Understanding benzenes resonance

Examples of reactions where benzene does not behave like an alkene. 

The pi-electrons are delocalized over the entire ring structure, not localized between two carbons. This contributes to the observed stability of benzene. 

---

It is understandable that dihydro adducts should be formed by polycyclic compounds and not by benzene or pyridine, because the loss of aromatic resonance energy is smaller in the former than in the latter process, (c) When dibenzoyl peroxide is decomposed in very dilute solution (0.01 Af) in benzene, 1,4-dihydro biphenyl is produced as well as biphenyl, consistent with addition of the phenyl... 

Here the operator af creates (and the operator a, removes) an electron at site i the nn denotes near-neighbors only, and /i,y = J drr/),/l(j)j denotes a Coulomb integral if i = j and a resonance integral otherwise. The second quantization form of this equation clearly requires a basis set. It is a model for the behavior of benzene - not a terribly accurate one, but one that helps us understand many things about its spectroscopy, its stability, its binding patterns, and other physical and chemical properties. 

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

The partial rate factors af and /3f for the a- and /3-positions of thiophene have been calculated for a wide range of electrophilic reactions these have been tabulated (71 AHC(13)235, 72IJS(C)(7)6l). Some side-chain reactions in which resonance-stabilized car-benium ions are formed in the transition states have also been included in this study. A correspondence between solvolytic reactivity and reactivity in electrophilic aromatic substitution is expected because of the similar electron-deficiency developed in the aromatic system in the two types of reactions. The plot of log a or log /3f against the p-values of the respective reaction determined for benzene derivatives, under the same reaction conditions, has shown a linear relationship. Only two major deviations are observed mercuration and protodemercuration. This is understandable since the mechanism of these two reactions might differ in the thiophene series from the benzene case. 

Most organic chemists are familiar with two very different and conflicting descriptions of the 7r-electron system in benzene molecular orbital (MO) theory with delocalized orthogonal orbitals and valence bond (VB) theory with resonance between various canonical structures. An attitude fostered by many text books, especially at the undergraduate level, is that the VB description is much easier to understand and simpler to use, but that MO theory is in some sense more fundamental . 

The question is whether or not these reliable predictions of quantitative VB theory may also arise from a qualitative VB theory. Early semiempirical HLVB calculations by Wheland (14,15) and for that matter any VB calculations with only HL structures, incorrectly predict that CBD has resonance energy larger than that of benzene. Wheland, who analyzed the CBD problem, concluded that ionic structures play an important role, and that their inclusion would probably correct the VB predictions. Indeed the above mentioned successful ab initio VB calculations implicitly include ionic structures due to the use of CF orbitals in the VB descriptions of benzene, CBD, and COT. As will be immediately seen, ionic structures are indeed essential for understanding the difference between aromatic and antiaromatic species, such as benzene, CBD, and COT. Furthermore, the inclusion of ionic structures bring in some novel insight into other features of these molecules, such as ring currents, and so on (see Exercise 5.4). 

On the basis of this resonance picture only, organic chemists initially expected that cyclobutadiene, like benzene, would have a large resonance stabilization and would be especially stable. Yet cyclobutadiene proved to be an extraordinarily elusive compound. Many unsuccessful attempts were made to prepare this compound before it was finally synthesized at very low temperature in 1965. The compound is quite unstable and reacts rapidly at temperatures above 35 K. As we shall see, cyclobutadiene is a member of an unusual group of compounds that are actually destabilized by resonance. To understand why benzene is so stable while cyclobutadiene is so unstable, we must examine a molecular orbital picture for these compounds. 

Visualizing benzene as a resonance hybrid of two Kekule structures cannot fully explain the unusual stability of the aromatic ring. As we have seen with other conjugated systems, molecular orbital theory provides the key to understanding aromaticity and predicting which compounds will have the stability of an aromatic system. 

Pauling s use of valence bond theory had a direct connection with the types of structures commonly used by organic chemists, and was relatively easy to understand, provided one did not delve too deeply into its details. The basic postulate was that compounds having n-electron systems that can be described by more than one structure will be stabilized by "resonance" and will have a lower energy than any of the contributing structures. Thus, for benzene one would write... 

We begin with the basic features and mechanism of electrophilic aromatic substitution (Sections 18.1-18.5), the basic reaction of benzene. Next, we discuss the electrophilic aromatic substitution of substituted benzenes (Sections 18.6-18.12), and conclude with other useful reactions of benzene derivatives (Sections 18.13-18.14). The ability to interconvert resonance structures and evaluate their relative stabilities is crucial to understanding this material. 

To understand why some substituents make a benzene ring react faster than benzene itself (activators), whereas others make it react slower (deactivators), we must evaluate the rate-determining step (the first step) of the mechanism. Recall from Section 18.2 that the first step in electrophilic aromatic substitution is the addition of an electrophile (E ) to form a resonance-stabilized carbo-cation. The Hammond postulate (Section 7.15) makes it pos.sible to predict the relative rate of the reaction by looking at the stability of the carbocation intermediate. 

The last three resonance forms are similar to the first three the change is that the electrons are shown in alternate positions in the benzene ring. To be rigorously correct, these three resonance forms should be included, but most chemists would not write them since they do not reveal extra charge delocalization understand that they would still be significant, even if not written with the others. 

To understand the concept of resonance energy better, let s take a look at the resonance energy of benzene. In other words, let s see how much more stable benzene (with three pairs of delocalized tt electrons) is than the unknown, unreal, hypothetical compound cyclohexatriene (with three pairs of localized tt electrons). 

Because benzene and cyclohexatriene have different energies, they must be different compounds. Benzene has six delocalized tt electrons, whereas hypothetical cyclohexatriene has six localized tt electrons. The difference in their energies is the resonance energy of benzene. The resonance energy tells us how much more stable a compound with delocalized electrons is than it would be if its electrons were localized. Benzene, with six delocalized tt electrons, is 36 kcal/mol more stable than hypothetical cyclohexatriene, with six localized tt electrons. Now we can understand why nineteenth-century chemists, who didn t know about delocalized electrons, were puzzled by benzene s unusual stability (Section 7.1). 

In resonance theory, the electronic stmcture of benzene can be expressed by two Kekule formulas as in resonance hybrid 6.1 with the understanding that neither is real, but the electrons are delocalized and shared by the molecule as a whole. Thus there are no single or double bonds in benzene all bonds have the same experimental length of 1.40 A, which is between the values for an sp -sp single bond (1.46 A) and a double bond (1.34 A). The ring is planar with all internal bond angles that of a hexagon, 120°. 

---