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Ronsenmund Reduction Mechanism - Explanation, Reaction with FAQs

Ronsenmund Reduction Mechanism - Explanation, Reaction with FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:53 PM IST

Rosenmund Reaction Class 12

Rosenmund Reduction reaction Mechanism deals with the selective reduction of acyl chlorides into aldehydes. Karl Wilhelm Rosenmund was the first person who reported the path in which acyl chlorides are selectively reduced into aldehydes in the year1918. thus, the reaction was named after the name of the scientist Rosenmund. The Rosenmund reaction is, basically, a hydrogenation process.

This Story also Contains
  1. Rosenmund Reaction Class 12
  2. Rosenmund Catalyst
  3. Catalytic Reduction Mechanism
  4. Explanation of Mechanism
  5. Acid chloride to Aldehyde

Here, a molecule of hydrogen reacts with the acyl chloride in the presence of active palladium on barium sulfate catalyst. The role that barium sulfate plays is to reduce the activity of the palladium due to its low surface area. As a result, over reduction is prevented. If there arises further need of reduction in palladium activity (in case of more reactive acyl chlorides), a catalytic poison should be added to deactivate the palladium catalyst completely.

Background wave

Thioquinanthrene and thiourea are two common poisons which can be used to reduce palladium activity in the Rosenmund’s reaction. The need for deactivation of the catalyst arises due to the subsequent aldehyde, formed from the reduction of the acyl chloride, further be reduced to form a primary alcohol by the catalyst. The primary alcohol thus formed, will react with the residual acyl chloride to produce an ester. Thus, yield of the desired product will be decreased subsequently.

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Rosenmund Catalyst

The Rosenmund catalyst (palladium on barium sulfate) is prepared by reducing palladium(II) chloride (PdCl2) solution in presence of a reducing agent, such as, formaldehyde in the presence of barium sulfate. A Rosenmund catalyst is, usually used to carry out selective reduction of acyl chlorides to their corresponding aldehydes. This is characteristically composed of palladium supported on BaSO4, and sometimes, poisoned by sodium acetate, N,N-dimethylaniline, thiourea, thiophene, dibenzothiophene, ethyldiisopropyl amine, or, most commonly, quinoline.

Catalytic Reduction Mechanism

Addition of hydrogen to a double bonded carbon moiety to produce an alkane is a reduction reaction. This can happen in the presence of a catalytic amount of hydrogen, thus called catalytic hydrogenation. Catalytic Hydrogenation reaction is a thermodynamically stable reaction since, more stable (lower energy) product is obtained. Thus, it is an exothermic reaction (heat is released) as the product is more stable than reactant.

The heat thus released is termed as the heat of hydrogenation. The reaction between molecular hydrogen (H2) and an alkene (saturated hydrocarbon) requires an active metal catalyst. A catalyst enhances the rate of the reaction by lowering the activation energy of the transition state. Catalysts which are commonly used in catalytic reduction techniques are: platinum, palladium, and nickel.

The metal catalyst acts as a surface for adsorption of hydrogen on which the reaction takes place. In the presence of these catalyst, the sigma bond of H2 breaks, and the two hydrogen atoms instead bind to the metal. The π bond of the alkene also interacts with the active metal surface which insists the weakening of the π bond,

Now, both the reactants are bound to the metal surface. the hydrogen atoms can be easily added, one at a time, to the previously unsaturated (double bonded) carbons.

catalytic hydrogenation mechanism

energy profile diagram of catalytic hydrogenation reaction

Catalytic hydrogenation of aldehydes and ketones

In the catalytic hydrogenation of Aldehydes, corresponding primary alcohols are produced.

here, Raney Ni is used as a catalyst.

primary alcohols

In the catalytic hydrogenation of ketones, a molecule of hydrogen is added across the carbon-oxygen double bond which leads to form a secondary alcohol as a final product.

here, Raney Ni is used as catalyst

secondary alcohol

Rosenmund reduction mechanism (Rosenmund reaction mechanism)

Rosenmund reduction mechanism

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Explanation of Mechanism

step 1: Hydrogenated Palladium (active metal) in presence of barium sulphate converts acid chlorides to acetaldehyde by selective reduction.

Initially, acetyl chloride binds with Pd surface and Pd binds in between C and Cl. the Pd(0) becomes Pd(II) after being bonded.

step 2: Cl binds with molecular hydrogen and gets eliminated by forming HCL. and another H atom binds with the carbonyl C which generates aldehyde as a major product.

step 3: Pd(0) is regenerated and further acts as a catalyst

step 4: in case of highly reactive acid chlorides, the over reduction by catalyst is prevented by using catalyst poisons, such as, thiourea, xylene etc.

step 5: barium sulphate reduces the activity of palladium metal by decreasing the surface are of adsorption.

