Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

Hydrogenation of benzoyl chloride in the presence of \(\mathrm{Pd}\) and \(\mathrm{BaSO}_{4}\) gives (a) benzyl alcohol (b) benzaldehyde (c) benzoic acid (d) phenol

Step-by-Step Solution

Answer

The answer is (b) benzaldehyde.

Step 1: Understanding the Reaction

The problem asks about the product of the hydrogenation of benzoyl chloride using \( \mathrm{Pd} \) and \( \mathrm{BaSO}_{4} \), known as the Rosenmund reduction. This is a catalytic hydrogenation process that converts acyl chlorides to aldehydes.

Step 2: Identifying the Reaction Reagents

In this reaction, the reagents used are palladium (\( \mathrm{Pd} \)) as the hydrogenation catalyst, and barium sulfate (\( \mathrm{BaSO}_{4} \)) which is used as a surface to disperse the \( \mathrm{Pd} \) catalyst.

Step 3: Recognizing the Product Formation

During the Rosenmund reduction, the benzoyl chloride is reduced to its corresponding aldehyde. Thus, benzoyl chloride (\( \mathrm{C_6H_5COCl} \)) will be converted to benzaldehyde (\( \mathrm{C_6H_5CHO} \)).

Step 4: Choosing the Correct Option

The product benzaldehyde corresponds to option (b). Therefore, the correct answer is option (b) benzaldehyde.

Key Concepts

Catalytic Hydrogenation

Catalytic hydrogenation is a crucial process in organic chemistry that involves the addition of hydrogen to compounds with the aid of a catalyst. This process is widely used to reduce multiple bonds like carbon-carbon double bonds (alkenes) or carbon-oxygen double bonds (carbonyls). By doing so, it helps transform unsaturated compounds into saturated ones.
This reaction is particularly important because it enables the conversion of various functional groups, ultimately transforming the structure and properties of the original compound.

• Catalysts commonly used in hydrogenation include metals like palladium, platinum, and nickel.
• The catalyst speeds up the reaction by lowering the energy barrier, making it more efficient.

In the context of Rosenmund reduction, catalytic hydrogenation is employed to achieve the selective reduction of acyl chlorides to aldehydes, rather than continuing the reduction to alcohols or completely different products.

Benzoyl Chloride

Benzoyl chloride is an organic compound with the formula \( \mathrm{C_6H_5COCl} \). It is an acyl chloride derived from benzoic acid and is a colorless liquid that emits a pungent odor. In chemical reactions, benzoyl chloride is highly reactive due to its effective leaving group, the chloride ion.
This reactivity makes it a valuable intermediate in the synthesis of other chemical compounds.

• It is commonly used in acylation reactions to introduce the benzoyl group into compounds.
• Besides, it serves as a starting material in the formation of benzaldehyde via Rosenmund reduction.

Handling benzoyl chloride requires caution, as it reacts aggressively with water, releasing hydrochloric acid, and should be managed with proper safety measures.

Benzaldehyde

Benzaldehyde \( (\mathrm{C_6H_5CHO}) \) is the simplest aromatic aldehyde and is characterized by its pleasant almond-like scent. It is a significant compound both in the laboratory and in various industries, especially in flavorings and as a precursor to several pharmaceuticals.

• In the Rosenmund reduction, benzoyl chloride is hydrogenated to benzaldehyde, highlighting the reaction's specificity.
• Benzaldehyde is known for maintaining its aromatic ring structure, making it stable and versatile.

It serves as a prime example of how selective reduction processes like Rosenmund can precisely adapt the reactant's functional group without altering other parts of the molecule.

Palladium Catalyst

The palladium catalyst is a significant component in hydrogenation reactions such as the Rosenmund reduction. Palladium is a precious metal often used in its metallic form or supported on various surfaces to increase its effectiveness.

• It aids in the activation of molecular hydrogen, facilitating its addition to the reactant.
• In Rosenmund reduction, palladium is typically supported on barium sulfate \((\mathrm{BaSO_4})\)\.

Using a support like barium sulfate helps in dispersing the palladium particles, enhancing the catalyst's surface area and efficiency. Its unique properties ensure that it not only accelerates the reaction but also provides selectivity, usually transforming acyl chlorides to aldehydes without progressing to further reductions.