Hydrogenation of an alkyne with a poisoned catalyst yields which product?

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Multiple Choice

Hydrogenation of an alkyne with a poisoned catalyst yields which product?

When an alkyne is hydrogenated with a poisoned catalyst, the reaction stops at the alkene stage rather than going all the way to an alkane. The poisoning reduces the catalyst’s activity so only one equivalent of hydrogen is added, and the hydrogen atoms add to the same side of the carbon–carbon bond (syn addition). This syn delivery fixes the geometry of the new double bond as cis (Z), giving a cis-alkene. A poisoned catalyst like Lindlar’s specifically promotes this semihydrogenation to the cis alkene, whereas using an active, unpoisoned catalyst would typically push the reaction further to the alkane.

When an alkyne is hydrogenated with a poisoned catalyst, the reaction stops at the alkene stage rather than going all the way to an alkane. The poisoning reduces the catalyst’s activity so only one equivalent of hydrogen is added, and the hydrogen atoms add to the same side of the carbon–carbon bond (syn addition). This syn delivery fixes the geometry of the new double bond as cis (Z), giving a cis-alkene. A poisoned catalyst like Lindlar’s specifically promotes this semihydrogenation to the cis alkene, whereas using an active, unpoisoned catalyst would typically push the reaction further to the alkane.

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