Benzene, structure resonance model

The most common example of the resonance theory is the description of the benzene structure. The experimentally precisely determined and accurately known carbon-carbon bond length is consistent with the model as average of the resonance structures. When Pauling s resonance description of the benzene structure was criticized, the physicist Edward Teller and his colleagues provided spectroscopic evidence to support it [40]. The Nobel laureate physicist Philip Anderson was oblivious of Teller s and his co-workers paper (Private communication from Philip Anderson to the author by e-mail in 2009), and 68 years after Teller s contribution, in 2008, Anderson communicated another supportive paper for Pauling s model [41]. 

Figure 15.2 A calculated structure for the arenium ion intermediate formed by electrophilic addition of bromine to benzene (Section 15.3). The electrostatic potential map for the principal location of bonding electrons (indicated by the solid surface) shows that positive charge (blue) resides primarily at the ortho and para carbons relative to the carbon where the electrophile has bonded. This distribution of charge is consistent with the resonance model for an arenium ion. (The van der Waals surface is indicated by the wire mesh.)...

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. 

As pointed out in Chapter 7, the atomic orbital (valence bond) model regards benzene as a resonance hybrid of the two structures... 

In aromatic compounds the effect of a functional group on retention may be enhanced or diminished by resonance. As illustrate in Fig. 4 the curves for monofunctional benzene derivatives exhibit a mo e or less parallel slope on the plot of log k against log eluent composition whereas the multifunctional derivatives, e.g., nitroanilines, cholestenotie, show distinctly different slopes. This demonstrates how difficult the prediction of retention behavior in adsorption chromatography is. The greater the deviation of the structure from the simple model compounds used for establishing the rules, the more difficult the prediction becomes. 

A true picture of benzene s structure was not determined until the 1930s when Linus Pauling produced his work on the chemical bond. Benzene does not exist as either of its resonance structures, and its structure should not be considered as either one or the other. A more appropriate model is to consider the... 

R. F. C url, R. F. Smalley, and 11. W. Kroto were awarded the Nobel prize m chemistry in 199fe for the discovery of the soccer ball-shaped molecule C, . The enthalpy of combustion of (J o is 25 937 kj-mol 1 and its enthalpy of sublimation is +233 kj-mol 1 There are 90 bonds in of which 60 are single and 30 are double bonds. is like benzene in that it has a set of multiple bonds for which resonance structures may be drawn, (a) Determine the enthalpy of formation of Cfj) from its enthalpy of combustion, (h) Calculate the expected enthalpy of formation of from bond enthalpies, assuming the bonds to be isolated double and single bonds, (c) Is CMI more or less stable than predicted on the basis of the isolated bond model ... 

Whenever there are two alternative Lewi.s structures, one alone will be an inaccurate representation of the molecular itructure. A more accurate picture will be obtained by the superposition of. the two structures into a new model, which lor benzene indicated by 3. The. superposition of two or more Lewis structures into a composite picture is called resonance. 

Valence bond theory is somewhat out of favour at present a number of the spectroscopic and magnetic properties of transition-metal complexes are not simply explained by the model. Similarly, there are a number of compounds (with benzene as an organic archetype) which cannot be adequately portrayed by a single two-centre two-electron bonding representation. Valence bond theory explains these compounds in terms of resonance between various forms. This is the origin of the tautomeric forms so frequently encountered in organic chemistry texts. The structures of some common ligands which are represented by a number of resonance forms are shown in Fig. 1-11. 

Benzene has two major resonance structures that contribute equally to the resonance hybrid. These are sometimes called Kekule structures because they were originally postulated by Kekule in 1866. You may also encounter benzene written with a circle inside the six-membered ring rather than the three double bonds. This representation is meant to show that the bonds in benzene are neither double nor single. However, the circle structure makes it difficult to count electrons. This text uses a single Kekule structure to represent benzene or its derivatives. You must recognize that this does not represent the true structure and picture the other resonance structure or call upon the MO model presented in Section 16.3 when needed. 

---