Resonance stabilization of benzene

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... 

The isodesmic reaction approach (see Section 4.1) has also been applied to calculation of the resonance stabilization of benzene. 

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

Canonical forms of benzene that are calculated to contribute about 22% to the resonance stabilization of benzene. Such resonance structures have no separate physical reality or independent existence. For the case of benzene, the two Kekule structures with alternating double bonds i.e., cyclohexatriene structures) contribute equally and predominantly to the resonance hybrid structure. A dotted circle is often used to indicate the resonance-stabilized bonding of benzene. Nonetheless, the most frequently appearing structures of benzene are the two Kekule structures. See Kekule Structures... 

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) ... 

The resonance stabilization of benzene is commonly estimated to be about 36 kcal/mol (151 kJ/mol), but this is not an experimentally verifiable number. The 4n pi electron systems (4, 8,12, 16,...), with half-filled energy levels, not only are less stable than the 4n + 2 systems but are destabilized relative to their open-chain analog and are called antiaromatic. 

An isodesmic reaction suitable for calculating the resonance stabilization of benzene relative to cyclohexene is... 

The isodesmic reaction method (see Section 1.2.6) has also been applied to the calculation of the resonance stabilization of benzene. Homodesmotic reactions, a special version of isodesmic reactions, can also be used. Homodesmotic reactions... 

Another molecular orbital approach is to calculate the energy of a model in which the tt bonds are constrained to be localized double bonds by the definition of the wave function. The calculated energy of this model can then be compared with the computed energy of the molecule in which delocalization is permitted. By this definition, butadiene has a resonance stabilization of about 9.3 kcal/mol, while benzene has a resonance energy of about 56 kcal/mol. To compare this value with that obtained with the polyene reference, one must subtract a correction for the butadiene resonance energy (3 x 9.3), which gives a value of about 28 kcal/mol as the resonance stabilization of benzene. 

AE + AH. The AE + AE energy decomposition tells us that the resonance stabilization of benzene is due to a positive AE which overpowers a negative AE. The... 

Several other experimental determinations of the resonance energy of benzene have been performed using different model compounds, and although these determinations differ somewhat in their results, they all agree that the resonance stabilization of benzene is large. Following are resonance energies for several other aromatic hydrocarbons. 

Figure 12.1 illustrates the resonance stabilization of benzene based on heat of hydrogenation data. 

Figure 12.1 Heats of Hydrogenation and the Resonance Stabilization of Benzene...

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