Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \stackrel{\mathrm{KMnO}_{4}}{\longrightarrow}(\mathrm{a}) \stackrel{\mathrm{SOCl}_{2}, \mathrm{NH}_{3}}{\longrightarrow}\) (b) \(\mathrm{Br}_{2}+\mathrm{NaOH}\) (c) the end product (c) is (a) Acetone (b) Ethylamine (c) Acetic acid (d) Methyl amine

Step-by-Step Solution

Answer

The end product (c) is methylamine.

Step 1: Analyzing the First Reaction

The first reaction involves the oxidation of ethanol (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \)) by \( \mathrm{KMnO}_{4} \). This strong oxidizing agent will convert ethanol to acetic acid (\( \mathrm{CH}_{3} \mathrm{COOH} \)). Thus, compound (a) is acetic acid.

Step 2: Understanding the Second Reaction

In the second reaction, acetic acid (\( \mathrm{CH}_{3} \mathrm{COOH} \)) reacts with \( \mathrm{SOCl}_{2} \) and \( \mathrm{NH}_{3} \). This is a process of converting acetic acid to an amide, specifically acetamide (\( \mathrm{CH}_{3} \mathrm{CONH}_{2} \)). Thus, compound (b) is acetamide.

Step 3: Analyzing the Final Reaction

In the final reaction with \( \mathrm{Br}_{2} + \mathrm{NaOH} \), the Hofmann rearrangement occurs. It transforms acetamide (\( \mathrm{CH}_{3} \mathrm{CONH}_{2} \)) into methylamine (\( \mathrm{CH}_{3} \mathrm{NH}_{2} \)). Thus, the end product (c) is methylamine.

Key Concepts

Oxidation Reactions

Oxidation reactions are a fundamental concept in organic chemistry, where a molecule loses electrons, often involving the addition of oxygen or removal of hydrogen. In the given sequence, oxidation plays a crucial role in converting ethanol

• Reactant: Ethanol (\(\mathrm{CH_3CH_2OH}\))
• Oxidizing Agent: Potassium permanganate (\(\mathrm{KMnO_4}\))
• Product: Acetic Acid (\(\mathrm{CH_3COOH}\))

During this process, the ethanol molecule undergoes a change in its oxidation state, where the original alcohol (\(\mathrm{OH}\) group) is converted to a carboxylic acid (\(\mathrm{COOH}\) group). This happens because \(\mathrm{KMnO_4}\), a strong oxidizing agent, facilitates the breaking of the \(\mathrm{C-H}\) bond and the formation of a \(\mathrm{C=O}\) bond. These transformations are critical in synthetic organic chemistry, allowing chemists to transform simple alcohols into more reactive acid derivatives, paving the way for further chemical transformations.

Hofmann Rearrangement

The Hofmann rearrangement is an iconic reaction in organic synthesis, utilized for the conversion of primary amides into primary amines with one fewer carbon. This rearrangement involves the use of

• Reactants: Acetamide (\(\mathrm{CH_3CONH_2}\)), Bromine (\(\mathrm{Br_2}\)), and Sodium Hydroxide (\(\mathrm{NaOH}\))
• Product: Methylamine (\(\mathrm{CH_3NH_2}\))

In this intriguing transformation, an amide molecule is treated with bromine and a strong base like sodium hydroxide. This triggers a series of steps where the amide's carbonyl group rearranges and loses a carbon atom, eventually turning into an amine (\(\mathrm{NH_2}\) group) that is shorter by a single carbon atom.The Hofmann rearrangement is particularly useful for producing amines from naturally occurring or readily accessible amides in one seamless reaction sequence. It showcases the power of chemical rearrangement in reducing molecular complexity while providing high efficiency and selectivity in chemical transformations.

Alcohol to Amine Conversion

Converting alcohols to amines involves several chemical steps, requiring careful selection of reagents and conditions to ensure high yields and the desired transformations. Here's how it happens in the exercise:

• Initial Alcohol: Ethanol (\(\mathrm{CH_3CH_2OH}\))
• Intermediates: Acetic Acid (\(\mathrm{CH_3COOH}\)) and Acetamide (\(\mathrm{CH_3CONH_2}\))
• Final Product: Methylamine (\(\mathrm{CH_3NH_2}\))

The transformation begins by oxidizing ethanol to acetic acid. This activated form allows for further conversion to an amide through reaction with \(\mathrm{SOCl_2}\) and ammonia. Finally, Hofmann rearrangement takes place, transforming the amide to an amine.Such multi-step conversions demonstrate the utility of sequential reactions where each step is carefully planned to provide the right environment for the next reaction. Each step— from oxidation to amide formation and ultimately amine production—illustrates the seamless transition of functional groups in organic synthesis. These transformations are vital in synthetic chemistry for constructing more complex molecules from simple starting materials.