Influence of a second reducible function on the mechanism of the hydrogenation of an ethylenic double bond

Abstract

The hydrogenation of 2-cyclohexenone and 2-methyl-2-cyclohexenone was carried out with a Pt/SiO 2 catalyst under stirred batch reactor conditions. The selectivity in the hydrogenation of the double bond is very high (> 90%). A negative order in ketone is observed, in contrast with the positive experimental order for olefins like cyclohexene, but in accordance with the negative order already found for 1,4-cyclohexanedione and acetophenone. Comparison with the hydrogenation of 3,4-dihydro-2 H-pyran whose behavior is identical to that of cyclohexene (kinetic order: + 1.0) shows that the juxtaposition of electrons in α position to the double bond does not change the kinetic parameters which are modified when there is a second reducible function (CO). This result is probably due to an interaction of the π system of the second function with the active sites of the catalyst leading to an inhibition by the reactant itself.