18.21 : Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation

Recall that catalytic hydrogenation of an alkene double bond occurs under normal reaction conditions. Under similar conditions, however, a benzene double bond will not be reduced.

This is illustrated in the hydrogenation of stilbene, where under normal conditions, only the olefinic bond is selectively reduced, leaving the benzene rings unaffected.

Hydrogenation of an alkene double bond is exothermic, whereas hydrogenating the first unsaturated bond in benzene is an endothermic process. This is because the benzene ring is highly stabilized by resonance and resists hydrogenation.

Hydrogenation of the benzene ring becomes possible if extreme conditions of temperature and pressure and specific catalysts are used.

For instance, reducing the benzene double bonds to give a cyclohexane ring requires 3 moles of hydrogen and a nickel catalyst at 100 atm and 150 °C.

The reaction proceeds through highly reactive intermediates that are far more reactive than benzene, and therefore, cannot be isolated.

Benzene undergoes step-wise hydrogenation, with the ΔH° of the first hydrogenation step being positive. This explains why extreme reaction conditions are required to completely reduce benzene to cyclohexane.