Rosenmund Reduction: Synthesis of Aldehydes
The Rosenmund Reduction is a selective hydrogenation reaction used to convert acid chlorides (acyl chlorides) into aldehydes. It utilizes hydrogen gas over a palladium catalyst supported on barium sulfate, which is deliberately poisoned to prevent over-reduction.
1. General Reaction
Acid chlorides are reduced by passing Hydrogen gas ($H_2$) through a boiling solution of the acid chloride in Xylene, in the presence of a poisoned catalyst.
Key Components:
- Substrate: Acid Chloride ($RCOCl$).
- Catalyst: Palladium ($Pd$) deposited on Barium Sulfate ($BaSO_4$).
- Catalyst Poison: Sulfur ($S$) or Quinoline.
- Product: Aldehyde ($RCHO$).
2. The Role of the Catalyst Poison
This is the most critical concept for exams.
Why Poison the Catalyst?
Palladium is a strong reducing agent. Without a poison, it would reduce the resulting aldehyde further into a Primary Alcohol ($RCH_2OH$).
The addition of Barium Sulfate ($BaSO_4$) and a poison like Sulfur or Quinoline lowers the activity of the Palladium catalyst. This stops the reduction selectively at the aldehyde stage.
3. Examples
A. Synthesis of Benzaldehyde
Reduction of Benzoyl Chloride.
$$ C_6H_5COCl + H_2 \xrightarrow{Pd/BaSO_4} C_6H_5CHO + HCl $$B. Synthesis of Acetaldehyde
Reduction of Acetyl Chloride.
$$ CH_3COCl + H_2 \xrightarrow{Pd/BaSO_4} CH_3CHO + HCl $$4. Limitations
Formaldehyde Cannot be Prepared
Formaldehyde ($HCHO$) requires Formyl Chloride ($HCOCl$) as the starting material. However, Formyl Chloride is unstable at room temperature, making this synthesis impossible via Rosenmund reduction.
5. Comparison with Lindlar's Catalyst
The catalyst composition ($Pd/BaSO_4$ or $Pd/CaCO_3$ + Poison) is very similar to Lindlar's Catalyst.
- Rosenmund: Reduces Acid Chlorides to Aldehydes.
- Lindlar: Reduces Alkynes to Cis-Alkenes.
Rosenmund Reduction Quiz
Test your concepts on selective reduction. 10 MCQs with explanations.
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