Substituent effects in heterogeneous catalysis: IX. Adsorption strength and reactivity of 4-substituted cyclohexanones in platinum-metal-catalyzed hydrogenation

Abstract

Substituent effects in platinum-metal-catalyzed hydrogenation were studied using 4-substituted cyclohexanones (substituent = OH, OMe, Cl, COOEt, Ph, C 6H 11, COOH, NMe 2) as reaction substrates. The hydrogenation rates of these derivatives were compared with that of cyclohexanone in both individual and competitive reactions. Despite the employment of several different metals and a variety of substituents, including acidic COOH and basic NMe 2, all of the relative rates fell within a rather limited range: 0.05–6.3 for competitive reactions and 0.10–2.1 for individual reactions. An analysis of these relative rate data indicates that in most cases the adsorption strength of the substituent-bearing cyclohexanones is greater than that of cyclohexanone, thus suggesting an attractive substituent-metal surface interaction. Palladium was distinguished from other platinum-group metals in its unusual weakness of carbonyl adsorption. In order to better understand the substituent effects of COOH and NMe 2, the effects of acid and base addition were also examined.