THE MECHANISM OF THE REACTIONS OF AROMATIC HYDROCARBONS WITH HYDROGEN ON METALLIC CATALYSTS.

The mechanism of the catalytic hydrogenation of the benzene ring is clarified by examining the kinetics of the reaction of a number of tert-butyl substituted benzenes t-butyl-1, 1,3-di-t-butyl-2, and 1,4-di-t-butylbenzene 3 and 4-t-butyltoluene 4 over an alumina supported rhodium catalyst. The rates are zero order in substrate, first order in hydrogen, and increase in the sequence 1,2,4,3. In competitive experiments the order of reactivity is 1,4,2,3 which, via a quantitative treatment of a conventional mechanistic scheme, yields the relative adsorption coefficients of 1.0, 0.10, 0.02 and 0.01 respectively. Apparently, the substitution of a t-butyl-group for a hydrogen atom affects the energy of the transition state for the reduction to a much lesser extent than it increases the energy of the absorbed aromatic hydrocarbon. An unusually large amount of the intermediate cycloalkene, 1,4-di-t-butylcyclohexene, is formed from 3. The reduction of 2 is similar to 3, although the maximum concentration of the intermediate cycloalkene is much less so that the kinetics of its formation and disappearance is not easily measured. Author