Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following pairs is not correctly matched? (a) \(>\mathrm{C}=\mathrm{O} \longrightarrow \mathrm{CH}_{2}\) Clemmensen reduction (b) \(-\mathrm{COCl} \longrightarrow-\mathrm{CHO}\) Rosenmund reduction (c) \(>\mathrm{C}=\mathrm{O} \longrightarrow \mathrm{CHOH}\) Wolff - Kishner reduction (d) \(-\mathrm{C} \equiv \mathrm{N} \longrightarrow \mathrm{CHO}\) Stephen reduction

Step-by-Step Solution

Answer

Option (c) is not correctly matched.

Step 1: Analyze Option (a)

Clemmensen reduction is used to reduce a carbonyl group (like a ketone or aldehyde) to a methylene group. The reaction is typically represented as \( >\mathrm{C}=\mathrm{O} \to \mathrm{CH}_2 \). This matches the given pair, meaning it's correctly matched.

Step 2: Analyze Option (b)

Rosenmund reduction involves the reduction of an acyl chloride to an aldehyde using hydrogen gas and a palladium catalyst over barium sulfate. The transformation \(-\mathrm{COCl} \rightarrow -\mathrm{CHO}\) corresponds to this reaction, so it's correctly matched.

Step 3: Analyze Option (c)

Wolff-Kishner reduction converts a carbonyl group into a methylene group, \( >\mathrm{C}=\mathrm{O} \to \mathrm{CH}_2 \), using hydrazine and a strong base. The transition \( >\mathrm{C}=\mathrm{O} \to \mathrm{CHOH} \) is incorrect for Wolff-Kishner reduction, thus it's incorrectly matched.

Step 4: Analyze Option (d)

Stephen reduction is used to reduce nitriles to aldehydes, represented as \(-\mathrm{C} \equiv \mathrm{N} \rightarrow \mathrm{CHO} \). This matches the description, so it is correctly matched.

Key Concepts

Clemmensen Reduction

Clemmensen Reduction is a widely used reaction in organic chemistry, especially when dealing with carbonyl compounds like ketones and aldehydes. This reaction focuses on reducing a carbonyl group (\( >\mathrm{C}=\mathrm{O} \)) to a methylene group (\( \mathrm{CH}_2 \)). The process requires the presence of zinc amalgam (Zn/Hg) and concentrated hydrochloric acid (HCl). This combination creates a highly reducing environment that facilitates the removal of the carbonyl oxygen and the addition of hydrogens to form the methylene group.

Clemmensen Reduction is particularly useful when you want to avoid the use of strong bases that might affect sensitive functional groups in the substrate. However, it is less effective on compounds that are sensitive to acidic conditions. It's important to note that during this process, aromatic rings and other stable structures in the molecule remain unaffected, making it a highly selective reduction method.

Rosenmund Reduction

The Rosenmund Reduction is a seminal method used to reduce acyl chlorides to aldehydes. This reaction utilizes hydrogen gas as a reducing agent, along with palladium catalysts supported on barium sulfate (Pd/BaSO4). The presence of this specific catalyst is crucial because it moderates the activity of palladium, preventing the full reduction of the acyl chloride beyond the aldehyde stage to the alcohol.

The reaction is conducted under controlled conditions to ensure the subtle balance achieved by the catalyst is maintained.

• Acyl chlorides (\(-\mathrm{COCl}\)) are reactive derivatives of carboxylic acids.
• The aldehyde product (\(-\mathrm{CHO}\)) is much more moderate and versatile for further synthetic applications.

Therefore, the Rosenmund Reduction is particularly valued in organic synthesis for its ability to create reactive aldehyde intermediates efficiently.

Wolff-Kishner Reduction

The Wolff-Kishner Reduction provides an alternative method for reducing carbonyl groups to methylene groups, but unlike Clemmensen, it uses a different mechanism and set of conditions. In this reduction, the carbonyl group (\( >\mathrm{C}=\mathrm{O} \)) is treated with hydrazine (\( \mathrm{NH}_2\mathrm{NH}_2 \)) and a strong base, typically KOH or NaOH.

This reaction is carried out in a high-boiling solvent like diethylene glycol, under high temperatures. The process involves the formation of a hydrazone intermediate, which is then decomposed to give out a methylene group and nitrogen gas (\( \mathrm{N}_2 \)).

• Wolff-Kishner is advantageous when dealing with substances that do not tolerate acid, making it complementary to Clemmensen Reduction.
• It is also immensely valuable when maintaining high pH is necessary.

Despite the requirement for harsh conditions, Wolff-Kishner is renowned for producing clean products without the risk of forming alcohols, making it particularly useful in synthetic chemistry where purity is quintessential.

Stephen Reduction

The Stephen Reduction is a lesser-known but highly useful reaction that focuses on converting nitriles (\(-\mathrm{C} \equiv \mathrm{N}\)) into aldehydes (\(-\mathrm{CHO}\)). This reduction involves using tin(II) chloride (SnCl2) in the presence of hydrochloric acid and water to achieve the desired conversion. Initially, the nitrile is reduced to an imine, which upon hydrolysis, yields the corresponding aldehyde.

The Stephen Reduction is especially useful in synthetic pathways where a direct method to produce aldehydes from nitriles is required.

• Nitriles are valuable precursors in synthetic chemistry due to their relative stability and availability.
• The reduction offers a direct route to aldehydes without the corresponding alcohol formation.

This method allows for versatility in complex synthetic sequences, providing a strategic advantage in multi-step processes where functional group transformations must be tightly controlled.