Stability of Benzene Class 12 Chemistry || Stability of Benzene on the Base of Heat of Hydrogenation

The heats of hydrogenation provide insight into the relative thermodynamic stability of various compounds. Notably, 1,3-cyclohexadiene is slightly more stable than anticipated, by about 2 kcal, likely due to the conjugation of its double bonds. In contrast, benzene is remarkably stable, being 36 kcal/mole (150.5 kJ/mol) more stable than expected.

Benzene’s stability allows it to exist indefinitely in a container. Energetically, it is more stable than a typical triene, a molecule with three double bonds, which is a characteristic of aromatic compounds. The estimated energy difference between benzene and the theoretical non-aromatic cyclohexatriene is around 37 kcal/mol (150.5 kJ/mol).

However, the heat of formation for benzene is reported to be +49 kJ/mol for the liquid state and +83 kJ/mol for the vapor state. The positive values indicate that one mole of benzene is less stable than six moles of graphitic carbon and three moles of diatomic hydrogen. It is the high activation energy barrier for the reaction C6H6 → 6 C + 3 H2 that grants benzene its practical stability. Thus, we can conclude that while benzene is thermodynamically unstable, it is kinetically stable.

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