The Kekule Structure for Benzene

A problem soon arose with the Kekule structure, however. The Kekule structure predicts that there should be two different 1,2-dibromobenzenes, but there are not. In one of these hypothetical compounds (below), the carbon atoms that bear the bromines would be separated by a single bond, and in the other they would be separated by a double bond. 

To accommodate this objection, Kekule proposed that the two forms of benzene (and of benzene derivatives) are in a state of equilibrium and that this equilibrium is so rapidly established that it prevents isolation of the separate compoimds. Thus, the two 1,2-dibro-mobenzenes would also be rapidly equilibrated, and this would explain why chemists had 

There is no such equiiibrium between benzene ring bond isomers. 

Nonetheless, the Kekule formulation of benzene s structure was an important step forward and, for very practical reasons, it is still used today. We understand its meaning differently, however. 

The tendency of benzene to react by substitution rather than addition gave rise to another concept of aromaticity. For a compound to be called aromatic meant, experimentally, that it gave substitution reactions rather than addition reactions even though it was highly unsaturated. 

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It is important to note that the Kekule structure for benzene and related compounds has not been abandoned - it is still widely used. 

The resonance theory is very useful in accounting for, and in many cases predicting, the behavior of substances with tt bonds. However, it is not omnipotent. One example where it fails is cyclobutadiene, for which we can write two equivalent valence-bond structures corresponding to the Kekule structures for benzene ... 

Scheme 33. Generation of the Twin States by Mixing of the Kekule Structures for Benzene (a) and Cyclobutadiene (b) as Archetypal Aromatic and Antiaromatic Species, (c) Transformation Properties of Ki and K2 with Respect to Symmetry Operations of the Dgh and Dih Point Groups...

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

The Discovery of Benzene 627 Nomenclature of Benzene Derivatives 628 Reactions of Benzene 630 The Kekule Structure for Benzene 631 The Thermodynamic Stability of Benzene 632 Modern Theories of the Structure of Benzene 634 Huckel s Rule The 4n +2 tt Electron Rule 637 Other Aromatic Compounds 645... 

Kekul4 structure (Sections 2. ID and 14.4) A structure in which lines are used to represent bonds. The Kekule structure for benzene is a hexagon of carbon atoms with alternating single and double... 

One can usually write two or more classical structures for a given cyclic conjugated molecule the Kekule structures for benzene are of course the classic example. However, one often finds that even in the case of molecules containing conjugated rings, certain bonds are single or double in all the possible classical structures that one can write. Such a bond is described as an... 

The Kekule structure for benzene was accepted for many years as the best explanation available. However it steadily became evident through the early 20th century that there were a range of problems in reconciling mounting experimental evidence with this structure. Some of these problems were ... 

The two Kekule structures for benzene have the same arrangement of atoms but differ m the placement of electrons Thus they are resonance forms and neither one by Itself correctly describes the bonding m the actual molecule As a hybrid of the two Kekule structures benzene is often represented by a hexagon containing an inscribed circle... 

Only four years after Kekule proposed the ring structure for benzene, and before the configuration of salicylic acid was established, Kraut concluded on the basis of sound chemical evidence that the products obtained on heating acetylsalicylic acid possess chain structures formed through intermolecular esterification. He assigned the dimeric and tetrameric formulas... 

Solving the simultaneous equations (2a) and (2b) leads to y = 3 and t = n — 2, implying the presence of three B-B bonds and n — 2 B-B-B bonds in the boron skeleton. Since a deltahedron with n vertices has In — 4 faces, the n — 2 B-B-B bonds cover exactly half of the faces. In this sense a Kekule-type structure for the deltahedral boranes B H 2- has exactly half of the faces covered by B-B-B bonds just as a Kekule structure for benzene has half of its edges covered by C=C double bonds. In 1977 Lipscomb and co-workers [29] reported a variety of such Kekule-type localized bonding structures with the lowest energies for deltahedral boranes. These structures were computed using wave functions in the differential overlap approximation. 

It is often asked whether or not the constituent structures of a resonating system, such as the KekulS structures for the benzene molecule ... 

The members of a set of structures, as the two Kekule structures for benzene, have no individual reality. They are hypothetical structures representing different electron-pairing schemes. We are not to think of benzene as a 50 50 mixture of equilibrating Kekule forms. 

Another feature of the resonance mixing, already alluded to, is the sign inversion which is caused by the different nature of the matrix elements that mix the Kekule structures for aromatics and antiaromatics. Thus, in the case of benzene (part a), the ground state is the positive combination of the two Kekule structures, while in cyclobutadiene (part b), the ground state is the negative combination.15116158210 214 Consequently, the twin excited states are the negative and positive linear combin-tions, respectively, for aromatics and antiaromatics.13-15-115-209 212 This relationship of the ground and excited states to the fundamental Kekule structures has been derived early on by the pioneers of VB theory.211-214... 

Pasteur s famous experiments with tartrate crystals have been described,161 the sesqui-centenary of their start being marked by a paper that examined missed opportunities in stereochemistry and, in particular, the work by Mitscherlich which nearly anticipated Pasteur.162 The centenary in 1995 of Pasteur s death prompted publication of several papers.163-165 His observations, and those of le Bel and van t Hoff, are described in a paper outlining the reasoning that led to the momentous recognition of the tetrahedral carbon atom.166 The reception of a tetrahedral carbon atom was varied. Its application to the relatively new Kekule structure for benzene was made by van t Hoff himself and more particularly by Wilhelm Komer.167 Eventual support came from Wislicenus, a widely ranging organic chemist, noted specially for his work on isomerism.168... 

FIGURE 6.20 The VBSCDs showing the crossing and avoided crossing of the Kekule structures of benzene along the bond alternating mode, b2u for (a) TT-only curves, (b) full ct+tt curves. 

It is to be noted that Ave can draw tAvo Kekule structures for benzene and its derivati i s. For example, for ortho xylene the two Kekule structures are... 

The procedure for counting S3 and D3 3 is based on the extended vertex-adjacency matrix and the adjacency bonding array. The extended vertex-adjacency matrix of a Kekule structure contains elements 1 or 2 depending on the single or double bond between carbon atoms in the Kekule structure and it is called extended because of the added column and row in which elements are set to 1 or 0 depending whether the carbon atom in the Kekule structure is connected to the hydrogen bond or not. For example, the extended vertex-adjacency matrix for the Kekule structure of benzene (see Figure 15) is ... 

Valence isomer A type of structural isomer that has the same basic skeleton, but differs in the exact bonding arrangement, e.g. the prismanes (or Ladenburg structure for benzene), and the Dewar and Kekule structures for benzene (i.e. the bicyclo[2.2.0]hexadienes), are all valence isomers of each other. 

One of the preeminent creators of structural theory was August Kekule, probably best known for advancing the hexagonal structure for benzene, C Hg. Any structural formula for benzene had ultimately to take account of the following facts the well-established quadrivalency of carbon the complete equivalence of benzene s six... 

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