Acid chlorides can be selectively reduced to their corresponding aldehydes using hydrogen gas in the presence of a partially deactivated palladium catalyst, known as the Rosenmund catalyst. This catalyst is prepared by depositing palladium (Pd) on barium sulfate (BaSO₄) and then poisoning it with additives such as 2,6-dimethylpyridine, quinoline–sulfur compounds, or others. The low surface area of BaSO₄ and the addition of catalyst poisons reduce the activity of palladium, thereby preventing over-reduction of the aldehyde to alcohol.

This reaction was first reported in 1918 by Karl Wilhelm Rosenmund, and is named in his honor.

• Reagents: Rosenmund catalyst, solvent

• Reactants: Acid chloride, hydrogen gas

• Product: Aldehyde

• Reaction Type: Reduction reaction

Experimental Tips:

• The Rosenmund reduction is suitable for synthesizing a wide range of aldehydes. However, formaldehyde cannot be produced via this method because formyl chloride is unstable at room temperature;

• The reaction must be carried out under anhydrous conditions, as acid chlorides hydrolyze to carboxylic acids in the presence of water. Additionally, carboxylic acids can react with acid chlorides to form anhydrides;

• If the palladium catalyst is not deactivated, it may further reduce the acid chloride to an alcohol. In some cases, even reduction to the corresponding hydrocarbon may occur;

• Functional groups such as nitro, halides, and esters generally remain intact and are not reduced under Rosenmund conditions.

Reaction Mechanism

Original Literature:

• K. W. Rosenmund, Chemische Berichte, 1918, 51, 585–593.

Notable References:

• Discovery and Development of an Efficient Process to Atovaquone, Org. Process Res. Dev., 2012, 16(10), 1607–1617.

• Synthesis of m-Phenoxybenzaldehyde Starting from Chlorobenzene and m-Cresol: Some Aspects of Process Development, Org. Process Res. Dev., 1999, 3(6), 416–424.