Problem 189 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Compounds to be reduced) 1\. \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\) 2\. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COCH}_{3}\) 3\. \(\mathrm{PhCH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{COCH}_{2} \mathrm{CH}_{3}\) 4\. \(\mathrm{PhCOCH}_{3}\) List II (most suitable reduction) (i) Clemmensen reduction (ii) Wolf-Kishner reduction (iii) Both The correct matching is:

Step-by-Step Solution

Answer

1 - (iii), 2 - (i), 3 - (ii), 4 - (i)

Step 1: Analyze compound 1

Compound 1, \( \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CHO} \), contains an aldehyde group. For aldehydes and ketones, zinc amalgam in hydrochloric acid (Clemmensen reduction) can be used, but the presence of \( \mathrm{Br} \) may cause complications. Wolf-Kishner reduction is generally preferred for compounds with sensitive groups like halogens. Thus, suitable reduction: (iii) Both.

Step 2: Analyze compound 2

Compound 2, \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CH}_{2}\mathrm{CH}_{2} \mathrm{COCH}_{3} \), contains a secondary alcohol and a ketone. Both Clemmensen and Wolf-Kishner reductions are applicable to ketones, with Clemmensen being more preferred for this type of ketone. Thus, suitable reduction: (i) Clemmensen reduction.

Step 3: Analyze compound 3

Compound 3, \( \mathrm{PhCH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{COCH}_{2}\mathrm{CH}_{3} \), contains both an alcohol and a ketone. The Wolf-Kishner reduction is more suitable for deoxygenating ketones without affecting secondary alcohols. Thus, suitable reduction: (ii) Wolf-Kishner reduction.

Step 4: Analyze compound 4

Compound 4, \( \mathrm{PhCOCH}_{3} \), is a straightforward ketone. Both Clemmensen and Wolf-Kishner reductions are suitable, but Clemmensen is generally preferred for such aromatic ketones. Thus, suitable reduction: (i) Clemmensen reduction.

Key Concepts

Clemmensen reduction

The Clemmensen reduction is a chemical reaction widely used in organic chemistry to reduce ketones and aldehydes to alkanes. This reaction employs zinc amalgam and hydrochloric acid as reagents.
The mechanism involves the addition of zinc metal to the carbonyl group, converting the carbonyl carbon to a hydrocarbon, effectively removing the oxygen and replacing it with hydrogen.

This reduction is highly effective for aliphatic and aromatic ketones. However, it involves strong acidic conditions. Therefore, it may not be suitable for compounds sensitive to acids, such as those containing alcohol or amine groups.

• Commonly used for: aromatic ketones
• Not suitable for: compounds with acid-sensitive groups
• Reagents: zinc amalgam, hydrochloric acid

Wolf-Kishner reduction

The Wolf-Kishner reduction is another method used to convert ketones and aldehydes to alkanes, but it operates under basic conditions using hydrazine and a strong base like sodium or potassium hydroxide.
A key advantage of this method is that it is suitable for compounds that are sensitive to acidic conditions.

During this reduction, the carbonyl group reacts with hydrazine to form a hydrazone. Then, in the presence of a base, nitrogen gas is released, removing the oxygen and leading to the formation of a hydrocarbon.

• Commonly used for: acid-sensitive compounds
• Advantages: operates under basic conditions
• Reagents: hydrazine, strong base

Aldehydes and ketones

Aldehydes and ketones are fundamental organic compounds characterized by the presence of the carbonyl functional group (C=O).
Aldehydes have the carbonyl group at the end of the carbon chain, whereas in ketones, the carbonyl group is within the carbon chain.

These compounds are highly reactive, making them valuable intermediates in various chemical reactions, including reductions. Their reactivity stems from the polarized carbonyl group, where the carbon is electrophilic and the oxygen is nucleophilic.

• Functional group: carbonyl (C=O)
• Aldehydes: carbonyl at chain end
• Ketones: carbonyl within chain

Chemical analysis

Chemical analysis involves techniques and methodologies used to identify and quantify the composition of substances.
For aldehydes and ketones, spectroscopic methods such as infrared (IR) spectroscopy and nuclear magnetic resonance (NMR) can be employed to analyze functional groups.

These analyses help in confirming the structure and purity of organic compounds, critical for predicting their reactions and properties in reduction reactions.

• Techniques: IR spectroscopy, NMR spectroscopy
• Purpose: identify and quantify substances
• Application: structure and purity confirmation

Organic compounds

Organic compounds are chemical substances that primarily contain carbon atoms bonded with other elements like hydrogen, oxygen, and nitrogen.
These compounds form the basis of all living organisms and are fundamental to organic chemistry.

The vast diversity of organic compounds arises from the ability of carbon to form stable covalent bonds with many other elements and itself, resulting in a variety of structures like chains, rings, and branches.

• Key component: carbon
• Include: hydrocarbons, alcohols, acids
• Diversity: due to carbon bonding