When bezoyl chloride si reduced by `H_(2)` in the presence of Pd catalyst, the product is benzyl alcohol. If `BaSO_(4)` is mixed with the catalyst before passing the gas, the product is benzaldehyde. Assign reason.

Explanation

This is an example of Rosenmund's reduction. `H_(2)` causes the hydrogenation of benzoyl chloride initially to benzaldehyde and then to benzyle alcohol in the presence of Pd catalyst.

But `BaSo_(4)` if added to the catalyst causes its posoning and reduces its efficiency. The dydrogenation (controlled) takes place only to the aldehydic stage and not to the alcohlic stage.