The Kekule Structure of Benzene

PROBLEM 4.2 Write out eqs. 4.2 and 4.4 using a Kekule structure for benzene. Does this model explain the existence of only one monobromobenzene Only three dibromobenzenes  

PROBLEM 4.3 How might Kekule explain the fact that there is only one dibromobenzene with the bromines on adjacent carbon atoms, even though we can draw two different structures, with either a double or a single bond between the bromine-bearing carbons  

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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. 

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. 

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 ... 

The Kekule structures of benzene account for the molecular formula of benzene and for the number of isomers obtained as a result of substitution. However, they fail to account for the unusual stability of benzene and for the observation that the double bonds of benzene do not undergo the addition reactions characteristic of alkenes. That benzene had a six-membered ring was confirmed in 1901, when Paul Sabatier (Section 4.11) found that the hydrogenation of benzene produced cyclohexane. This, however, still did not solve the puzzle of benzene s structure. 

The constant 225 results from the normalization conditions to obtain a zero value for the Kekule structure of benzene and 1 for any system with all bonds of equal length. 

Figure 16 The VBSCDs showing the crossing and avoided crossing of the Kekule structures of benzene along the bond alternating mode, biu for (a) rt-only curves, (b) full rt + CT curves, and (c) re-only curves in a putative situation where the avoided crossing leads to a ground state with a re-symmetrizing tendency. In this latter case, the excited state will have a distortive re-state and hence, a low frequency for the f>2 mode.

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