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

Question

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

Step-by-Step Solution

Answer

Option (c) is not correctly matched; Wolff-Kishner reduction reduces to CH2, not CHOH.

Step 1: Review each reaction type

Each option describes a chemical reaction process, where specific functional groups undergo certain transformations. These reactions are well-defined named reactions in organic chemistry.

Step 2: Clemmensen Reduction Analysis

The Clemmensen reduction is used to reduce carbonyl groups (C=O) to methylene groups (CH2) using zinc amalgam and hydrochloric acid. This matches option (a), which correctly transforms (C=O) to (CH2).

Step 3: Rosenmund Reduction Analysis

The Rosenmund reduction is used to reduce acyl chloride (-COCl) to an aldehyde (-CHO) using hydrogen gas in the presence of a palladium catalyst poisoned with barium sulfate. Option (b) correctly describes this transformation.

Step 4: Wolff-Kishner Reduction Analysis

The Wolff-Kishner reduction converts carbonyl groups (C=O) to methylene groups (CH2) using hydrazine and a strong base. Therefore, option (c) incorrectly shows (C=O) converting to (CHOH) when it should show (CH2).

Step 5: Stephen Reduction Analysis

Stephen reduction is used to convert nitriles (–C≡N) to aldehydes (-CHO) by using stannous chloride and hydrochloric acid. Option (d) correctly matches this description.

Key Concepts

Clemmensen Reduction

The Clemmensen reduction is an important reaction in organic chemistry, utilized primarily to convert carbonyl compounds (such as ketones and aldehydes) into hydrocarbons. The reaction uses zinc amalgam (a combination of zinc with mercury) and concentrated hydrochloric acid as catalysts.
The aim of this reduction is straightforward: it transforms a carbonyl group (C=O) into a methylene group (CH2). This can be particularly useful when desiring to reduce a ketone or an aldehyde without affecting other sensitive functional groups present in the molecule.

• Typical conditions: Heating is often applied to facilitate the reaction.
• Limitations: Clemmensen reduction is not suitable for compounds containing acid-sensitive groups as harsh acidic conditions are required.

This reaction is beneficial in the synthesis of hydrocarbons from more reactive carbonyl compounds, making it a fundamental tool in organic synthesis.

Rosenmund Reduction

The Rosenmund reduction provides a method to selectively reduce acyl chlorides (COCl) to aldehydes (CHO). This reaction utilizes hydrogen gas in the presence of a palladium catalyst. However, to ensure that the reaction does not reduce the aldehyde further to an alcohol, the palladium is poisoned using barium sulfate.
To execute a Rosenmund reduction, careful control of conditions is vital to maintain the presence of the aldehyde rather than reducing it all the way to an alcohol.

• Specificity: This reduction is specific to acyl chlorides and is often chosen for its reliability in aldehyde synthesis from acyl chlorides.
• Safety Note: Hydrogen gas handling should be executed with caution due to its flammable nature.

Its selectivity and simplicity make the Rosenmund reduction a mainstay in synthesizing aldehydes in organic chemistry.

Wolff-Kishner Reduction

The Wolff-Kishner reduction is a chemical reaction that transforms carbonyl groups (such as ketones and aldehydes) into methylene groups (CH2). This is achieved by using hydrazine and a strong base, often potassium hydroxide, under basic conditions with heat.
The process occurs under neutral to basic conditions, making it suitable for substrates that are sensitive to acids.

• Major application: Used primarily for substrates where acidic conditions (like those in Clemmensen reduction) would lead to decomposition.
• Procedure: The reaction begins with the formation of a hydrazone, which then decomposes to form nitrogen gas and the target hydrocarbon.

While offering versatility, the Wolff-Kishner reduction is more suitable for reactions where heat and a basic environment will not destabilize other functional groups.

Stephen Reduction

Stephen reduction is a special technique used to convert nitriles (e.g., CN) into aldehydes. The reaction involves the use of stannous chloride (SnCl2) and hydrochloric acid.
This reduction method is vital as it enables the conversion of nitrogen-containing organic compounds into aldehydes, opening pathways for various synthetic applications.

• Chemistry in action: The process works by initially converting the nitrile into an iminium salt, which is then hydrolyzed to yield the desired aldehyde.
• Importance: The Stephen reduction is extremely useful when working with nitriles, aiming to obtain aldehydes without over-reducing to alcohols.

An essential method in organic chemistry, the Stephen reduction provides a unique pathway to synthesize aldehydes under mild conditions, different from those needed for acyl chlorides or carbonyl group reductions.