Rosenmund Reduction Reaction
In Rosenmund Reaction, acyl chloride is hydrogenated to get reduced into aldehyde and palladium-barium sulphate is used as catalyst. It is a reduction reaction involves addition of hydrogen.
Rosenmund Reaction can be written as follows –
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Acyl Chloride Alkyl Aldehyde
R can be an alkyl or aryl group in the above reaction.
Examples of Rosenmund Reduction Reaction –
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Rosenmund Catalyst
As we have discussed above, Palladium on barium sulphate is called rosenmund catalyst. Use of palladium induces the reduction process while use of barium sulphate reduces the activity of palladium as barium sulphate has low surface area, thus prevents the over reduction. Over reduction should be stopped for the desired product which is aldehyde in this reaction. If over reduction takes place, then aldehyde converts into alcohol which would the react with remaining acyl chloride and form ester. To prevent further hydrogenation, catalyst is mixed with poison.
Rosenmund catalyst is prepared by reduction of palladium (II) chloride solution in the presence of barium sulphate.
Rosenmund Reduction Reaction Mechanism
1. Preparation of Acyl Chloride From an Acid –
(Image to be added soon) SOCl2 or PCl3or PCl5→ (Image to be added soon)
Carboxylic Acid Acyl Chloride
2. Preparation of Aldehyde –
We use a poisoned rosenmund catalyst to get the desired product which is aldehyde. So, we use palladium with barium sulphate.
(Image to be added soon) Pd-BaSO4 (partial hydrogenation)→ (Image to be added soon) + HCl
If we don’t use poisoned palladium then reaction will take place as follows –
(Image to be added soon) Pd(non poisoned catalyst)→ (Image to be added soon) Pd (Image to be added soon) (Image to be added soon)
In this condition we get ester in place of aldehyde.
Applications of Rosenmund Reduction Reaction
1.Rosenmund reduction reaction is used for the production of aldehydes.
2. It is used for the production of saturated fatty aldehydes.
3. It is used for the production of alkyl or aryl aldehydes.
Limitation of Rosenmund Reaction
We can prepare many aldehydes by Rosenmund Reduction Reactions but formaldehydes cannot be prepared. As formyl chloride is unstable at room temperature.
Rosenmund reduction reaction is one of the important name reactions of CBSE Class 12 Chemistry for your final board examinations. You need to practice twice all the name reactions to score high marks in the examination. We at Vedantu have provided “Important Chemical Reactions of Class 12 Chemistry” page also for you so that you can score maximum. You can also find similar articles on other name reactions such as diazotization reaction, hofmann elimination reaction, Friedel crafts reaction etc. at Vedantu.
FAQs on Rosenmund Reduction Mechanism
1. What is the Rosenmund Reduction reaction in organic chemistry?
The Rosenmund Reduction is a chemical reaction where an acyl chloride is selectively reduced into an aldehyde. This is achieved through hydrogenation using gaseous hydrogen in the presence of a specific catalyst, typically palladium supported on barium sulphate. The general equation is: R-COCl + H₂ → R-CHO + HCl.
2. What is the specific catalyst used in the Rosenmund Reduction and what is its function?
The catalyst used is known as the Rosenmund catalyst, which is palladium supported on barium sulphate (Pd/BaSO₄). In this setup:
- Palladium (Pd) acts as the primary hydrogenation catalyst that facilitates the addition of hydrogen.
- Barium Sulphate (BaSO₄) serves as a support with a low surface area, which helps control the activity of the palladium.
3. Why is the catalyst 'poisoned' in the Rosenmund Reduction?
The catalyst is intentionally 'poisoned' with substances like sulphur or quinoline to partially deactivate it. This is a critical step to prevent the over-reduction of the newly formed aldehyde into a primary alcohol. An overly active catalyst would continue the hydrogenation process, leading to an unwanted alcohol product instead of stopping at the desired aldehyde stage.
4. Can you provide a common example of the Rosenmund Reduction?
A classic example of the Rosenmund Reduction is the synthesis of benzaldehyde from benzoyl chloride. In this reaction, benzoyl chloride is treated with hydrogen gas over a poisoned palladium-barium sulphate catalyst to yield benzaldehyde and hydrochloric acid.
Reaction: C₆H₅COCl + H₂ (gas) --(Pd/BaSO₄, Sulphur)--> C₆H₅CHO + HCl
5. What is the main difference between the Rosenmund Reduction and the Stephen Reaction for preparing aldehydes?
The key difference lies in the starting material and reagents used:
- Starting Material: The Rosenmund Reduction starts with an acyl chloride (R-COCl), while the Stephen Reaction begins with a nitrile (R-CN).
- Reagents: Rosenmund Reduction uses H₂ with a poisoned Pd/BaSO₄ catalyst. The Stephen Reaction uses stannous chloride (SnCl₂) and hydrochloric acid (HCl) to form an imine intermediate, which is then hydrolysed to produce the aldehyde.
6. Why is it not possible to prepare formaldehyde using the Rosenmund Reduction method?
Formaldehyde cannot be prepared using this method due to the instability of the required starting material. The corresponding acyl chloride, formyl chloride (H-COCl), is extremely unstable at room temperature and readily decomposes. Its inability to exist under normal reaction conditions makes the synthesis of formaldehyde via this route impractical.
7. What are the primary applications of the Rosenmund Reduction?
The Rosenmund Reduction is an important method used in organic synthesis, primarily for:
- The preparation of a wide variety of aliphatic and aromatic aldehydes.
- The synthesis of saturated fatty aldehydes from their corresponding acid chlorides.
