Why is the catalyst poisoned in Rosenmund Reduction?

In the Rosenmund Reduction, the catalyst used is palladium on barium sulfate (Pd/BaSO₄), and it is "poisoned" with sulfur (S) or quinoline. The purpose of poisoning the catalyst is to decrease its activity. Without poisoning, the palladium catalyst is highly active and would reduce the aldehyde further to an alcohol.

By poisoning the catalyst, its activity is moderated, ensuring that the reduction stops at the aldehyde stage and does not proceed further to form alcohols. This selective reduction is crucial for obtaining aldehydes from acyl chlorides without over-reducing them.

This concept is explained in the NCERT syllabus under the section of reduction reactions in organic chemistry.