Illustrated Glossary of Organic Chemistry

Poisoned catalyst: A hydrogenation catalyst whose activity has been reduced by the presence of substance(s) other than the catalyst itself.

	H₂Pd Fast		H₂Pd Fast	CH₃CH₂CH₂CH₂CH₂CH₃
3-Hexyne An internal alkyne Starting material		Cis-2-hexene An internal cis alkene Cannot be isolated		Hexane An alkane Reaction product

When subjected to catalytic hydrogenation using palladium as the catalyst, 3-hexyne (an alkyne) is reduced to hexane (an alkane). Under these conditions the intermediate alkene (cis-3-hexene) is reduced more quickly than the starting alkyne, so this alkene cannot be isolated in any useful amount, even if only one equivalent of hydrogen is used.

	H₂ Lindlar catalyst Fast		H₂ Lindlar catalyst Slow	CH₃CH₂CH₂CH₂CH₂CH₃
3-Hexyne An internal alkyne Starting material		Cis-2-hexene An internal cis alkene Reaction product		Hexane An alkane Not formed

When subjected to catalytic hydrogenation using the Lindlar catalyst, reduction of 3-hexyne stops at cis-3-hexene. This alkene is further reduced only very slowly (if at all) to hexane, and so this alkene can be easily isolated from the reaction.