Resonance of benzene

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

Chemical shifts for the C-H proton resonances of benzene, pyridine and pyrrol e (spectra recorded in CDC13)... 

In this they somewhat resemble the curly arrows used to show resonance. in benzene, where the arrows show where to draw the new bonds, and which ones not to draw in the canonical structure but in this case there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not far fetched, but it is as well to be clear that one is a reaction, with starting materials and a product, and the other is not. 

The resonance energies of some aromatic hydrocarbons are given in Table CXIII. Further insight into the resonance of benzene may be obtained from a study of the three successive stages of the hydrogenation reaction. We have... 

The shape resonances of benzene and some of its derivatives have been explored by a number of investigators using ETS (16,17,20, 36). With the exception of questions regarding pseudo-Jahn-Teller problems (37), for example in the alkyl-benzenes, substituent effects on the doubly-degenerate ground state anion near 1.1 eV... 

The curly arrows can be drawn in either direction. Here they are drawn so as to imply a clockwise movement of electrons, but the arrows could equally well have been drawn anti-clockwise. There is no absolute sense to the direction in which the electrons flow. Similarly, there is no absolute sense in which the hydrogen atom that moves from one carbon atom to the other in the ene reaction is a hydride shift, as seems to be implied by the curly arrows, or a proton shift, as it would seem to be if the arrows were to have been drawn in the opposite direction. In other words, neither component can easily be associated with the supply of electrons to any of the new bonds. The curly arrows therefore have a somewhat different meaning from those used in ionic reactions. In this they resemble somewhat the curly arrows used to show resonance in benzene, where the arrows show where to draw the new bonds and which ones not to draw in the canonical structure, but in the drawing of arrows interconnecting resonance structures there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not farfetched, but it is as well to be clear that one, the Diels-Alder reaction, is a reaction, with starting materials and a product, and the other, resonance in benzene, is not. 

A similar comparison is provided below to illustrate the deshielding/shielding properties of the primary, secondary and tertiary amine groups on the carbon resonances of benzene. The aromatic additivity constants are ... 

Fig. 1. The solvent dependence of the aromatic proton resonances of benzene,...

Figure 16. Spectra of natural rubber cross-linked with 25 phr ROOR. Spectrum (A) swollen in benzene to equilibrium swelling. Spectrum obtained under conditions of NFT experiment. Spectrum (B) same sample as (A), obtained under CP-MASS. The asterisk marks resonance of benzene solvent. Spectrum (C) the difference between (A-B).

You have learned that the resonance of benzene, C5H5, gives the compound special stabUity. 

Twentieth century theories of bonding m benzene gave us a clearer picture of aromatic ity We 11 start with a resonance description of benzene... 

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

Members of a class of arenes called polycyclic aromatic hydrocarbons possess subslanlial resonance energies because each is a colleclion of benzene rings fused logelher... 

In general the most stable resonance structure for a polycyclic aromatic hydro carbon is the one with the greatest number of rings that correspond to Kekule formula tions of benzene Naphthalene provides a fairly typical example... 

One explanation for the structure and stability of benzene and other arenes is based on resonance according to which benzene is regarded as a hybrid of the two Kekule structures... 

Each of the following may be represented by at least one alternative resonance structure in which all the six membered nngs correspond to Kekule forms of benzene Write such a resonance form for each... 

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate In spite of being allylic a cyclohexadienyl cation IS not aromatic and possesses only a fraction of the resonance stabilization of benzene... 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Azoles containing a free NH group react comparatively readily with acyl halides. N-Acyl-pyrazoles, -imidazoles, etc. can be prepared by reaction sequences of either type (66) -> (67) or type (70)->(71) or (72). Such reactions have been carried out with benzoyl halides, sulfonyl halides, isocyanates, isothiocyanates and chloroformates. Reactions occur under Schotten-Baumann conditions or in inert solvents. When two isomeric products could result, only the thermodynamically stable one is usually obtained because the acylation reactions are reversible and the products interconvert readily. Thus benzotriazole forms 1-acyl derivatives (99) which preserve the Kekule resonance of the benzene ring and are therefore more stable than the isomeric 2-acyl derivatives. Acylation of pyrazoles also usually gives the more stable isomer as the sole product (66AHCi6)347). The imidazole-catalyzed hydrolysis of esters can be classified as an electrophilic attack on the multiply bonded imidazole nitrogen. 

The most impressive example of resonance stabilization is benzene, in which the delocalization is responsible for a stabilization of 30-36 kcal/mol, the resonance energy of benzene. 

We will return to the aromatic stabilization of benzene in more detail in Chapter 9, but substituted benzenes provide excellent examples of how proper use of the resonance concept can be valuable in predicting reactivity. Many substituents can be readily classified... 

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