovered Kekule s structure requires that 1 2 and 1 6 disubstitution patterns create different compounds (isomers)... 

The two substituted carbons are connected by a double bond in one structure but by a single bond in the other Because no such cases of isomerism m benzene derivatives were known and none could be found Kekule suggested that two isomeric structures could exist but mterconverted too rapidly to be separated... 

At ceremonies in Berlin in 1890 celebrating the twenty fifth anniversary of his proposed struc ture of benzene August Kekule recalled the origins of his view of the benzene structure... 

Concluding his remarks Kekule merged his advocacy of creative imagination with the rigorous standards of science by reminding his audience... 

The Kekule quotes are taken from the biographical article of K Hafner published n Angew Chem Internat ed Engl 18 641-651 (1979)... 

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