Examples:

Barium Sulphate

Hydrogenation of benzoyl chloride in presence of pd on BaSo4

Some catalytic poisons are added to the Rosenmund catalyst to stop further reduction once the desired product is obtained. An example for the method to completely deactivate the palladium over barium sulfate catalyst given below-


Hydrogenation of benzoyl chloride

Acid chloride to Aldehyde

Acid chloride to aldehyde

In the presence of active palladium metal in barium sulphate, acetyl chloride is reduced selectively to form acetaldehyde.

Also check-

NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. What is rosenmund reaction?

The conversion of acid chlorides into aldehydes in low temperature and low-pressure hydrogenation over poisoned Pd/BaSo4 is the classical Rosenmund reduction reaction.

2. What is the role of BaSO4 in the Rosenmund reduction reaction?

The Rosenmund reduction reaction is catalysed by palladium on surface of barium sulphate. The barium sulphate decreases the activity of the palladium due to its low surface area, hence, prevents unwanted reduction.

3. Why poisoned catalyst is used in Rosenmund reaction?

Use of palladium, as a catalyst, persuades the reduction process. Whereas, use of barium sulphate decreases the activity of palladium. since, barium sulphate has low surface area, it prevents the over reduction of reactant and we get our desired product. So, To prevent further hydrogenation, palladium is mixed with poison.

4. Which catalyst is used in Rosenmund reaction?

The most abundant catalyst which is used in Rosenmund reaction is palladium on barium sulfate, which is sometimes called the Rosenmund catalyst. Barium sulfate is used as catalyst poison.

5. Does lindler’s catalyst reduces alkanes?

Lindlar's catalyst is a palladium catalyst which is poisoned with pinch of lead and quinoline. It helps to reduce its activity such that it can only reduce alkynes, not alkenes.

