Rosenmund reduction is a chemical reaction used to convert an acid chloride into an aldehyde. It involves the use of a palladium catalyst, which is sensitive to poisoning by certain impurities. One such impurity is barium sulphate (BaSO4), which is commonly used as an inert material in the reaction.

Explanation:

I. Rosenmund reduction and its purpose:
Rosenmund reduction is a well-known organic reaction named after Karl Wilhelm Rosenmund. It is primarily used to convert acid chlorides (RCOCl) into aldehydes (RCHO). The reaction is carried out in the presence of a palladium catalyst, typically Pd/C, and a hydrogen source like molecular hydrogen (H2). The aldehyde product is often used in the synthesis of various organic compounds.

II. The role of barium sulphate:
In Rosenmund reduction, barium sulphate (BaSO4) is added to the reaction mixture as an inert material. Its purpose is to prevent the formation of toxic and reactive intermediates called organobarium compounds. These compounds can be highly reactive and can potentially damage the catalyst or interfere with the desired reaction.

III. Catalyst poisoning:
The palladium catalyst used in Rosenmund reduction is highly sensitive to poisoning by various impurities and contaminants. These impurities can deactivate the catalyst or alter its reactivity, leading to reduced conversion and yield of the desired product. Barium sulphate acts as a poison for the catalyst, rendering it less effective in facilitating the desired conversion of acid chloride to aldehyde.

IV. Mechanism of catalyst poisoning:
The exact mechanism of how barium sulphate poisons the catalyst is not fully understood. However, it is believed that the barium ions (Ba2+) released from the BaSO4 can adsorb onto the surface of the palladium catalyst, blocking the active sites and inhibiting the catalytic activity. This adsorption of barium ions may also promote the formation of inactive palladium-barium compounds, further reducing the catalyst's effectiveness.

V. Importance of catalyst activity:
The effectiveness of the palladium catalyst is crucial for the success of the Rosenmund reduction. A highly active and selective catalyst ensures efficient conversion of acid chlorides to aldehydes without the formation of undesired byproducts or side reactions. Therefore, minimizing catalyst poisoning by impurities like barium sulphate is essential to achieve the desired reaction outcome.

In conclusion, barium sulphate is added to the Rosenmund reduction reaction as an inert material, but it also acts as a catalyst poison. The barium ions released from BaSO4 can adsorb onto the palladium catalyst, inhibiting its activity and reducing the efficiency of the reaction. Therefore, option D, "Poisons the catalyst," is the correct answer.