18.26: 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.
