91Ó°ÊÓ

The catalytic hydrogenation of alkynes involves the reaction of molecular hydrogen with alkynes in the presence of a catalyst.

The product obtained depends on the type of catalyst used and the amount of hydrogen gas supplied.

A Lindlar catalyst is used to reduce alkyne to obtain a cis alkene, and the metal reduction method (Na in ammonia) is used to obtain a trans alkene.

Both the catalysts give a partial reduction of alkyne to a cis or trans alkene, whereas hydrogen palladium catalyst gives a complete reduction of an alkyne to an alkene.

(a) An excess of hydrogen in the presence of palladium catalyst results in an incomplete reduction of alkyne.

Complete reduction of alkyne

(b) The Lindlar catalyst results in the formation of a cis alkene using an equivalent of a catalyst.

Partial reduction giving cis alkene

(c) There is no change in the product obtained even when an excess of hydrogen is used, as it cannot completely reduce the triple bond.

Partial reduction giving cis alkene

(d) The metal reduction method results in the formation of a trans alkene.
Partial reduction of alkene giving trans alkene