Problem 76 - 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 C is 2-methylpropanol-2.

Step 1: First Reaction - Formation of A

The starting compound is \(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3}\) (2-aminopropane). It reacts with \(\mathrm{HNO}_{2}\) to form propanol-2 (\(\mathrm{CH}_{3}-\mathrm{CH(OH)}-\mathrm{CH}_{3}\)) as \(\mathrm{HNO}_{2}\) converts the amino group into an alcohol group. Therefore, A is \(\mathrm{CH}_{3}-\mathrm{CH(OH)}-\mathrm{CH}_{3}\).

Step 2: Oxidation - Formation of B

Compound A, which is propanol-2, undergoes oxidation. Oxidation of a secondary alcohol like propanol-2 gives a ketone. Therefore, B is the ketone propan-2-one \(\left(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3}\right)\), commonly known as acetone.

Step 3: Grignard Reaction - Formation of C

Now, B, which is acetone, reacts with \(\mathrm{CH}_{3} \mathrm{MgI}\). The Grignard reagent \(\mathrm{CH}_{3} \mathrm{MgI}\) adds a methyl group to the carbonyl carbon of the ketone, leading to the formation of \(\mathrm{CH}_{3}-\mathrm{C(OH)}(\mathrm{CH}_{3})-\mathrm{CH}_{3}\) after subsequent hydrolysis (\(\mathrm{H}^{-} / \mathrm{H}_{2} \mathrm{O}\)). This compound is 2-methylpropanol-2.

Key Concepts

Amine to Alcohol Conversion

Converting an amine to an alcohol can be achieved by using nitrous acid (\(\mathrm{HNO}_{2}\)). In the given reaction sequence, our starting compound is 2-aminopropane (\(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{NH}_{2}\right)-\mathrm{CH}_{3}\)).

When this compound reacts with nitrous acid, a chemical transformation occurs in which the amine group (\(\mathrm{NH}_{2}\)) is replaced by a hydroxyl group (\(\mathrm{OH}\)). This process takes advantage of the chemical instability of primary amines with nitrous acid, forming nitrogen gas and water during the transformation. The result is the production of propanol-2 (\(\mathrm{CH}_{3}-\mathrm{CH(OH)}-\mathrm{CH}_{3}\)).

Understanding this transformation is vital, as it is a classic example of converting functional groups in organic chemistry, allowing for the creation of new molecules with different properties.

Oxidation of Secondary Alcohols

Oxidation plays a crucial role in the transformation of alcohols to ketones and aldehydes. Here, the secondary alcohol, propanol-2, undergoes oxidation to form a ketone.

Secondary alcohols like propanol-2 (\(\mathrm{CH}_{3}-\mathrm{CH(OH)}-\mathrm{CH}_{3}\)) can be oxidized using oxidizing agents such as chromic acid or potassium dichromate. The oxidation process involves the removal of hydrogen from the alcohol group, resulting in the formation of a carbonyl group (\(\mathrm{CO}\)).

This results in the molecule acetone (propan-2-one), represented by \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3}\). This step is significant in synthetic pathways, especially in building complex organic compounds, as ketones act as versatile intermediates in various reactions.

Grignard Reagent Reactions

In organic chemistry, Grignard reactions are essential tools for forming carbon-carbon bonds. In the given sequence, acetone (propan-2-one) serves as a substrate for the Grignard reagent \(\mathrm{CH}_{3} \mathrm{MgI}\).

The Grignard reagent adds a methyl group to the carbonyl carbon of acetone. This addition results in an alkoxide ion intermediate, which upon hydrolysis (treatment with \(\mathrm{H}^{-} / \mathrm{H}_{2}\mathrm{O}\)), results in the alcohol 2-methylpropanol-2. The final product, 2-methylpropanol-2, is characterized by its tertiary alcohol structure \(\mathrm{CH}_{3}-\mathrm{C(OH)}(\mathrm{CH}_{3})-\mathrm{CH}_{3}\).

Grignard reactions are widely used for synthesizing alcohols and other functionalized organic molecules, showcasing the power of this reaction in constructing complex, carbon-based molecules.