Stereochemistry as a Criterion of Mechanism

Studies on the variation in the distribution of stereoisonieric products as a function of the accessible independent variables permit deductions concerning the multiplicity of the reaction paths. A classic example is found in Hughes and Ingold s studies into the mechanism of the solvolysis of alkyl halides in aqueous-alcoholic media, an important variable, being the concentration of base or other nucleophilic reagent (56 ). The obvious variable in hydrogenation studies is the pressure of 

The stereochemistry of hydrogenation of 1,2-dimethylcyclQhexene and 1,2-dimethylcyclopentene is instructive. Each of these substances would be expected to yield only the ci5-l,2-dimethylcycloalkane via cis addition. Both cis and trans isomers, however, are formed from either of these two cycloalkenes when hydrogenated in the liquid phase (acetic acid) over reduced platinum oxide—one of the more stereoselective catalysts [57, 58). The ratio of isomers which is produced is a function of the pressure of hydrogen, the proportion of cis increasing with increasing pressure (Fig. 5). This fact implies that the trans isomer is formed via a 

Variation with the pressure of hydrogen of the proportion of cis- and trans-diinethylcycloalkanes obtained from 1,2-dimethylcyclohexene (0), 2,3-dimethyIcycIo-hexene (A), 1,2-dimethylcyelopentene ( ), and 2,3-dimethylcyclopentene (V) reduced Pt02 in glacial acetic acid at 25°. 

Hydrogen is assumed to dissociate upon adsorption on the surface, reaction (1), 

The remaining steps consist in the union of the adsorbed olefin and its derivative, the half-hydrogenated state, with an adsorbed hydrogen atom, reactions which occur with retention of configuration of the substituted carbon atom, reactions (3) and (4), 

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