The Role of Catalyst Poison in the Rosenmund Reaction

What is the Rosenmund reduction Mechanism?

The Rosenmund reduction mechanism is a term used to explain how acyl chlorides get selectively reduced into aldehydes. Karl Wilhelm Rosenmund first reported the reaction in 1918. Karl Wilhelm Rosenmund was a famous German chemist credited with the Rosenmund-von Braun reaction.  

The Rosenmund reaction is a hydrogenation procedure where molecular hydrogen reacts with an acyl chloride in the presence of a catalyst, which is usually palladium on barium sulphate. Palladium on barium sulphate is therefore also referred to as the Rosenmund catalyst. 

Interestingly, the Rosenmund catalyst, palladium on barium sulphate, is also used for the decarboxylation of aliphatic esters to produce biodiesel. Of course, it is also used in hydrogenation reactions.

What is the Role of Catalyst Poison in the Rosenmund Reaction?

As mentioned above, the Rosenmund reaction is when acid chlorides get converted into aldehyde through catalytic reduction with palladium and barium sulphate in either quinoline or sulphur. The addition of barium sulphate helps reduce the activity of the palladium because of barium sulphate’s low surface area. 

This prevents the reaction from over-reduction. Suppose there is a need for furher reduction in the activity of palladium. As is the case with more reactive acyl chlorides, poison is also added to completely deactivate the palladium catalyst. 

Some common catalyst poisons used to restrict palladium’s activity in the Rosenmund reaction are thiourea and thioquinanthrene. This need for further deactivation of palladium arises because the aldehyde formed from the reduction of the acyl chloride also gets reduced to primary alcohol by the reaction. 

This is an unwanted reaction in the Rosenmund technique because the primary alcohol reacts with the remaining acyl chloride to form an ester. The Rosenmund catalyst, which is palladium on barium sulphate, is prepared by reducing palladium (II) chloride solution with the inclusion of reducing agent formaldehyde in the presence of barium sulphate.

Deactivation is necessary for the reaction because the system needs to reduce the acyl chloride but not the produced aldehyde. So if further reduction continues to occur, it will create primary alcohol. This primary alcohol would react with the remaining acyl chloride and produce an ester.

This is why catalyst poison is required to achieve the desired product, which is an aldehyde. If the poisoned palladium is not used, the reaction will produce an ester instead of aldehyde.

How does the Rosenmund Reaction Work?

Let us look at the mechanism of the Rosenmund reaction: