Structure Kekule

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. 

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... 

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... 

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... 

The extent to which benzene is more stable than either of the Kekule structures is its resonance energy, which is estimated to be 125-150 kJ/mol (30-36 kcal/mol) from heats of hydrogenation data... 

Kekule structure (Section 112) Structural formula for an aro matic compound that satisfies the customary rules of bond mg and is usually characterized by a pattern of alternating single and double bonds There are two Kekule formula tions for benzene... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

A single Kekule structure does not completely describe the actual bonding in the molecule. 

Benzene could be represented by two resonating Kekule structures. 

Kekule structure (Section 1.4) A method of representing molecules in wrhich a line between atoms indicates a bond. 

Resonance hybrid (Section 2.4) A molecule, such as benzene, that can t be represented adequately by a single Kekule structure but must instead be considered as an average of two or more resonance structures. The resonance structures themselves differ only in the positions of their electrons, not their nuclei. 

Chen, R. S., Cyvin, S. J., Cyvin, B. N., Brunvoll, J., and Klein, D. J. Methods of Enumerating Kekule Structures, Exemplified by Applified by Applications to Rectangle-Shaped Benzenoids. 153, 227-254 (1990). 

The difficulty with a single Kekule structure is that it does not fit all the experimental evidence ... 

A Kekule structure suggests that benzene should have two different bond lengths three longer single bonds (154 pm) and three shorter double bonds (134 pm). Instead, all the bonds are found experimentally to have the same intermediate length (139 pm). 

If the Kekule structure were correct, there would be two distinct dichlorobenzenes in which the chlorine atoms are attached to adjacent carbon atoms (13), one in which the carbon atoms are joined by a single bond and one with a double bond. In fact, only one such compound is known. 

We can use the concept of resonance to explain these characteristics of the benzene molecule. There are two Kekule structures with exactly the same energy they differ only in the positions of the double bonds. As a result of resonance... 

FIGURE 3.21 Unhybridized carbon 2p-orbitals can form a ir-bond with either of their immediate neighbors. Two arrangements are possible, each one corresponding to a different Kekule structure. One Kekule structure and the corresponding ir-bonds are shown here. 

FIGURE 3.22 As a result of resonance m between two structures like the one. g shown in Fig. 3.21 (corresponding to 3 resonance of the two Kekule structures), 3 the ir-electrons form a double doughnut- j. shaped cloud above and below the plane of the ring. w... 

Benzene is more stable and less reactive than would be predicted from its Kekule structures. Use the mean bond enthalpies in Table 6.8 to calculate the lowering in molar energy when resonance is allowed between the Kekule structures of benzene. 

Kekule structures Two Lewis structures of benzene, consisting of alternating single and double bonds, kelvin (K) The SI unit of temperature. See also Appendix IB. 

The convenience and usefulness of the concept of resonance in the discussion of chemical problems are so great as to make the disadvantage of the element of arbitrariness of little significance. Also, it must not be forgotten that the element of arbitrariness occurs in essentially the same way in the simple structure theory of organic chemistry as in the theory of resonance — there is the same use of idealized, hypothetical structural elements. In the resonance discussion of the benzene molecule the two Kekule structures have to be described as hypothetical it is not possible to synthesize molecules with one or the other of the two Kekule structures. In the same way, however, the concept of the carbon-carbon single bond is an idealization. The benzene molecule has its own structure, which cannot be exactly composed of structural elements from other molecules. The propane molecule also has its own structure, which cannot be composed of structural elements from other molecules — it is not possible to isolate a portion of the propane molecule, involving parts of two carbon atoms... 

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