The Reaction of Aromatic Hydrocarbons with Hydrogen

Studies of the kinetics of the addition and exchange of deuterium with benzene resulted in the first detailed mechanistic proposals of Polanyi and his associates (50,83) and of Farkas and Farkas (J3, 84,85). 

Farkas and Farkas suggested that the critical step in hydrogenation involved the simultaneous addition of two hydrogen atoms to an adsorbed benzene molecule, whereas exchange with deuterium required the prior dissociation, on the surface, of benzene to form a phenyl radical and a hydrogen atom. The phenyl radical then combined with a deuterium atom, which had been produced by the dissociation of a deuterium molecule, and the monodeuterobenzene was desorbed. 

Horiuti and Polanyi (50) argued differently and concluded that the exchange reaction had an associative mechanism as does the addition of hydrogen to benzene. However, they assumed that in the latter the two atoms of hydrogen added consecutively. 

Later Greenhalgh and Polanyi (14) formulated kinetic expressions for this mechanism. They showed that the dissociative mechanism for exchange according to Farkas and Farkas would yield the same mathematical function as the associative mechanism if the combination of deuterium atoms with phenyl radicals was the slow step. However, they continued to favor the associative mechanism. 

Recently Harper and Kemball 90) have elaborated the associative mechanism in a manner which avoids the above geometrical limitations. Much of the reasoning upon which they base their ideas is recorded in papers by Gault et al. 91) and by Rooney 80). They postulate that TT-bonded benzene or molecular species such as B can combine either with a hydrogen atom from the surface or with a hydrogen molecule from the gas or physically adsorbed phase  

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