Hydrogenation theory: Some aspects

The model of associate adsorption of olefinic groups to give the half‐hydrogenated state suggested first by Horiuti and Polanyi, gives the simplest possible description of the hydrogenation process. By adding the requirement that a cis selective process is favored by preventing the free rotation around the C‐C bond of olefinic parentage, one can rationalize many observations in fat hydrogenation. Examples given in the present paper show that this prevention is achieved if two coordination sites are available for an effective locking of the diene during the hydrogenation. From this very idea, it follows that the trans selectivity must increase when the iodine value has been reduced to such a point that the proportion of dienes is very small. Another indication of the usefulness of the presented ideas is the spectroscopic observation that poisoning of the sites on a metallic surface that make two coordination places available for olefin coordination increases the trans ‐selectivity. All in all, this review emphasizes the important contributions of coordination chemistry and spectroscopy to the understanding of the mechanism of hydrogenation processes on solid metal surfaces.