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) yields a product with a three-membered ring.

Step 1: Analyze Reaction (a)

Reaction (a) is a substitution reaction involving \(\mathrm{CH}_{3}-\mathrm{C}(\mathrm{O})-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \) in a base such as KOH in water. This is a typical setting for an intramolecular SN2 reaction to form a ring structure, potentially a three-membered ring if the conditions permit.

Step 2: Evaluate Reaction (b)

Reaction (b) involves a base \( \mathrm{t}-\mathrm{BuO}^{-} / \mathrm{t}-\mathrm{BuOH} \) with \( \mathrm{PhCHO} + \mathrm{Br}-\mathrm{CH}_{2}-\mathrm{C}(\mathrm{O})-\mathrm{OEt} \). This is characteristic of an aldol condensation reaction, which does not form three-membered rings but rather results in the formation of larger aldol products.

Step 3: Analyze Reaction (c)

Reaction (c) utilizes mCPBA, which is known for converting alkenes into epoxides. An epoxide has a three-membered ring structure, formed by adding an oxygen atom across the double bond of \( \mathrm{Ph}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \). This indicates that the product will indeed have a three-membered ring structure.

Step 4: Consider Reaction (d)

In reaction (d), 3-bromobutan-2-ol reacts with \(-\mathrm{OH} / \mathrm{H}_{2}\mathrm{O}\). This reaction typically results in the formation of a different type of structure called a vicinal diol but is not known to produce a three-membered ring.

Key Concepts

Intramolecular SN2 Reaction

An intramolecular SN2 reaction is a fascinating type of substitution reaction, where a nucleophile and an electrophile are located within the same molecule. This sets the stage for the formation of ring structures. Here's how it works:

• The nucleophile attacks the electrophile, leading to the displacement of a leaving group.
• Since both the nucleophile and electrophile are part of the same molecule, the reaction can result in the formation of a cyclic compound.
• For this type of reaction to result in a three-membered ring, the nucleophile and the electrophile need to be appropriately positioned apart in the molecular chain.

Intramolecular reactions are typically faster and more favored compared to their intermolecular counterparts due to the close proximity of reacting sites in the molecule. This proximity reduces the entropy loss typically associated with bringing reactants together.

Aldol Condensation

Aldol condensation is a reaction between aldehydes or ketones involving the addition of a nucleophilic enolate to a carbonyl carbon. While it is a popular method to form carbon-carbon bonds, it does not produce three-membered rings. Instead, it creates larger, more complex structures. Here's how aldol condensation unfolds:

• The reaction begins with the formation of an enolate ion from a carbonyl compound.
• This enolate ion attacks another carbonyl group, leading to the formation of a β-hydroxy aldehyde or β-hydroxy ketone, known as an aldol product.
• If allowed to proceed under certain conditions, dehydration may occur, resulting in an α,β-unsaturated carbonyl compound.

Due to the multi-step process leading to extended chain compounds or cyclic structures larger than three-membered rings, aldol condensation is distinct from reactions yielding three-membered rings.

Epoxide Formation

Epoxides are three-membered cyclic ethers, characterized by an oxygen atom forming a bridge between two adjacent carbon atoms. Formation of epoxides typically involves:

• Converting alkenes into epoxides using reagents like mCPBA (meta-Chloroperoxybenzoic acid).
• Adding an oxygen atom across the double bond of an alkene, resulting in a strained, three-membered ring structure.

This reaction type is not only efficient but also quite popular due to the unique reactivity of epoxides. The three-membered ring is strained, making epoxides superb intermediates in synthesis for further reactions, such as opening the ring to form various products. This aspect makes epoxide formation a pivotal tool in organic synthesis.

Three-Membered Ring Structures

Three-membered ring structures in organic chemistry are notable for their strain and reactivity. They are less stable than larger rings due to the angle strain from smaller bond angles deviating from the ideal tetrahedral angle of 109.5 degrees. Some common three-membered rings include:

• Cyclopropanes in hydrocarbons, which have all carbon atoms.
• Epoxides, where two carbons and one oxygen form the ring.

These small rings are quite common in nature and synthetic chemistry due to their interesting properties. For example: - The strain in the rings often makes them reactive, useful for various chemical transformations. - Despite their instability, technologies exist to manipulate them further, opening rings to form more stable compounds. Through understanding the positioning and the inherent strain, chemists can exploit three-membered rings to synthesize complex molecules efficiently.