Rosenmunds reduction is the catalytic hydrogenation of acyl chlorides to produce aldehydes. Without a catalyst hydrogenation proceeds too slowly. That is why a hydrogenation catalyst is used. Palladium (Pd) is an efficient hydrogenation catalyst strongly speeding the reaction. So, Pd is used in Rosenmunds reduction as a catalyst to speed the reaction. Such expensive metals as Pd are used in supported state, i.e. as a thin layer of the metal on cheap non-metal material – support. Barium sulfate is not a catalyst. It is a support. Most often materials with great specific surface area are used as a support (the greater the surface area the greater the reaction rate), but BaSO4 is a low surface area material. In case of Rosenmunds reduction acyl chloride must be reduced to aldehyde (target product). Subsequent reduction of the aldehyde formed (overreduction) results in primary alcohol (by-product), which can react with the remaining acyl chloride to form an ester (by-product), and it is strongly undesirable. To prevent overreduction a support of low surface area should be used (with a low surface area, the contact time of the substrate with the catalyst is shorter). So, BaSO4 is used in Rosenmunds reduction as a low surface area support of Pd catalyst. Sulfur is used for additional regulation of the catalyst activity. Sulfur is a poison for Pd catalyst, so its addition reduces catalyst activity that prevents overreduction. So, S is used in as a reducer of the Pd catalyst activity.
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