The hydrogenation of alkynes to alkanes occurs through an intermediate that is the alkene. It is possible to stop the hydrogenation in the alkene using a poisoned palladium catalyst, called Lindlar's catalyst.
The poisons decrease the activity of the catalyst and allow to stop the hydrogenation in the alkene.
Hydrogenation with the Lindar catalyst is SYN, the hydrogens add to the triple bond from the same side, yielding cis alkenes.
[1] 2-Butyne
[2] cis-But-2-ene
[3] Undec-5-ine
[4] cis-Undec-5-ene
