Benzene resonance stabilization energy

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. 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

Another example is the polymerization of the dimer of o-xvlylene (77), which is also characterized by P-scission of the carbon-carbon bond in a key intermediate species 72. Acquisition of the resonance-stabilization energy of the benzene ring is the driving force of the reaction. 

This difference of 3.3 kcal is due to resonance stabilization energy. This energy is much lower than the resonance energy for the aromatic compounds, for example, benzene with a resonance stabilization energy of 36 kcal. 

Calculate from appropriate bond and stabilization energies the heats of reaction of chlorine with benzene to give (a) chlorobenzene and (b) 1,2-dichloro-3,5-cyclohexadiene. Your answer should indicate that substitution is energetically more favorable than addition. Assume the bond dissociation energy for a C=C it bond to be 6 5 kcal the resonance stabilization energy of benzene to be 36 kcal, and that of 1,2-dichloro-3,5-cyclohexadiene to be 3 kcal. 

The 36 kcal of resonance stabilization energy is added to the equation because that amount of energy is expended when we break the aromatic structure of benzene. 

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 polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

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