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: \(\begin{array}{llll}1 & 2 & 3 & 4\end{array}\) (a) (iii) (ii) (ii) (i) (b) (i) (ii) (ii) (iii) (c) (i) (iii) (iii) (ii) (d) (ii) (iii) (ii) (i)

Step-by-Step Solution

Answer

The correct matching is option (d): (ii), (iii), (iii), (i).

Step 1: Identify the Functional Groups for Clemmensen Reduction

Clemmensen reduction is used to reduce ketones and aldehydes to alkanes under acidic conditions. It's suitable for compounds that contain carbonyl groups like ketones (C=O).

Step 2: Identify the Functional Groups for Wolff-Kishner Reduction

Wolff-Kishner reduction is suitable for reducing carbonyl groups to alkanes, similar to Clemmensen, but under basic conditions. It's often used when the compound is sensitive to acids.

Step 3: Analyze Compound 1

Compound 1 is \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\), containing an aldehyde group \(\mathrm{CHO}\). Both Clemmensen and Wolff-Kishner are suitable. However, the bromine group may participate in side reactions under acidic conditions, so Wolff-Kishner is preferred.

Step 4: Analyze Compound 2

Compound 2 is \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CH}_{2}\mathrm{CH}_{2} \mathrm{COCH}_{3}\), containing a tertiary alcohol and a ketone. Both Clemmensen and Wolff-Kishner can reduce the ketone group, so both methods can work here.

Step 5: Analyze Compound 3

Compound 3 is \(\mathrm{PhCH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{COCH}_{2}\mathrm{CH}_{3}\), featuring an alcohol and a ketone group. Similar to Compound 2, both Clemmensen and Wolff-Kishner can be used to reduce the ketone group.

Step 6: Analyze Compound 4

Compound 4 is \(\mathrm{PhCOCH}_{3}\), which is a simple ketone. Clemmensen reduction is typically used for such cases.

Step 7: Match the Answers

- For Compound 1, option (ii) Wolff-Kishner reduction is suitable. - For Compound 2, option (iii) both methods are suitable. - For Compound 3, option (iii) both methods are suitable. - For Compound 4, option (i) Clemmensen reduction is suitable.

Key Concepts

Clemmensen Reduction

Clemmensen reduction is a chemical reaction used to transform carbonyl groups into hydrocarbons. It operates under acidic conditions to convert both aldehydes and ketones to their corresponding alkanes. This is achieved by using zinc amalgam and hydrochloric acid (HCl). The reaction is particularly useful when the compound can withstand the acidic environment without other functional groups becoming reactive or degraded. Clemmensen reduction is versatile for molecules that do not contain acid-sensitive functional groups, making it valuable in synthesizing alkanes from various synthetic intermediates.

Wolff-Kishner Reduction

The Wolff-Kishner reduction is a powerful method used to convert carbonyl compounds like aldehydes and ketones into alkanes, similar to the Clemmensen reduction. However, the key difference is that it occurs under basic conditions rather than acidic. Generally, this process uses hydrazine ( ext{N}_2 ext{H}_4) and a strong base, often potassium hydroxide ( ext{KOH}), at high temperatures. The choice of using the Wolff-Kishner reduction is usually dictated by the presence of acid-sensitive groups in the compound that could react unfavorably under acidic conditions. Thus, it serves as a complementary technique to Clemmensen, offering flexibility depending on the substrate's compatibility with acids.

Functional Groups Analysis

Functional group analysis is a fundamental step in determining which reduction method to use. By carefully analyzing the functional groups present in a compound, chemists can predict both the reactivity of the compound and the outcome of the reduction reaction. In the context of reductions, the presence of carbonyl groups, such as aldehydes ( ext{CHO}) and ketones ( ext{C=O}), are of particular interest since both the Clemmensen and Wolff-Kishner reductions target these groups to convert them to hydrocarbons. It's also crucial to consider other functional groups present in the molecule. For example, compounds with acid-sensitive groups might not be suitable for reduction via Clemmensen method. Similarly, some functional groups might be resistant to high temperatures required in the Wolff-Kishner reduction.

Reduction of Carbonyl Compounds

Carbonyl compounds are characterized by the carbon-oxygen double bond (C=O) and include both aldehydes and ketones. These compounds are common in organic chemistry and versatile in synthetic applications, serving as intermediates for various transformations. Reductions of these carbonyl groups to alkanes are significant reactions. Clemmensen and Wolff-Kishner reductions are prominent methods used for these conversions. Each method has its own applicability depending on the conditions required and the nature of the compound. In practical settings, the reduction of carbonyls greatly influences the design of synthesis pathways in organic chemistry, often requiring careful selection between acidic (Clemmensen) or basic conditions (Wolff-Kishner) based on the stability and reactivity of the substrate.