6. What is the main purpose of the Rosenmund reduction?
The main purpose of the Rosenmund reduction is to selectively convert acyl chlorides to aldehydes without further reduction to primary alcohols. This reaction is valuable in organic synthesis when aldehydes are the desired product.
7. What is the role of hydrogen gas in the Rosenmund reduction?
Hydrogen gas serves as the reducing agent in the Rosenmund reduction. It provides the hydrogen atoms needed to replace the chlorine in the acyl chloride, converting it to an aldehyde.
8. Can the Rosenmund reduction be used to produce ketones?
No, the Rosenmund reduction cannot be used to produce ketones. It is specific for converting acyl chlorides derived from carboxylic acids to aldehydes. Ketones cannot be produced because they lack the necessary hydrogen atom on the carbon adjacent to the carbonyl group.
9. What is the general reaction equation for the Rosenmund reduction?
The general reaction equation for the Rosenmund reduction is:
10. Why is the Rosenmund reduction considered a selective reduction?
The Rosenmund reduction is considered selective because it stops at the aldehyde stage and doesn't continue to reduce the product to a primary alcohol. This selectivity is crucial in organic synthesis when aldehydes are the desired end product.
11. Can the Rosenmund reduction be used on an industrial scale?
While the Rosenmund reduction is valuable in laboratory settings, its use on an industrial scale is limited. This is due to:
12. How can over-reduction be prevented in the Rosenmund reduction?
Over-reduction can be prevented in the Rosenmund reduction by:
13. What is the significance of using an inert solvent in the Rosenmund reduction?
An inert solvent like toluene or xylene is used in the Rosenmund reduction to:
14. How does the structure of the acyl chloride affect the Rosenmund reduction?
The structure of the acyl chloride can affect the Rosenmund reduction in several ways:
15. What are some alternatives to the Rosenmund reduction for producing aldehydes?
Some alternatives to the Rosenmund reduction for producing aldehydes include:
16. What is the mechanism of the Rosenmund reduction?
The mechanism of the Rosenmund reduction involves:
17. What are the typical reaction conditions for the Rosenmund reduction?
Typical reaction conditions for the Rosenmund reduction include:
18. What are some common side reactions in the Rosenmund reduction?
Common side reactions in the Rosenmund reduction include:
19. How does the Rosenmund reduction differ from other reduction methods like LiAlH4 or NaBH4?
The Rosenmund reduction differs from LiAlH4 or NaBH4 reductions in its selectivity. While LiAlH4 and NaBH4 would reduce an acyl chloride all the way to a primary alcohol, the Rosenmund reduction stops at the aldehyde stage. This makes it useful when aldehydes are the desired product.
20. Can the Rosenmund reduction be applied to all acyl chlorides?
While the Rosenmund reduction can be applied to many acyl chlorides, it works best with aliphatic and aromatic acyl chlorides. Some acyl chlorides may be too reactive or unstable for this method, and others might require modified conditions.
21. What type of catalyst is used in the Rosenmund reduction?
The Rosenmund reduction uses a heterogeneous palladium catalyst, typically palladium on barium sulfate (Pd/BaSO4). This catalyst is crucial for the selective reduction of the acyl chloride to an aldehyde.
22. Why is barium sulfate used as a support for the palladium catalyst?
Barium sulfate is used as a support for the palladium catalyst because it's inert and doesn't promote further reduction of the aldehyde to an alcohol. This helps maintain the selectivity of the reaction towards aldehyde formation.
23. Why is it important to use a poisoned catalyst in the Rosenmund reduction?
A poisoned catalyst (like Pd/BaSO4) is important in the Rosenmund reduction because it helps prevent over-reduction of the aldehyde to an alcohol. The poison (usually sulfur compounds) partially deactivates the catalyst, making it less active for further reduction steps.
24. What is the importance of the Rosenmund reduction in organic synthesis?
The Rosenmund reduction is important in organic synthesis because:
25. How does the Rosenmund reduction compare to the Lindlar reduction?
While both use palladium catalysts, they differ significantly:
26. What is the Rosenmund reduction?
The Rosenmund reduction is a chemical reaction that converts acyl chlorides (acid chlorides) to aldehydes using hydrogen gas and a palladium catalyst. It's an important method for selectively reducing acyl chlorides without further reducing them to alcohols.
27. Who discovered the Rosenmund reduction?
The Rosenmund reduction was discovered by German chemist Karl Wilhelm Rosenmund in 1918. He developed this method as a way to selectively produce aldehydes from acyl chlorides.
28. How does the Rosenmund reduction fit into the broader context of carbonyl chemistry?
The Rosenmund reduction is an important tool in carbonyl chemistry because:
29. How does the Rosenmund reduction compare to the McFadyen-Stevens reaction?
Both can produce aldehydes, but they differ significantly:
30. Can the Rosenmund reduction be used to reduce carboxylic acids directly?
No, the Rosenmund reduction cannot be used to reduce carboxylic acids directly. It is specific for acyl chlorides. To reduce a carboxylic acid using this method, it must first be converted to an acyl chloride using reagents like thionyl chloride (SOCl2) or oxalyl chloride ((COCl)2).
31. How does the Rosenmund reduction compare to the Soai autocatalytic reaction?
These reactions are quite different:
32. Can the Rosenmund reduction be used in total synthesis of natural products?
Yes, the Rosenmund reduction can be valuable in total synthesis:
33. How does temperature affect the Rosenmund reduction?
Temperature plays a crucial role in the Rosenmund reduction:
34. What is the role of pressure in the Rosenmund reduction?
Pressure affects the Rosenmund reduction by:
35. How does the electronic nature of substituents affect the Rosenmund reduction?
The electronic nature of substituents can significantly impact the Rosenmund reduction:
36. What precautions should be taken when performing a Rosenmund reduction?
Important precautions for the Rosenmund reduction include:
37. Can the Rosenmund reduction be used in asymmetric synthesis?
The standard Rosenmund reduction is not typically used in asymmetric synthesis as it doesn't create new stereogenic centers. However, modified versions using chiral ligands or supports for the palladium catalyst have been developed for some asymmetric transformations, though these are not widely used.
38. What are some common workup procedures after a Rosenmund reduction?
Common workup procedures after a Rosenmund reduction include:
39. How does the Rosenmund reduction compare to the Fukuyama reduction?
While both can produce aldehydes, they differ significantly:
40. Can the Rosenmund reduction be monitored by TLC (Thin Layer Chromatography)?
Yes, the Rosenmund reduction can be monitored by TLC:
41. What is the effect of solvent polarity on the Rosenmund reduction?
Solvent polarity can affect the Rosenmund reduction:
42. Can the Rosenmund reduction be applied to α,β-unsaturated acyl chlorides?
The Rosenmund reduction can be challenging with α,β-unsaturated acyl chlorides:
43. What is the role of quinoline-S in some Rosenmund reductions?
Quinoline-S can be used as a catalyst poison in Rosenmund reductions:
44. Can the Rosenmund reduction be performed under flow conditions?
Yes, the Rosenmund reduction can be adapted to flow chemistry:
45. What is the effect of using deuterium gas instead of hydrogen in the Rosenmund reduction?
Using deuterium (D2) instead of hydrogen (H2) in the Rosenmund reduction:
46. How does the presence of other functional groups affect the Rosenmund reduction?
Other functional groups can significantly impact the Rosenmund reduction:
47. What are some common methods for preparing the acyl chlorides used in Rosenmund reduction?
Common methods for preparing acyl chlorides include:

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