Problem 60 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Cyclobutyl bromide on treatment with magnesium in dry ether forms an organometallic (A). The organometallic reacts with ethanal to give an alcohol (B) after mild acidification. Prolonged treatment of alcohol (B) with an equivalent amount of HBr gives 1 -bromo-1methylcyclopentane (C). Write the structures of (A), (B) and explain how \((\mathrm{C})\) is obtained from (B).

Step-by-Step Solution

Answer

(A): Cyclobutylmagnesium bromide, (B): 1-(2-hydroxyethyl)cyclobutane, (C): 1-bromo-1-methylcyclopentane.

Step 1: Identify the Organometallic Compound (A)

Cyclobutyl bromide reacts with magnesium in dry ether to form a Grignard reagent. The product of this reaction is cyclobutylmagnesium bromide, which is labeled as compound (A). The structure of (A) is 1-cyclobutylmagnesium bromide.

Step 2: React Organometallic with Ethanal to Form Alcohol (B)

The Grignard reagent (A) reacts with ethanal (acetaldehyde). The nucleophilic carbon in the Grignard reagent attacks the carbonyl carbon of ethanal, forming an alkoxide intermediate. After protonation during the work-up step with a mild acid, an alcohol is formed. The resulting alcohol (B) is 1-(2-hydroxyethyl)cyclobutane.

Step 3: Convert Alcohol (B) to Halide (C)

Alcohol (B) is treated with HBr. The hydroxyl group is protonated by HBr, making it a good leaving group. Subsequent nucleophilic attack by bromide ion leads to the displacement of the hydroxyl group, forming 1-bromo-1-methylcyclopentane (compound C). This reaction involves an intramolecular rearrangement, where the five-membered cyclopentane ring is formed from the original cyclobutane ring.

Key Concepts

Organometallic Compounds

In chemistry, organometallic compounds hold significant importance, particularly in reaction mechanisms like the Grignard reaction. An organometallic compound is defined by an organic group directly bonded to a metal atom. Here, we encounter cyclobutylmagnesium bromide as the organometallic compound formed. This Grignard reagent is crucial as it acts as a powerful nucleophile.
Grignard reagents are typically used in the synthesis of alcohols, among other compounds. These organometallics react with various electrophiles to make new carbon-carbon bonds.
Their high nucleophilicity is mainly due to the carbon-metal bond. This bond gives the carbon a partial negative charge, enabling it to readily attack electrophilic centers, such as carbonyl carbons in aldehydes or ketones.
Remember, for successful Grignard reagent formation, maintaining dry conditions is key. This ensures that the reagent remains reactive, as contact with water would deactivate it by converting it to a hydrocarbon.

Alcohol Synthesis

Alcohol synthesis through Grignard reagents is a fundamental chemical process. After forming the organometallic compound, it reacts with ethanal, also known as acetaldehyde, to form an alcohol.
The Grignard reagent attacks the carbonyl carbon in ethanal. This step is called a nucleophilic attack, where the carbon-metal bond of the Grignard breaks to form a new C-C bond.
This results in the generation of an alkoxide ion, which, after protonation with a mild acid, yields the desired alcohol. The compound (B) formed here is 1-(2-hydroxyethyl)cyclobutane.

• The nucleophilic attack involves a transfer of electrons, forming a stable tetrahedral intermediate.
• Acidic conditions then help convert the alkoxide to alcohol by providing a proton.

This method is lauded for its ability to create complex alcohol structures by merely adjusting the Grignard reagents and aldehydes or ketones used.

Cycloalkane Transformation

Transforming alcohol (B) to 1-bromo-1-methylcyclopentane (C) involves a fascinating process of chemical rearrangement. Initially, the alcohol undergoes protonation when treated with hydrobromic acid (HBr), allowing the hydroxyl group to become a good leaving group.
The reaction proceeds through an intriguing intramolecular rearrangement. The departure of the hydroxyl group paves the way for a ring expansion from the original cyclobutane structure to a cyclopentane ring.
Here’s a simplified rundown of the process:

• Step 1: Hydroxyl group in alcohol is protonated by HBr.
• Step 2: Bromide ion acts as a nucleophile and displaces the protonated hydroxyl group.
• Step 3: As this substitution occurs, the four-membered ring (cyclobutane) rearranges into a five-membered cyclopentane ring.

This transformation is a classic example of a rearrangement reaction where mechanistic aspects like relieving ring strain play a significant role. It demonstrates how relatively simple starting materials can be converted into more complex structures through strategic synthetic routes.