Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of \(\mathrm{CH}_{3} \mathrm{MgBr}\) with acetone and hydrolysis of the resulting product gives (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

The reaction yields \\((\mathrm{CH}_{3})_{3} \mathrm{COH}\\), option (a).

Step 1: Understanding the Reaction

The reaction involves the Grignard reagent (CH₃MgBr) reacting with acetone (CH₃COCH₃). This is a typical Grignard reaction where the nucleophilic carbon of the Grignard reagent attacks the carbonyl carbon of acetone, forming a new bond.

Step 2: Writing the Reaction Equation

The initial reaction can be written as: \[ ext{CH}_3 ext{MgBr} + ext{CH}_3 ext{COCH}_3 ightarrow ( ext{CH}_3)_3 ext{C}^- ext{Mg}^+ ext{Br}\]Here, a tertiary alkoxide ion is formed as the intermediate after the Grignard reagent attacks the carbonyl carbon.

Step 3: Hydrolysis of the Tertiary Alkoxide

The alkoxide ion undergoes hydrolysis when it reacts with water or a dilute acid. This step replaces the -MgBr portion with an -OH group:\[( ext{CH}_3)_3 ext{C}^- ext{Mg}^+ ext{Br} + ext{H}_2 ext{O} ightarrow ( ext{CH}_3)_3 ext{COH}\]This results in the formation of a tertiary alcohol, which is trimmed by the -OH group attaching to the carbon previously bonded to MgBr.

Step 4: Identifying the Product

The final organic compound obtained after hydrolysis is \[( ext{CH}_3)_3 ext{COH}\]This corresponds to option (a), indicating that the tertiary alcohol with the structural formula \[ ext{2-Methylpropan-2-ol}\] or \[ ext{tert-Butanol}\] is formed.

Key Concepts

Nucleophilic Addition

Nucleophilic addition is an important reaction mechanism in organic chemistry, particularly with Grignard reagents. This mechanism involves the addition of a nucleophile to an electron-deficient electrophile. In the context of our reaction, the Grignard reagent, represented by \( \text{CH}_3\text{MgBr} \), acts as a strong nucleophile due to the presence of the highly polar carbon-magnesium bond. This carbon part is very negatively charged, making it eager to bond with positive or partially positive elements.

In our case, the nucleophilic carbon attacks the electrophilic carbon in the carbonyl group \((\text{C=O})\) of acetone. This is because the carbonyl carbon is slightly positive due to the electron-withdrawing nature of the oxygen. When the \( \text{CH}_3\) group of the Grignard reagent attacks this positively charged carbonyl carbon, a new \(\text{C-C}\) bond forms. This step effectively converts the \(\text{C=O}\) double bond into a \(\text{C-O}\) single bond.

The result of this attack is an intermediate in the form of a magnesium alkoxide complex, temporarily neutralizing the reaction site, but also preparing it for further transformation in subsequent steps.

Tertiary Alcohol Formation

The formation of tertiary alcohols in this Grignard reaction follows the nucleophilic addition stage. Once the alkoxide intermediate is formed, it requires hydrolysis to become a stable alcohol product. The initial product of the nucleophilic addition of methyl magnesium bromide to the acetone carbonyl group is a tertiary alkoxide ion.

During the hydrolysis step, water donates a proton, and the \( -\text{MgBr} \) group is replaced with an \( \text{OH} \) group. This transformation is crucial, as it stabilizes the molecule and gives rise to a tertiary alcohol. The structure of this type of alcohol is defined by the carbon bonded to three other carbon groups and an \( \text{OH} \) group, making it a tertiary configuration.

• Hydrolysis stabilizes the alkoxide intermediate.
• Replacement of \( -\text{MgBr} \) with \( \text{OH} \) completes the alcohol formation.
• The final product from the given reaction is tert-butanol.

In this specific Grignard reaction, the tertiary alcohol formed is \(\text{tert-Butanol}\), because the central carbon is attached to three methyl groups and an \(\text{OH}\) group, highlighting its tertiary nature.

Carbonyl Compound

Carbonyl compounds are characterized by their \( \text{C=O} \) double bond, and they play a central role in many organic reactions, including the Grignard reaction described here. The carbon in the carbonyl group of acetone acts as an electrophilic center, ready to be attacked by nucleophiles like the methyl group in \( \text{CH}_3\text{MgBr} \).

Carbonyl carbon is electron-deficient because the oxygen atom in the \( \text{C=O} \) group is highly electronegative, pulling electron density towards itself. This electron deficiency makes it a prime candidate for reactions with nucleophiles. In particular, the polar nature of the carbonyl group makes it highly reactive.

• Acetone acts as a typical carbonyl compound in Grignard reactions.
• The electrophilic carbon in acetone's \( \text{C=O} \) paves the way for nucleophilic attack.
• Grignard reagents exploit this vulnerability to attach carbon groups.

This ability to attract nucleophiles makes carbonyl compounds like acetone indispensable starting materials in the formation of alcohols during Grignard reactions. Their versatile nature allows them to be transformed into a wide range of functionalized organic compounds.