Mechanisms of hydrogenation. Part V. The hydrogenation of olefinic and acetylenic substances. The influence of stereochemistry and substitution on chemisorption and on chromatography
Chemisorption in a series of cyclic olefins is related to the molecular geometry of the parent olefin, whilst the rates of hydrogenation follow the heats of hydrogenation. From this and other evidence it is inferred that chemisorption of an olefin compares closely with olefin–transition-metal-ion complex formation in which the olefinic ligand is known to retain its molecular geometry.
For a series of alkyl-substituted pentenes the rates of hydrogenation parallel the value of the equilibrium constant for complexing of the olefin with silver nitrate. In this, and similar series, alkyl substitution is found to exert an essentially physical, steric influence on hydrogenation. For substituted olefins, the rate of hydrogenation is not seriously influenced by the polarity of the substituent, X, except where X = CHO or COCH3, when the rate is increased, due probably to conjugative addition.
For acetylenes, which are very strongly adsorbed, increasing the size of the substituents assists reaction by reducing competitive exclusion of hydrogen.
In a series of butene derivatives: R–CHCH–R, the cis-isomer is generally hydrogenated more rapidly than the trans-, but the cis-series is more sensitive to the size of the group R. With large groups the trans-isomer reacts the more rapidly. Within some of the groups of substances examined a close parallel is found between the influence of structural change on chromatographic distribution and rate of hydrogenation.