Concept:The reaction illustrates the Rosenmund reduction, where an acid chloride is selectively reduced to an aldehyde using a poisoned palladium catalyst.Explanation:Benzoic acid reacts with thionyl chloride (SOCl2​) to form benzoyl chloride, which is labelled as A.The by-products are sulphur dioxide and hydrogen chloride gases.C6​H5​COOH+SOCl2​→C6​H5​COCl+SO2​+HClIn the next step, benzoyl chloride (A) is subjected to catalytic hydrogenation in the presence of Pd/BaSO4​.This special reduction is called the Rosenmund reduction.The catalyst is poisoned so that the reduction stops at the aldehyde stage and does not proceed to benzyl alcohol.C6​H5​COCl+H2​Pd/BaSO4​​C6​H5​CHO+HClHence, the product formed (B) is benzaldehyde, an aromatic aldehyde.
