Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is an example of \(\mathrm{S}_{\mathrm{N}^{2}}\) reaction? (a) \(\mathrm{CH}_{3} \mathrm{Br}+\mathrm{OH} \longrightarrow \mathrm{CH}_{3} \mathrm{OH}+\mathrm{Br}\) (b) CC(Br)CC(O)CBr (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH} \underset{\text { Heat }}{\mathrm{H}_{2}} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}\) (d)

Step-by-Step Solution

Answer

The reaction in option (a) is an example of an \( \mathrm{S}_{\mathrm{N}^{2}} \) reaction.

Step 1: Identify Key Reaction Type

An \( \mathrm{S}_{\mathrm{N}^{2}} \) reaction involves a bimolecular nucleophilic substitution, where the nucleophile attacks the substrate, forcing the leaving group to depart simultaneously. It typically occurs in one step involving a transition state.

Step 2: Analyze Option (a)

The reaction \( \mathrm{CH}_{3} \mathrm{Br} + \mathrm{OH}^{-} \longrightarrow \mathrm{CH}_{3} \mathrm{OH} + \mathrm{Br}^{-} \) shows a nucleophile \( \mathrm{OH}^{-} \) attacking the carbon of \( \mathrm{CH}_{3} \mathrm{Br} \) and displacing the \( \mathrm{Br}^{-} \) in a single concerted step. This is characteristic of an \( \mathrm{S}_{\mathrm{N}^{2}} \) reaction given the primary carbon and strong nucleophile.

Step 3: Evaluate Option (b)

The given SMILES string represents a complex molecule, \( \mathrm{CC(Br)CC(O)CBr} \), where recognizing an \( \mathrm{S}_{\mathrm{N}^{2}} \) if it occurs is complex due to multiple possible reaction sites and the given format. The absence of any specific attacking nucleophile indicates it is less likely an intentional \( \mathrm{S}_{\mathrm{N}^{2}} \) example.

Step 4: Review Option (c)

This reaction \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH} \underset{\text { Heat }}{\rightarrow} \mathrm{H}_{2} \mathrm{O} + \mathrm{H}_{2} \mathrm{C} = \mathrm{CH}_{2} \) is an elimination reaction (\( \mathrm{E1} \) or \( \mathrm{E2} \)), where a molecule of water leaves, forming a double bond. Such reactions are not \( \mathrm{S}_{\mathrm{N}^{2}} \) type.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental concept in organic chemistry. It involves the exchange of one functional group in a molecule for another. Here, the incoming group is known as the nucleophile, and it essentially 'attacks' the substrate. The leaving group is the one that is displaced in this chemical process.

In an SN2 reaction, the nucleophile and the leaving group exchange places in one smooth process. This means everything happens in a single, swift step. It’s like a dance, where one partner leaves as another steps in all in the same beat.

• The nucleophile: It is often rich in electrons and can donate a pair to form a new bond.
• The leaving group: This is usually a group that can stabilize itself by accepting electrons as it leaves.

Because the reaction is concerted, there’s no pause or intermediate formation; instead, the nucleophile attacks and bonds while the leaving group simultaneously departs.

Reaction Mechanism

Understanding reaction mechanisms in organic chemistry is crucial because it explains how and why reactions proceed. It illustrates the step-by-step sequence of elementary reactions by which overall chemical change occurs. In the context of an SN2 reaction, the mechanism is particularly unique.

The SN2 mechanism is distinguished by its 'bimolecular' nature. This means that the reaction rate depends on the concentration of both the nucleophile and the substrate. In other words, when both reactants collide with proper orientation, the nucleophile attacks from the opposite side of the leaving group. This back-side attack causes inversion of configuration, resembling an umbrella flipping inside-out.

• Bimolecular: The name SN2 where '2' stands for bimolecular, highlighting that two species are involved in the rate-determining step.
• Backside Attack: Essential for causing the specific stereochemical outcomes, like inversion of configuration.

This mechanism is especially common with primary carbon substrates where less steric hindrance allows easier access for the nucleophile.

Organic Chemistry

Organic chemistry is the branch of chemistry dealing with carbon-containing compounds, their properties, compositions, reactions, and syntheses. It plays a crucial role in the development of everyday compounds and complex biological processes. SN2 reactions are a type of organic reaction critical in synthesizing a wide array of chemical products.

These reactions are vital in transforming molecules and enable chemists to build complex structures from simpler pieces. SN2 reactions are favored in organic synthesis for their simplicity and predictability in many cases.

• Chemical Synthesis: SN2 reactions can be used to synthesize a broad range of products, modifying functional groups effectively.
• Carbon-Based Chemistry: Carbon’s ability to form stable bonds allows endless possibilities for compound creation through different organic reactions.

A good grasp of organic chemistry and reaction mechanisms like SN2 is essential for anyone interested in chemistry related fields or industrial applications.