Benzene: Introduction to Aromatic Compounds
Stability of Benzene
The cyclohexatriene structure, which was proposed by Kekulé in 1865, was not sufficient enough to answer all questions concerning benzene. Nevertheless, benzene does not react like a polyene, but is rather a remarkably stable compound. The reaction enthalpy of the hydrogenation of cyclohexene, which yields cyclohexane, amounts to -28.6 kcal/mol. About twice that amount is therefore expected for the hydrogenation of cyclohexadiene, namely -57.2 kcal/mol. Experimentally, a reaction enthalpy of -55.4 kcal/mol is found for the hydrogenation of cyclohexadiene, which corresponds well to the theoretically expected value. The small difference may be explained by the high stability of two conjugated double bonds compared to that of two isolated double bonds. The hydrogenation enthalpy of benzene (formally cyclohexatriene) may have been expected to be about -57.2 kcal/mol - 28.6 kcal/mol = -85.8 kcal/mol; due to the conjugation of the double bonds, it could also be only about -80 kcal/mol. However, the hydrogenation enthalpy of benzene actually amounts to only -49.8 kcal/mol. This difference (36.0 kcal/mol) is called resonance energy.
Single and double bonds in the benzene ring are indistinguishable. Carbon-carbon single bonds between the double bonds of non-aromatic conjugated dienes are 1.47 Å long, while their double bonds are 1.33 Å long. The carbon-carbon bonds in the benzene ring are all 1.39 Å long. Thus, their length is between the lengths of the diene's single and double bonds.