The Origin of Selectivity in Alkyne Hydrogenation

Work described in the literature can be divided into two broad categories  

It will also become clear that the behaviour of alkynes closely resembles that of the 1,2- and 1,3-alkadienes already discussed. Their strong chemisorption was due to the simultaneous interaction of both double bonds in the latter case, and probably to the large release of strain that accompanied the opening of one of the double-bonds (or at least the disengagement of one set of tt orbitals) in the latter case. The heat of hydrogenation reflects the magnitude of the release of strain and hence the strength of the interaction with surface atoms for ethyne to ethene it is 172 kJ moU compared to 137 kJ moU for ethene to ethane. These reactions are thus quite exothermic, and in industrial use care must be taken to avoid temperature excursions, as these would lead to loss of selectivity. 

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