Abstract
A kinetic study of substituent effects in the liquid-phase hydrogenolysis of benzyl oxygen compounds has been made using carbon-supported palladium as the catalyst. The reaction is zero order in substrate and is progressively inhibited by its product. Proton catalysis is essential for the hydrogenolysis of benzyl alcohols and alkyl benzyl ethers. For the hydrogenolysis of benzyl alcohols and 2-aryl-3-methyl-2-butanols the effects of substituents in the aromatic ring may be expressed in terms of a Hammett-Yukawa relationship with ϱ = −0.37 and −1.43, respectively. α-Substituents retard the rate of hydrogenolysis of benzyl alcohol. The order of reactivity for different leaving groups is: OH, OAlkyl ⪡ OAryl < +OHAlkyl < +OH 2, OAc < OCOCF 3. It is concluded that the hydrogenolysis reaction involves hydride attack at the benzylic carbon displacing the leaving group as its anion. For primary alcohols an S N2 mechanism is operative, whereas tertiary alcohols show s N1 type character. The adsorbed state is discussed on the basis of deuterolysis experiments.
Published Version
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