The Rosenmund reaction is a hydrogenation procedure in which molecular hydrogen reacts with acyl chlorides, selectively reducing the acyl chlorides to aldehydes. The reaction uses a palladium catalyst. A catalyst poison, such as barium sulfate, is used to control the reaction and prevent over-reduction.
Barium sulfate has a low surface area, which reduces the activity of the palladium catalyst. This controlled activity is crucial to stop the reduction process at the aldehyde stage, ensuring the desired product. The primary role of barium sulfate in the Rosenmund reaction is to prevent the newly formed aldehyde from being further reduced to primary alcohol.
If catalyst poison is not used, the aldehyde produced will continue to be reduced to a primary alcohol, which can then react with the remaining acyl chloride, leading to the formation of an ester instead of the desired aldehyde.