Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct? 1\. SN mechanism is most commonly given by tertiary alkyl halides. 2\. \(\mathrm{SN}^{1}\) mechanism proceeds through the formation of a carbocation. 3\. SN \(^{2}\) mechanism involves retention of configuration. 4\. \(\mathrm{SN}^{2}\) mechanism proceeds through the formation of a transition state. (a) 1,2 and 4 (b) 1,3 and 4 (c) 2,3 and 4 (d) \(1,2,3\) and 4

Step-by-Step Solution

Answer

The correct choice is (a) 1, 2, and 4.

Step 1: Analyze Statement 1

Statement 1 claims that the SN mechanism is most commonly given by tertiary alkyl halides. Tertiary alkyl halides are most commonly associated with the SN1 mechanism due to the stability of the tertiary carbocation formed. Therefore, this statement is correct when considering the SN1 mechanism.

Step 2: Analyze Statement 2

Statement 2 states that the SN1 mechanism proceeds through the formation of a carbocation. This is true as the SN1 mechanism involves the formation of a carbocation in its first step, where the leaving group departs, creating a carbocation.

Step 3: Analyze Statement 3

Statement 3 mentions that the SN2 mechanism involves retention of configuration. This is incorrect because the SN2 mechanism involves a backside attack, leading to an inversion of configuration, not retention.

Step 4: Analyze Statement 4

Statement 4 claims that the SN2 mechanism proceeds through the formation of a transition state. This is correct because the SN2 reaction involves a concerted mechanism where the nucleophile attacks the substrate directly and simultaneously as the leaving group leaves, forming a transition state.

Step 5: Evaluate Which Statements Are Correct

Based on the analysis, statements 1, 2, and 4 are correct. Statement 3 is incorrect because SN2 results in inversion of configuration rather than retention. Therefore, the correct statements are: 1, 2, and 4.

Key Concepts

Tertiary Alkyl Halides

Tertiary alkyl halides are a central player in nucleophilic substitution reactions, especially the SN1 mechanism. These compounds are characterized by a carbon atom bonded to three alkyl groups and one halogen atom. The crowded environment around the carbon atom stabilizes the positive charge needed for carbocation formation. This makes them highly reactive in SN1 reactions.

Here's why tertiary alkyl halides are more favorable for SN1 reactions:

• The carbocation formed during the SN1 mechanism is stabilized by the surrounding alkyl groups through hyperconjugation and inductive effects.
• This stability reduces the energy barrier for the reaction to proceed, making the SN1 pathway more feasible.

Unlike primary and secondary alkyl halides, tertiary ones don't frequently undergo SN2 reactions due to steric hindrance. This crowding prevents the backside attack needed for the SN2 mechanism.

Carbocation Formation

Carbocation formation is a crucial and defining step in the SN1 mechanism. When the leaving group departs from the substrate, it leaves behind a positively charged carbon atom, known as a carbocation.

Carbocation stability is influenced by several factors:

• Alkyl groups attached to the carbocation can donate electron density through the inductive effect, which helps to stabilize the positive charge.
• In SN1 reactions, tertiary carbocations are more stable than secondary or primary ones due to the greater number of alkyl groups available for stabilization.

The formation of a stable carbocation is essential before the nucleophilic attack, making this step crucial in determining the reaction rate and pathway.

Stereochemistry in SN2 Reactions

The stereochemistry of SN2 reactions is quite unique compared to SN1. SN2 reactions involve a direct displacement where the nucleophile attacks the substrate from the opposite side of the leaving group. This is known as a backside attack, leading to an interesting phenomenon called "inversion of configuration."

Here's what happens during an SN2 reaction:

• The nucleophile approaches from the rear, behind the plane of the molecule.
• This results in flipping the configuration of the molecule, much like an umbrella turning inside out.

Therefore, the SN2 mechanism does not retain the original configuration but instead inverts it. This inversion is a distinguishing feature of SN2 reactions and is crucial for understanding the stereochemical outcomes involved.

Transition State in SN2 Mechanism

In the SN2 mechanism, the transition state is a fleeting and critical point where the nucleophile and the leaving group are both partially attached to the substrate. This creates a temporary, high-energy structure where bonds are simultaneously being broken and formed.

Key characteristics of the SN2 transition state include:

• The nucleophile partially bonds with the substrate while the leaving group begins to depart.
• This simultaneous interaction makes the transition state particularly unstable and short-lived.

Understanding the transition state is important because it represents the moment of highest energy in the reaction pathway, dictating the rate of the reaction. Unlike reactions requiring carbocation intermediates, the SN2 mechanism progresses in one continuous, concerted step, passing swiftly through this critical juncture.