Rosenmund Reduction - Mechanism, Uses, Limitations, Example and Reaction

The transformation of Acyl chlorides into aldehydes in the presence of lindlar’s catalyst is known as the Rosenmund reduction. Karl Wilhelm Rosenmund, who performed the reaction for the first time in 1918, is honored by the reaction's name.

German chemist Karl Wilhelm Rosenmund was born. Born in Berlin, he passed away in Kiel.

In 1906, Rosenmund graduated from the University of Berlin with a Ph.D. in chemistry for his collaboration with Otto Paul Hermann Diels. The Rosenmund reduction, which involves reducing acid chlorides to aldehydes using a palladium-on-carbon catalyst, was developed by him. He is also credited with creating the Rosenmund-von Braun reaction, which transforms an aryl bromide into an aryl nitrile.

Table of content

• What is Rosenmund reduction?
• Rosenmund reduction mechanism
• Uses of Rosenmund reduction
• Limitations of Rosenmund reduction
• Practice Problems
• Frequently asked Questions-FAQs

What is Rosenmund reduction? 

Acid chlorides(R-CO-Cl) are converted into aldehydes(R-CHO) by undergoing hydrogenolysis. 

In rosenmund reduction hydrogen gas(H2) is passed through palladium(Pd) on barium sulfate(BaSO4). Tertiary amine(R3N) is required to control the activity of the catalyst as well as to neutralize the HCl produced during the reaction (and prevent over reduction). Barium sulfate has less surface area; it restricts the activity of palladium. Thus, it reduces the ability of palladium to react in order to prevent over- reduction of acid chloride. Therefore BaSO4 or CaCO3 act as a support by facilitating easy escape of the product to prevent over-reduction. With the addition of poison, the activity is further diminished for the more reactive acyl chlorides. To avoid excessive hydrogenation, a toxin like thioquinanthrene or thiourea is utilised. If the deactivation does not occur, it could result in additional aldehyde reduction and the production of primary alcohol. If this primary alcohol is created, it will subsequently interact with the residual acyl chloride to generate ester.

Mechanism of Rosenmund reduction 

Step 1: Insertion
Pd gets inserted between C-Cl bond. 

Step 2: Oxidative addition
H2 adds to Pd.

Step 3: Reductive elimination
Pd and HCl gets eliminated to give aldehyde as the final product. 

Uses of Rosenmund reduction

1. Rosenmund reduction is used in the synthesis of saturated fatty aldehydes.
2. It is also useful in manufacturing alkyl halides and aryl halides.

Limitations of Rosenmund reduction

This reduction method cannot be used in the preparation of formaldehyde. This is because formyl chloride formed upon reduction, is highly unstable at room temperature.

Practice Problems

Q1. What is the major product of the following reaction?

A) 
B)
C)
D) None of the above

Answer: A
Solution: 

Q2. Which of the following acts as a poison in the Rosenmund reduction?

A) Thioquinanthrene
B) Thiourea 
C) Both A and B
D) None of the above

Answer: C

Solution: In rosenmund reduction hydrogen gas(H2) is passed through palladium(Pd) on barium sulfate(BaSO4) along with tertiary amine(R3N). With the addition of poison, the activity is further diminished for the more reactive acyl chlorides. To avoid excessive hydrogenation, a toxin like thioquinanthrene or thiourea is utilised.

Q3. Benzoyl chloride gets reduced to benzaldehyde in presence of —--.

A) H2, Pd
B). BaSO4
C) R3N
D) All of the above

Answer: D

Solution: Benzoyl chloride gets reduced when hydrogen gas(H2) is passed through a catalyst palladium(Pd) on barium sulphate(BaSO4) along with tertiary amine(R3N) to give benzaldehyde. 

Q4. What is the role of Pd in rosenmund reduction?

A) Reducing agent
B) Catalyst
C) Poison
D) Supporter

Answer: B

Solution: Palladium in rosenmund reduction acts as a catalyst. 

Frequently asked Questions-FAQs

Question 1. What is hydrogenation?
Answer. Hydrogenation is a process of reduction of compounds. In hydrogenation, a compound is reacted with H2, which is also called molecular hydrogen. This reaction usually occurs in the presence of a catalyst, most likely nickel, palladium or platinum. Hydrogenation reduces or sa turates organic compounds. It generally functions by adding a pair of hydrogen atoms to a functional group. The group with which it most commonly attaches itself is an alkene. Catalysts are very important for this reaction because, without a catalyst, a hydrogenation reaction only occurs at very high temperatures, as it reduces the double and triple bonds in hydrocarbons.

Under the impact of the catalysts used in the process, hydrogenation can also result in alkenes producing their isomers. In this situation, the alkenes' functional position changes from cis to trans.

Question 2. What is hydrogenolysis?
Answer. Hydrogenation of polar bonds is called hydrogenolysis. Breaking of bonds between carbon and carbon or carbon and any other heteroatom is known as hydrogenolysis. In this group, oxygen, nitrogen, and halogen are the most frequently occurring heteroatoms.

Question 3. How trans-fat, which is obtained by hydrogenation of vegetable oil, is harmful for us?
Answer. All of the trans fats we consume, especially in oils, are a result of this as well. Trans fat is extremely harmful to our health and could lead to heart-related problems. Every year, millions of individuals worldwide pass away as a result of heart attacks.

Question 4. How hydrogen gas is stored?
Answer. Different sources provide the hydrogen needed for the hydrogenation process. The gas itself, which is known to exist freely in the atmosphere and the air around us, is the most frequent source. In fact, hydrogen is one of the most prevalent gases in our environment. However, the gas for the procedure must be obtained from pressurized cylinders that are readily available in the marketplace. The process frequently requires up to one atmosphere of hydrogen, and booster pumps would be necessary to maintain a constant and adequate supply for the ongoing reaction. 

Hydrocarbons are employed to produce the hydrogen required for the process on an industrial scale. This is accomplished using a procedure known as steam reforming, which involves employing dehydrogenating chemicals like carbon dioxide, acetone, and anthracene to remove hydrogen from the molecules of substances like formic acid, isopropanol, and dihydroanthracene. 

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