Question

Write the reaction involved in: Rosenmund's reduction

Hint:

Rosenmund catalyst ($Pd/BaSO_4$) is specifically "poisoned" to stop reduction at the aldehyde stage.

Answer 1

Step 1: Concept
Preparation of aldehydes from acid chlorides.

Step 2: Meaning
It is a catalytic hydrogenation reaction.

Step 3: Analysis
An acyl chloride (acid chloride) is hydrogenated in the presence of palladium deposited on barium sulfate ($Pd/BaSO_4$). The barium sulfate acts as a catalyst poison, reducing the activity of palladium and preventing further reduction of the aldehyde to a primary alcohol.

• This selective reduction is known as the \[ \boxed{\text{Rosenmund reduction}}. \]

• In this reaction, an acyl chloride is treated with hydrogen gas over a palladium catalyst supported on barium sulfate: \[ RCOCl+H_2 \xrightarrow{Pd/BaSO_4} RCHO+HCl. \]

• Palladium is an efficient hydrogenation catalyst.

• However, if ordinary palladium is used, the aldehyde formed would undergo further reduction to a primary alcohol.

• To prevent this, palladium is deposited on barium sulfate.

• Barium sulfate acts as a catalyst poison, decreasing the catalytic activity of palladium.

• Additional catalyst poisons such as sulfur or quinoline are often employed to further suppress over-reduction.

• As a result, the reduction stops at the aldehyde stage.

• Therefore, Rosenmund reduction provides a convenient method for preparing aldehydes selectively from acid chlorides.

Step 4: Conclusion
The controlled reduction successfully stops at the aldehyde stage.

Final Answer:
$R-COCl + H_2 \xrightarrow{Pd / BaSO_4} R-CHO + HCl$
(Example: Benzoyl chloride to Benzaldehyde)