Problem 94 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An organic compound A, \(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}\) on reaction with \(\mathrm{CH}_{3} \mathrm{MgBr}\) followed by acid treatment gives compound \(\mathbf{B}\). The compound \(\mathbf{B}\) on ozonolysis gives compound \(\mathbf{C}\), which in presence of a base gives 1- acetylcyclopentene D. The compound \(\mathbf{B}\) on reaction with HBr gives compound \(\mathbf{E}\). Write the structures of \(\mathbf{A}, \mathbf{B}, \mathbf{C}\) and \(\mathbf{E}\). Show how \(\mathbf{D}\) is formed from \(\mathbf{C}\) ?

Step-by-Step Solution

Answer

Compound A is cyclopentanone. Compound B is 2-methylcyclopentanol. Compound C forms via ozonolysis and base treatment yields D, 1-acetylcyclopentene. Compound E is 2-bromomethylcyclopentane.

Step 1: Determine Compound A

Compound A, with the formula \(\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}\), is likely a ketone. Given that it reacts with \(\mathrm{CH}_{3} \mathrm{MgBr}\) (Grignard reagent) to form compound B, A is inferred to be cyclopentanone.

Step 2: Identify Product of Grignard Reaction – Compound B

Cyclopentanone reacts with \(\mathrm{CH}_{3} \mathrm{MgBr}\) to form a tertiary alcohol after acid workup. This suggests that compound B is 2-methylcyclopentanol.

Step 3: Perform Ozonolysis to Form Compound C

When compound B (2-methylcyclopentanol) undergoes ozonolysis, it cleaves the carbon-carbon double bond in the ring if fully unsaturated into compounds with carbonyl functionalities. Compound C would be a likely product of this reaction.

Step 4: Base Treatment to Form Compound D

Compound C, when treated with a base, rearranges to form 1-acetylcyclopentene D, which indicates the transformation from an alcohol to a conjugated ketone.

Step 5: Reaction of Compound B with HBr to Get Compound E

Compound B's hydroxyl group reacts with HBr in a substitution reaction, leading to the formation of bromo alcohol. This substitution creates compound E, expected to be 2-bromomethylcyclopentane.

Key Concepts

Organic Chemistry

Organic chemistry is a vast field within chemical sciences devoted to studying compounds primarily made up of carbon atoms, often bonded with hydrogen, oxygen, nitrogen, and other elements. It forms the basis for understanding more complex biochemical and industrial processes.

In organic chemistry, understanding mechanisms such as the Grignard reaction is crucial. Grignard reagents, like \(\mathrm{CH}_{3} \mathrm{MgBr}\), are organometallic compounds which have a carbon-magnesium bond. They are highly reactive and can form new carbon-carbon bonds.

These reagents are key players in synthesizing alcohols from ketones or aldehydes, as they react with the carbonyl groups present in these compounds. The mechanism generally involves nucleophilic addition, where the carbon atom of the Grignard reagent attacks the electrophilic carbon in the carbonyl group, followed by an acidic workup to yield the alcohol. Such transformations expand the diversity and complexity of organic molecules that can be synthesized.

Ozonolysis

Ozonolysis is a chemical reaction where ozone, \(\mathrm{O}_{3}\), is used to cleave carbon-carbon double bonds in alkenes, transforming them into carbonyl groups like ketones or aldehydes.

This process is particularly useful in determining the structure of unknown alkenes or for breaking down complex molecules in a controlled manner. During ozonolysis, ozone reacts with the alkene to form a molozonide intermediate, which rearranges to form an ozonide.

• The ozonide can then be reduced further in the presence of a reducing agent, such as zinc and water, to yield the final carbonyl-containing compounds.
• This step is crucial because it prepares the molecule for subsequent reactions, as seen in the transformation of compound B to compound C in the example problem.

Ozonolysis is a powerful tool in organic chemistry for altering and analyzing molecular structures, especially in research and industrial applications.

Ketone Reactions

Ketones are organic compounds characterized by a carbonyl group \(\mathrm{C}=\mathrm{O}\) bonded to two carbon atoms. Understanding their reactions is a fundamental part of organic chemistry, especially when discussing their interactions with other reagents.

The Grignard reaction is an essential reaction for understanding ketones. By utilizing Grignard reagents, ketones can be transformed into tertiary alcohols. This reaction is a two-step process that involves the nucleophilic addition of the Grignard reagent to the ketone, followed by protonation to convert the alkoxide into an alcohol.

• Ketones also participate in other notable reactions, such as ozonolysis, where they can form as breakdown products.
• Additionally, base-catalyzed reactions with ketones can lead to products like conjugated enones, shown in the example where compound D is derived from compound C.

Grasping the variety of reactions involving ketones helps in synthesizing more complex organic molecules and performing structural elucidation.