Resonance energy, of benzene

The Nomination and hydrogenation reactions just discussed, along with much other evidence, indicate that benzene has an especially large resonance stabilization. Can the magnitude of this resonance stabilization be determined One way to measure such a stabilizing effect in a compound is to measure the amount of heat evolved in some re- 

What compound should be used in the hydrogenation as the model for the hypothetical compound cyclohexatriene —that is, benzene without any resonance stabilization The model should be as similar to benzene as possible but without any possible resonance stabilization. The best that can be done is to use the double bond of cyclohexene as the model for one double bond of benzene. Hydrogenation of cyclohexene produces cyclohexane and 28.6 kcal/mol (120 kJ/mol) of heat  

According to this model, the hydrogenation of the three double bonds of benzene should produce 3 X 28.6 = 85.8 kcal/mol (359 kJ/mol) of heat if there were no resonance stabilization. The difference between the amount calculated on the basis of three cyclohexene double bonds and the amount that is actually produced is the resonance stabilization for benzene. By using these reactions, the resonance energy or resonance stabilization of benzene is calculated to be 36.0 kcal/mol (151 kJ/mol)  

Minus heat evolved from the reaction —49.8 kcal/mol (208 k)/mol) 

The catalytic hydrogenation of naphthalene produces 80.0 kcal/mol (335 kJ/mol) of heat. Calculate the resonance stabilization of naphthalene. Do you think naphthalene should be termed aromatic  

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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... 

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. 

The Nature of the Chemical Bond. V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon... 

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

Pauling, L. Wheland, G.W. The Nature of the Chemical Bond. V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals J. Chem. Phys. 1933, 1, 362-374 Errata, ibid, 1934, 2, 482. 

Those familiar with the long history of the attacks on the question of the resonance energy of benzene may be somewhat surprised at the small numbers in Table 15.3. The energy differences that are given there are for just the sort of process that might be expected to yield a theoretical value for the resonance energy, but experimental determinations deld numbers in the range 1.7-2.3 eV. This is an important question, which we will take up in Section 15.3, where it will turn out that some subtleties must be dealt with. 

There is still interest in the resonance energy of benzene. Beckhaus et al. [65] have synthesized a molecule with a strained benzene ring in it and measured heats of hychogenation. This is an ejq)erimental attempt to assay what we did theoretically. They found similar results. 

A quantitative estimate of the stabilization or resonance energy of benzene (which cannot be directly measured) may be obtained by determining the heat evolved when hydrogen is added to benzene and to cyclohexene to yield cyclohexane ... 

The three structures C, Z), and E are less stable than the Kekul6 structures and make much smaller contributions to the normal state of the benzene molecule. They increase the resonance energy from 0.9a to 1.11a. By equating this to the empirical resonance energy of benzene, 37 kcal/rncle, a is found to have the value 33 kcal/mole. 

Since a calculation of the resonance energy of benzene by the valence bond method shows that the greater part of it is a result of the resonance between the two Kekule structures shown, we might suppose that its homologs would also have significant resonance stabilization energies. Such conclusions are at variance with experimental fact, however, since cyclobutadiene appears to be too unstable to have any permanent existence, and cyclooctatetraene exists as a nonplanar tetraolefin, having no resonance stabilization of the sort considered. 

On the other hand, we should perhaps not be too surprised if values of fi derived from different kinds of measurements do not agree exactly, and indeed this is the case. We have seen above that/ obtained from the vertical resonance energy of benzene is about 37 kcal/mol, whereas Platt first showed that the best overall fit to the spectra of benzene and other unsaturated hydrocarbons was obtained in the framework of the Huckel approximation by taking / to be 55-60 kcal/mol. This high value, which has subsequently been widely adopted to estimate actual differences in energies between MOs, is generally known as the spectroscopic value of / . 

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