Lindlars catalyst and hydrogen gas (H2) will react with alkynes to form alkenes:

The H2 and alkyne are adsorbed onto the surface of Lindlars catalyst. This reaction adds a hydrogen to each carbon on the alkyne bond, reducing the bond to an alkene bond. The hydrogens are added in a cis fashion, not trans:

Cis Addition

NOT trans Addition

The reaction mechanism is depicted below:

In the first step, the H2 and alkyne adsorb to Lindlars catalyst, which catalyzes the reaction.

In the second step, the alkyne bond is able to react with H2, resulting in a cis addition of hydrogen to each carbon atom and the reduction of the triple bond to a double bond.

The reagent used for this reaction is H2 with Lindlars catalyst, represented by Pd in the mechanism figure.