Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions, \(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{A}\) \(\stackrel{\text { oxidation }}{\longrightarrow} \mathrm{B} \frac{\text { (i) } \mathrm{CH}_{3} \mathrm{Mgl}}{\text { (ii) } \mathrm{H}^{+} / \mathrm{H}_{2} \mathrm{O}} \mathrm{C}\) the compound \(\mathrm{C}\) formed will be (a) butanol-1 (b) 2-methylpropanol-1 (c) 2 -methylpropanol-2 (d) butanol-2

Step-by-Step Solution

Answer

The compound \( \text{C} \) formed is (c) 2-methylpropanol-2.

Step 1: Identify the Reaction with HNO2

The given sequence starts with the reaction of \( \text{CH}_3-\text{CH(NH}_2\text{)}-\text{CH}_3 \) with \( \text{HNO}_2 \). Primary amines convert to alcohols via diazotization followed by hydrolysis. Here, \( \text{A} \) will be \( \text{CH}_3-\text{CHOH}-\text{CH}_3 \), which is 2-propanol.

Step 2: Oxidation to form B

Next, compound \( \text{A} \) (2-propanol) undergoes oxidation. Secondary alcohols are typically oxidized to ketones. Thus, 2-propanol oxidizes to acetone \( (\text{CH}_3-\text{CO}-\text{CH}_3) \), which is compound \( \text{B} \).

Step 3: Reaction with Grignard Reagent

In the final step, acetone reacts with a Grignard reagent, \( \text{CH}_3\text{MgI} \), followed by hydrolysis. This forms a tertiary alcohol. The reaction is as follows: \( \text{CH}_3-\text{CO}-\text{CH}_3 + \text{CH}_3\text{MgI} \rightarrow \text{CH}_3\text{-C(OH)(CH}_3\text{)(CH}_3\text{)} \), which simplifies to 2-methylpropan-2-ol.

Key Concepts

Reaction Mechanism

Understanding reaction mechanisms is key in organic chemistry as it helps us know exactly how a reaction proceeds from start to finish. These mechanisms describe the step-by-step pathway and transformation of reactants to products.
Consider the conversion of a primary amine like \( \text{CH}_3-\text{CH(NH}_2)-\text{CH}_3 \) in our exercise. It reacts first with \( \text{HNO}_2 \), through a process known as diazotization. This transforms the amine into an alcohol, specifically 2-propanol (\( \text{CH}_3-\text{CHOH}-\text{CH}_3 \)).

The next step involves oxidizing this secondary alcohol. It's crucial to recognize that secondary alcohols generally become ketones when oxidized, turning 2-propanol into acetone (\( \text{CH}_3-\text{CO}-\text{CH}_3 \)).

• Diazotization: conversion of amines to diazonium salts.
• Hydrolysis: transformation of the salt to alcohols.

By understanding such mechanisms, you appreciate how different steps contribute to the overall reaction.

Grignard Reagent

A Grignard reagent is a vital tool in organic synthesis. It's typically represented as \( \text{R-MgX} \), where \( \text{R} \) is an alkyl or aryl group and \( \text{X} \) is a halide.
In the considered exercise, acetone reacts with \( \text{CH}_3\text{MgI} \) (a type of Grignard reagent).

This reaction adds the methyl group from the reagent to acetone, forming a new carbon-carbon bond.
The result of this is a tertiary alcohol once the mixture undergoes hydrolysis.

• Grignard reagents convert carbonyl compounds like ketones to alcohols.
• They are highly reactive and must be handled with care, avoiding moisture at all costs.

Ultimately, in our exercise, the Grignard reaction leads to the formation of 2-methylpropan-2-ol, a tertiary alcohol.

Oxidation of Alcohols

Understanding the oxidation process of alcohols is crucial for differentiating between primary, secondary, and tertiary alcohols.
Primary alcohols oxidize to aldehydes, and further to carboxylic acids. Secondary alcohols, as seen in our exercise with 2-propanol, oxidize to ketones like acetone.

Tertiary alcohols, however, do not oxidize in the same manner because they lack a hydrogen atom bonded to the carbon bearing the hydroxyl group.

• Primary Alcohol: \( \text{RCH}_2\text{OH} \to \text{RCHO} \to \text{RCOOH} \)
• Secondary Alcohol: \( \text{R}_2\text{CHOH} \to \text{R}_2\text{CO} \)
• Tertiary Alcohol: No oxidation without breaking carbon-carbon bonds.

In the provided exercise, this knowledge of oxidation helps track the transformation from 2-propanol to acetone, giving insight into how reactions progress naturally.