When my textbook talks about hydrogenation using Pt, Pd or Ni heterogeneous catalysts, it never mentions if it is anti or syn addition. It simply jumps on to say that for alkynes, NiB2 (P-2) catalysts and Lindlar's catalysts are syn hydrogenation. Can someone tell me if using Pt gives syn or anti for alkynes?
Using any transition metal catalyst (Pt, Pd, Ni) for hydrogenation of alkynes results in syn hydrogenation to not alkenes but alkanes. Using Lindlar's catalyst results in syn hydrogenation of an alkyne to an alkene.
In the case of sodium metal (not a transition metal) in ammonia, hydrogenation of alkynes results in a trans alkene.
All catalytic hydrogenations involving solid metal catalysts transfer hydrogen in a syn fashion to the alkyne to produce the corresponding cis-olefin. The following figure provides a mechanism that illustrates why the
addition must be syn.
But, with alkynes there are problems. First, reduction of the resultant alkene is faster than reduction of the starting alkyne, so product mixtures of alkenes and alkanes will result unless poisoned catalysts (Lindlar catalyst) are used or the reaction is stopped early. A second problem often observed is that the resultant alkene usually remains adsorbed on the catalyst surface. Consequently cis-trans isomerization of the alkene can take place leading to further contamination of the initial cis-product.