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.
18.21:
Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation
Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is because of resonance stabilization of the ring, that makes the ring extra stable the bond inert to regular hydrogenation conditions. Hydrogenation of the benzene ring requires extreme conditions of temperature and pressure, along with the use of specific catalysts. For example, benzene can be reduced to cyclohexane using three moles of hydrogen with nickel catalyst at 100 atm and 150 °C. The intermediates cyclohexadienes and cyclohexene are highly reactive and cannot be isolated because they are very reactive than benzene. In the case of disubsitutued benzenes, catalytic hydrogenation yields a mixture of cis and trans isomers.