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) \(\mathrm{CH}_{3}-\mathrm{C}(\mathrm{O})-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \quad \mathrm{KOH}, \mathrm{H}_{2} \mathrm{O}\) (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 three-membered ring (epoxide).

Step 1: Identify Ring-Forming Reactions

To find the reaction that yields a three-membered ring, we need to first identify the reactions that involve the formation of a cyclic compound. Cyclopropanes, a type of three-membered ring, are often formed through specific types of reactions.

Step 2: Evaluate Each Reaction for Ring Formation

(a) The reaction involves potassium hydroxide (KOH) and a chlorinated allyl ketone, which typically results in the formation of an alkene rather than a cyclopropane. (b) This reaction involves the formation of an enolate ion and an -elimination, typically leading to extended conjugation or additional alkene formation rather than cyclopropane formation. (c) This is an epoxidation reaction with mCPBA (meta-Chloroperoxybenzoic acid) on an alkene, which commonly forms epoxides, also known as three-membered cyclic ethers, a subset of cyclopropane structures. (d) This is a nucleophilic substitution reaction that will not result in ring formation.

Step 3: Conclusion - Identify the Ring-Forming Reaction

From the evaluations, it is apparent that option (c) involves the formation of a three-membered ring. The reaction of an alkene with mCPBA typically forms an epoxide, a three-membered cyclic ether, matching the criteria of the exercise.

Key Concepts

Epoxidation Reaction

In chemistry, the epoxidation reaction is a pivotal process for converting alkenes into epoxides, which are three-membered cyclic ethers. These reactions usually entail the addition of an oxygen atom across the double bond of an alkene, resulting in a strained three-membered ring. This is a particular type of cyclization that many students study due to its broad application in organic synthesis.

A classic example of an epoxidation reaction is the use of mCPBA (meta-Chloroperoxybenzoic acid) as an oxidizing agent. When applied to an alkene, the mechanism proceeds through a concerted process wherein the peroxy acid reacts with the π-bond to create an epoxide. This reaction is notable for its simplicity and efficiency, primarily forming epoxides with high regio- and stereoselectivity.

Epoxidations are useful in creating epoxides that can further undergo various transformations, making them versatile intermediates in organic synthesis. This formation of three-membered rings is crucial for the further functionalization of molecules into complex natural products and pharmaceuticals.

Three-Membered Rings

Three-membered rings are a fascinating subject due to their unique properties and strain energy. These rings, consisting of three connected atoms, are significantly strained compared to larger rings. This strain arises because the bond angles in a three-membered ring, such as in epoxides or cyclopropanes, deviate considerably from the ideal tetrahedral angle of 109.5 degrees.

For instance, in an epoxide, the oxygen atom forms bonds with two carbon atoms, creating a ring with bond angles around 60 degrees. This angle creates tension within the ring, leading to increased reactivity.

• Epoxides: Oxygen-containing three-membered rings.
• Cyclopropanes: Comprised of three carbon atoms forming a ring.

Because of the tension, these rings are reactive and can participate in additional reactions that are not possible for less strained rings. Their unique properties make three-membered rings an essential topic for chemists focusing on reaction mechanisms and synthetic applications.

Reaction Mechanisms

In organic chemistry, understanding reaction mechanisms is essential for predicting the outcomes of chemical reactions, such as the formation of three-membered rings. A reaction mechanism describes the step-by-step sequence of events that occur during a chemical reaction.

Mechanisms typically involve:

• Bond breaking and forming stages.
• Transition states and intermediates.
• Electron movement denoted by curved arrows.

The epoxidation mechanism using mCPBA, for example, involves such steps and is an excellent demonstration of a concerted pathway, where bond breaking and forming occur simultaneously, forming the epoxide directly with no intermediates.

Understanding these mechanisms is crucial for designing synthetic routes in chemistry, as they reveal how changes to reactants or conditions can influence the rate and outcome of a reaction. Clear comprehension of these processes allows chemists to create more efficient and selective syntheses of desired compounds.

Cyclic Compounds

Cyclic compounds, as opposed to linear ones, are a major category in organic chemistry and include structures where atoms are connected to form a loop or ring. These can range from small three-membered rings to large macrocyclic structures.

Cyclic compounds:

• Can be fully saturated or contain unsaturations like double bonds.
• Often exhibit special properties due to ring strain or aromaticity.

Small rings, like cyclopropanes or epoxides, are particularly interesting due to their high ring strain, which makes them more reactive. This reactivity is essential in organic synthesis, as it allows the introduction of new functional groups and the building of more complex structures.

By leveraging the unique properties of cyclic compounds, chemists can develop new materials, drugs, and molecules with specific desired functionalities, showcasing the importance and versatility of these compounds in scientific advancement.