Problem 176 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions, yield a product with a three membered ring? (a) \(\mathrm{CH}_{3}-\mathrm{C}(\mathrm{O})-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \stackrel{\mathrm{KOH}, \mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\) (b) \(\mathrm{PhCHO}+\mathrm{Br}-\mathrm{CH}_{2}-\mathrm{C}(\mathrm{O})-\mathrm{OEt}\) \(\mathrm{t}-\mathrm{BuO}^{-/} \mathrm{t}-\mathrm{BuOH}\) (c) \(\mathrm{Ph}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \stackrel{\mathrm{mCPBA}}{\longrightarrow}\) (d) 3- bromobutan \(-2\) - ol \({ }^{-\mathrm{OH} / \mathrm{H}_{2} \mathrm{O}}\)

Step-by-Step Solution

Answer

Reaction (c) forms a product with a three-membered ring.

Step 1: Review the general reaction types

Before analyzing the options, review some common reactions that form three-membered rings, such as epoxidation of alkenes, cyclopropanation, and reactions involving carbenes.

Step 2: Analyze Reaction (a)

The reaction involves a ketone, dichloride, and KOH, which typically does not result in a cyclopropane ring formation. This is often a routine mechanism for forming alcohols or E2 eliminations.

Step 3: Analyze Reaction (b)

This reaction is a phenyl aldehyde with a bromoester in the presence of a strong base. This could set up an alpha-halogen carbanion, usually leading to substitution or elimination, rather than ring formation.

Step 4: Analyze Reaction (c)

The reaction of an alkene with mCPBA (meta-chloroperoxybenzoic acid) is a classic epoxidation reaction. Reacting alkenes with peroxy acids forms epoxides, three-membered cyclic ethers.

Step 5: Analyze Reaction (d)

The reaction appears to involve a bromohydrin and a base, which generally results in dehydrohalogenation leading to alkenes rather than forming three-membered rings.

Step 6: Conclusion: Determine which reaction forms a three-membered ring

After analysis, only Reaction (c) involves an epoxidation, forming an epoxide which is a three-membered ring. Therefore, Reaction (c) is the correct answer.

Key Concepts

Epoxidation

Epoxidation is a chemical reaction that converts alkenes into epoxides, which are three-membered cyclic ethers. This reaction is typically carried out using a peroxy acid, such as mCPBA (meta-chloroperoxybenzoic acid). When an alkene is treated with mCPBA, the pi bond is attacked, and an oxygen atom is inserted, forming a strained, but stable, three-membered ring structure.

The process often includes the following steps:

• The peroxy acid approaches the alkene, and a concerted mechanism follows where the oxygen is transferred.
• The alkene's double bond electrons are responsible for forming a new bond with the oxygen.
• This transfer results in a cyclic transition state, leading directly to the formation of the epoxide product.

The stereochemistry of the alkene dictates the orientation of the epoxide, conserving the original configuration of substituents across the double bond. Epoxidation is widely used in the synthesis of natural products and pharmaceuticals due to its efficiency and selectivity.

Cyclopropanation

Cyclopropanation is the process of forming a cyclopropane ring, a small, three-membered carbon ring. This transformation often involves the reaction of an alkene with a carbene or a carbenoid species. The formation of cyclopropanes is crucial in organic synthesis due to their application in various complex molecules.

One popular method of cyclopropanation involves using a zinc-copper couple with a dihalomethane. The reaction proceeds via the generation of a carbenoid intermediate, which transfers to the alkene to form the three-membered ring. Some notable steps are:

• A carbene or carbenoid species forms, typically from a dihalomethane and zinc.
• The carbenoid reacts with the double bond of the alkene, forming two new carbon-carbon bonds simultaneously.

The reaction is stereospecific, often preserving the stereochemistry of the original alkene. Cyclopropanation expands the toolkit for organic chemists, allowing for the exploration of versatile synthetic pathways.

Carbene Reactions

Carbene reactions are essential in organic chemistry due to their ability to form reactive intermediates that can participate in forming three-membered rings among other transformations. Carbenes are neutral species containing a carbon atom with two non-bonding electrons and typically divalent carbon, making them extremely reactive.

Due to their unique electronic structure, carbenes can insert into C-H and C-X (where X is a halogen) bonds, or add across double bonds to form cyclopropanes. Here's how carbenes typically react:

• Carbenes can be generated through the decomposition of diazo compounds or through photolysis.
• Once formed, the carbene can interact with alkenes' double bonds, resulting in cyclopropane formation.
• Alternatively, carbenes may insert into existing bonds or participate in wider reaction pathways.

Carbene chemistry provides significant opportunities for constructing unique molecular architectures largely due to the carbene's reactivity and ability to form new rings and bonds efficiently.