Kekule structure of benzene

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... 

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. 

The second mechanism, due to the permutational properties of the electronic wave function is referred to as the permutational mechanism. It was introduced in Section I for the H4 system, and above for pericyclic reactions and is closely related to the aromaticity of the reaction. Following Evans principle, an aromatic transition state is defined in analogy with the hybrid of the two Kekule structures of benzene. A cyclic transition state in pericyclic reactions is defined as aromatic or antiaromatic according to whether it is more stable or less stable than the open chain analogue, respectively. In [32], it was assumed that the in-phase combination in Eq. (14) lies always the on the ground state potential. As discussed above, it can be shown that the ground state of aromatic systems is always represented by the in-phase combination of Eq. (14), and antiaromatic ones—by the out-of-phase combination. 

Owing to the 7tCc-ocrc CT interactions, the usual equivalence between the two Kekule structures of benzene is broken and the ring distorts strongly to D3h symmetry, with pronounced alternation (by 0.02 A) of C=C bond lengths. Complexes of this type are evidently closely related to the metallocene sandwich compounds discussed elsewhere in this book (Section 4.9.5), with the benzene molecule described as a tridentate ligand in the language of metal coordination chemistry. 

Figure 11.16 The Kekule structures of benzene (C6H6) (upper structures), showing the two possible arrangements of the double bonds around the ring. Below is the aromatic resonance structure with the rotation of the double bonds symbolized by a ring, and the hydrogen atoms assumed to be present at each of the six corners .

If we consider the Kekule structure of benzene, it is evident that the two proposed structures differ only in the positions of the electrons. Therefore, instead of being two separate molecules in equilibrium, they are indeed two resonance contributors to a picture of the real molecule of benzene. 

Exercise 21-1 Determine which of the following structures can be represented by one or more specific electron-pairing schemes similar to the Kekule structures of benzene ... 

To understand the directionality of the -displacement, we now consider the effect of an annelating ring that maintains ji—jr-interaction with the Kekule structures of benzene, shown in Figure 12. At its top, the figure shows the Kekule structures, K2 with the double bonds in the endo-positions and Ki with the... 

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. 

These two inputs will yield the energy of a Kekule structure of benzene. 

XMVB Input 10.7. Calculations of the Energy of a Single Kekule Structure of Benzene... 

In discussing the resonance concept (p. 54), which allows a molecule to be represented by two or more structures, it was seen that a bond represented as a single bond in one of the structures might well be represented as double bond in another structure. In benzene (one of the simplest cases) the three carbon-carbon double bonds in one Kekule structure become single bonds in the other. The bond number here is between 1 and 2, but a more exact specification of bond number would depend on how this number is described, and agreement is by no means universal on this point. Since, however, the two Kekule structures of benzene are equivalent, it is quite reasonable to assign a number of 1.5 to the carbon-carbon bonds. Similarly, for the carbonate ion,... 

The representation of the Kekule structure of benzene by six monkeys. Notice that the monkeys held by two hands use one leg and vice versa. 

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 ... 

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