Benzene resonance theory

Resonance Raman scattering, 21 326-327 Resonance stabilization of benzene, 3 599 Resonance theory, 20 774 Resonant cavity, 14 851 Resonant-cavity enhanced structures,... 

The resonance theory can be applied successfully to explain the structure of benzene. First of all, let us have a look at the resonance theory. According to this theory... 

The resonance theory accounts for the much greater stability of benzene (resonance energy) when compared with the hypothetical 1,3,5-cyclohexa-triene. It also explains why there is only one 1,2-dibromobenzene rather than two. Therefore, the structure of benzene is not really a 1,3,5-cyclohex-atriene, but a hybrid structure as shown above. 

The resonance theory is very useful in accounting for, and in many cases predicting, the behavior of substances with tt bonds. However, it is not omnipotent. One example where it fails is cyclobutadiene, for which we can write two equivalent valence-bond structures corresponding to the Kekule structures for benzene ... 

We have used the concepts of the resonance methods many times in previous chapters to explain the chemical behavior of compounds and to describe the structures of compounds that cannot be represented satisfactorily by a single valence-bond structure (e.g., benzene, Section 6-5). We shall assume, therefore, that you are familiar with the qualitative ideas of resonance theory, and that you are aware that the so-called resonance and valence-bond methods are in fact synonymous. The further treatment given here emphasizes more directly the quantum-mechanical nature of valence-bond theory. The basis of molecular-orbital theory also is described and compared with valence-bond theory. First, however, we shall discuss general characteristics of simple covalent bonds that we would expect either theory to explain. 

Modem descriptions of the benzene structure combine resonance theory with molecular orbital theory. 

With the advent of the computer era, it is now possible to reexamine and rethink the resonance theory at the ab initio level. For example, throughout Pauling and Wheland s books, benzene is supposed to be a hybrid of two Kekule structures, by noting that Dewar and other ionic structures make little contribution to the resonance in benzene. However, classical ab initio VB calculations with all possible 175 resonance structures by Norbeck et al. [51] and Tantardini et al. [3], where strictly atomic orbitals are used to construct VB functions, manifested that the five covalent Kekule and Dewar structures make even less contribution to the ground state of benzene than the other 170 ionic structures. This prompts us to reconsider the mathematical formulations for resonance structures [52]. 

Bearing in the mind that a real bond should be described by three classical VB structures, we return to the case of benzene. Across the whole history of resonance theory, Kekule structure has been treated as the hypothetical 1,3,5-cyclohexatiene whose double bonds are comparable to ethylene. However, it is clear from the previous paragraph that the n bond in ethylene should be expressed as a sum of three classical VB structures. Furthemore, there are three ji bonds in a Kekule structure. Consequently, from the mathematical point of view, the wave function for a Kekule structure should be expanded by 33=27 classical VB structures as follows ... 

Naphthalene, which has two benzene-type rings fused together, provides a more interesting test for the predictive powers of resonance theory. There are three resonance structures for naphthalene, each with alternating single and double bonds around the rings ... 

By analogy with the rules of substitution in benzene based on the resonance theory and by considering that the quinonoid positions in naphthalene relative to 1 and 2 are 2,4,5,7 and 1,6,8 respectively a less empirical rule of substitution can be established. 

In theory then, pyridine is aromatic. But is it in real life The most important evidence comes from the proton NMR spectrum. The six protons of benzene resonate at 8h 7.27 p.p.m., some 2 p.p.m. downfield from the alkene region, clear evidence for a ring current (Chapter 11). Pyridine is not as symmetrical as benzene but the three types of proton all resonate in the same region. 

As we have already seen these lone pairs can form part of the system of n electrons. The difference between chromo-phoric and auxochromic groups is in this way of secondary importance. Also the much discussed question whether a ben-zoid (benzene-like) or a quinonoid (quinone-like) structure should be attributed to dyestuffs becomes, in the light of the resonance theory, an incorrectly chosen alternative. It is the possibility of resonance which is reflected in the multiplicity of the valence structure that forms the true basis for light absorption. An isolated benzoid configuration is just as little a colouring matter as a quinonoid structure compare the uncoloured hydroquinone and the very weakly coloured quinone. 

Both the valence-bond and molecular-orbital methods show that there is delocalization in benzene. For example, each predicts that the six carbon-carbon bonds should have equal lengths, which is true. Since each method is useful for certain purposes, we will use one or the other as appropriate. Recent ab initio, SCF calculations confirms that the delocalization effect acts to strongly stabilize symmetric benzene, consistent with the concepts of classical resonance theory. ... 

Resonance theory (Sections 2.4-2.5) accounts for the stability and properties of benzene by describing it as a resonance hybrid of two equivalea forms. Neither form is correct by itself the true structure of benzene is somewhere in between the two resonance forms but is impossible to draw with our usual conventions. Many chemists therefore represent benzene by drawing it with a circle inside to indicate the equivalence of the carbon-carbon bonds. This kind of representation has to be used carefully, however, because it doesn t indicate the number of w electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of jr electrons this way, but we must be aware of the limii tions of the drawing. . 

Benzene is described by resonance theory as a resonance hybrid of two equivalent structures ... 

It would seem therefore, that the actual state is one somewhere between a carbon-carbon single bond and a double bond, though rather nearer a double bond in character. We may express this on resonance theory, by saying that the true structure of benzene is a resonance hybrid of several structures, Where the actual structure is more stable than any of the writable conventional ones. It should be pointed out that the hybrid is not the result of an oscillation between such structures, nor does the benzene molecule contain certain percentages of the structures. 

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