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Selective hydrogenation denotes the preferential attachment of hydrogen in presence of a catalyst to one of two or more centres of unsaturation. When it occurs, hydrogenation of all the unsaturated groups present is effected eventually, but the process is consecutive and, except to a minor extent, the unsaturated groups are not attacked concurrently. The phenomenon cannot be explained by variation in the rate of addition of hydrogen to different unsaturated groups because, although the rate of hydrogen absorption is in general constant during each stage of the process, the relative rates are not, as a rule, widely different. A linoleic ester absorbs hydrogen about three times as fast as an oleic ester; in other instances, as may be gathered from the present work (cf . p. 128), the relative rate of hydrogenation in the first stage may be equal to or as much as five or six times that of the subsequent stage. Thus, the almost quantitative nature of the selective process cannot, in any case, be ascribed to the simple effects of relative concentration and affinity, according to the usual application of the law of mass action.

proceedings of the royal society of london. series a, containing papers of a mathematical and physical character

A study of catalytic actions at solid surfaces. Part XIII.—Some factors controlling selective hydrogenation, with particular reference to certain terpene derivatives