Palladium polar effect, substituents

In many papers in which the reaction rates or adsorption coefficients of a series of substrates were obtained the effect of structure is discussed only qualitatively. Rilzicka and Cerveny (5) found that the rate of hydrogenation of 1-olefins C6-C17 in ethanol under usual conditions on platinum, palladium and rhodium catalysts decreased monotonically with the chain length. Since in this olefinic series the chain length did not affect (68) the relative adsorptivity of olefins, as demonstrated in Table II, the observed phenomenon may be explained only by the effect exerted by the chain length on the rate constants. Since, moreover, the polar effect of substituents in this series is virtually the same, an explanation can be sought in the steric hindrance of the adsorbed double bond by the free end of the olefin molecule. The trans isomers were adsorbed much more weakly than the respective cis... 

In an attempt to eliminate the steric effect of substituents, Kieboom and van Bekkum (75) hydrogenated substituted 2-aryl-3-methyl-2-butenes and 3,4-dihydro-l,2-dimethylnaphthalenes. They found that the polar effect of substituents on the rate of hydrogenation on a palladium catalyst was virtually negligible, and hence, that the character of the activated complex of the rate determining step was similar to the initial state. The adsorptivity of substrates was reduced by electron abstracting substituents, in agreement with the view that the substrate is adsorbed as a surface n complex. 

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

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