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 final product is \((\mathrm{CH}_3)_3\mathrm{COH} \), option (a).

Step 1: Understand the Reactants

We have a Grignard reagent, specifically methyl magnesium bromide, \( \mathrm{CH}_3\mathrm{MgBr} \), and acetone, \( \mathrm{(CH}_3\mathrm{)}_2\mathrm{CO} \). The Grignard reagent is an organomagnesium compound used in the formation of carbon-carbon bonds.

Step 2: Analyze the Reaction

The Grignard reagent \( \mathrm{CH}_3\mathrm{MgBr} \) will react with acetone. The nucleophilic methyl group from the Grignard reagent attacks the carbonyl carbon of acetone, transferring its lone pair to the oxygen atom. This results in the formation of a tertiary alkoxide intermediate.

Step 3: Identify the Alkoxide Intermediate

After the methyl group from \( \mathrm{CH}_3\mathrm{MgBr} \) attacks acetone, the intermediate formed is a tertiary alkoxide: \( (\mathrm{CH}_3)_3\mathrm{CO}^-\mathrm{MgBr} \). This intermediate is a key step before final hydrolysis.

Step 4: Hydrolysis of the Alkoxide Intermediate

Upon hydrolysis, the alkoxide undergoes a reaction with water. This causes the replacement of the magnesium halide group with a hydrogen atom, forming a tertiary alcohol. The product from the hydrolysis of the alkoxide \( (\mathrm{CH}_3)_3\mathrm{CO}^-\mathrm{MgBr} \) is \((\mathrm{CH}_3)_3\mathrm{COH} \).

Step 5: Conclude the Reaction Product

From the reaction description, the final product after hydrolysis of the initial Grignard reaction is \((\mathrm{CH}_3)_3\mathrm{COH} \), also known as tert-butanol, corresponding to option (a).

Key Concepts

Nucleophilic Addition

In a Grignard reaction, nucleophilic addition is a fundamental step. Here, the Grignard reagent, methyl magnesium bromide (\( \mathrm{CH}_3\mathrm{MgBr} \)), plays a crucial role as a nucleophile. A nucleophile is a species that donates an electron pair to form a chemical bond. When methyl magnesium bromide reacts with acetone (\( \mathrm{(CH}_3\mathrm{)_2CO} \)), the nucleophilic methyl group carries out an attack on the carbonyl carbon of acetone.

• This attack results in the breaking of the double bond between carbon and oxygen in the carbonyl group.
• The electrons from the carbon-oxygen bond shift towards the oxygen, creating a tetrahedral intermediate with a negatively charged alkoxide ion.

The nucleophilic addition is essential as it sets the foundation for creating new carbon-carbon bonds, which is a key feature in many organic synthesis reactions. By understanding this primary step, one can appreciate the versatility of the Grignard reaction in forming complex alcohol structures.

Tertiary Alcohol

Formation of a tertiary alcohol is the outcome of the reaction between a Grignard reagent and a ketone, such as acetone. After the nucleophilic addition phase, a tertiary alkoxide intermediate is formed. This intermediate consists of a central carbon connected to three methyl groups and an oxygen atom with a negative charge. Upon hydrolysis, this intermediate will yield a tertiary alcohol.
In our example, the final product is tert-butanol (\( (\mathrm{CH}_3)_3\mathrm{COH} \)):

• Tertiary alcohols are characterized by the hydroxyl group (OH) attached to a carbon atom bonded to three other carbon atoms.

Hydrolysis is a crucial step that completes the conversion of the alkoxide to a tertiary alcohol. It involves introducing water to replace the bonded magnesium bromide with a hydrogen atom. Tertiary alcohols are essential in various chemical processes and syntheses due to their higher reactivity compared to primary and secondary alcohols.

Reaction Mechanism

The detailed reaction mechanism of the Grignard reaction elucidates each transformation from reactants to the final product in a very structured manner. Initially, the methyl group from the Grignard reagent acts as a nucleophile. This methyl group attacks the carbonyl carbon of acetone, creating a tetrahedral intermediate.

Steps in the Reaction Mechanism:

• The nucleophilic addition involves breaking the double bond of the carbonyl group and forming a new C-C bond.
• This formation generates the tertiary alkoxide, a negatively charged species.
• Upon addition of water (hydrolysis), the alkoxide ion reacts to form a hydroxyl group, converting it into the final alcohol product.

Tracking each phase of this reaction mechanism helps in understanding how simple reactants are transformed into valuable compounds through systematic structural changes. The Grignard reaction mechanism is a prime example of how strategic utilisation of reagents leads to the successful synthesis of complex organic molecules.