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

Question

Which of the following statements are correct? 1\. \(\mathrm{SN}^{1}\) mechanism is most commonly given by tertiary alkyl halides. 2\. SN \(^{1}\) mechanism proceeds through the formation of a carbocation. 3\. \(\mathrm{SN}^{2}\) 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

Correct statements are 1, 2, and 4; option (a) is correct.

Step 1: Understanding SN1 Mechanism

The SN1 mechanism typically involves two steps. First, the leaving group departs, forming a carbocation intermediate. The second step involves a nucleophile attacking the carbocation. Tertiary alkyl halides usually undergo SN1 reactions because the steric bulk and the ability to stabilize the carbocation favors this mechanism.

Step 2: Analyzing Statement 1

Statement 1 claims that the SN1 mechanism is most commonly given by tertiary alkyl halides. This is true because tertiary carbocations are more stable and therefore more likely to form, facilitating the SN1 pathway.

Step 3: Analyzing Statement 2

Statement 2 asserts that the SN1 mechanism proceeds through the formation of a carbocation. This is correct since the carbocation formation is a key step in the SN1 process.

Step 4: Understanding SN2 Mechanism

The SN2 mechanism is a single-step process where the nucleophile attacks the substrate as the leaving group leaves, forming a transition state. This concerted mechanism results in an inversion, not retention, of configuration at the carbon center.

Step 5: Analyzing Statement 3

Statement 3 suggests that the SN2 mechanism involves retention of configuration. This is false because SN2 reactions actually result in the inversion of configuration due to the backside attack of the nucleophile.

Step 6: Analyzing Statement 4

Statement 4 claims that the SN2 mechanism proceeds through the formation of a transition state. This is true because the SN2 mechanism involves a single, concerted step where a transition state is formed as the nucleophile and leaving group are simultaneously involved.

Step 7: Selecting the Correct Statements

Based on the analysis, Statements 1, 2, and 4 are correct, while Statement 3 is incorrect. Hence, the correct answer is option (a) 1, 2, and 4.

Key Concepts

Carbocation Intermediate

In organic chemistry, a carbocation intermediate plays a pivotal role, especially in the \(\text{SN}^1\) mechanism. When we discuss carbocations, we're talking about positively charged carbon atoms. The formation of a carbocation occurs in the first step of the \(\text{SN}^1\) reaction, after the leaving group departs.

This intermediate is very reactive due to its positive charge, which makes it ready to react quickly with a nucleophile. The stability of a carbocation is crucial. More stable carbocations lead to faster reaction rates. Generally, the stability order is:

• Tertiary carbocations, which are attached to three other carbons, are the most stable.
• Secondary carbocations, connected to two carbons, are less stable.
• Primary carbocations, attached to only one carbon, are the least stable.

This hierarchy helps explain why tertiary alkyl halides favor the \(\text{SN}^1\) mechanism, as they form more stable carbocation intermediates.

Tertiary Alkyl Halides

Tertiary alkyl halides are organic compounds where a halogen atom is bonded to a tertiary carbon atom. This is a carbon attached to three other carbons. Due to their structure, tertiary alkyl halides are less prone to steric hindrance.

In \(\text{SN}^1\) reactions, the bulky nature of tertiary carbons stabilizes the carbocation intermediate through both inductive effects and hyperconjugation. This makes the formation of the carbocation more energetically favorable, thus promoting \(\text{SN}^1\) pathways over \(\text{SN}^2\).

• Due to less steric hindrance, tertiary alkyl halides easily lose the leaving group, forming a carbocation.
• The stability and abundance of such intermediates ensure a swift and favorable nucleophilic attack.

Consequently, these compounds exhibit behavior that aligns well with the characteristics of \(\text{SN}^1\) rather than \(\text{SN}^2\) reactions.

Transition State

In an \(\text{SN}^2\) mechanism, the concept of a transition state is vital. Unlike the two-step process of the \(\text{SN}^1\) mechanism, \(\text{SN}^2\) occurs in one concerted step. During this, no carbocation is formed, and instead, a transition state emerges.

The transition state is a high-energy, unstable arrangement where the nucleophile is partially bonded to the carbon, and the leaving group is partially detaching. This temporary state represents the peak energy point along the reaction pathway.

• The role of the transition state is crucial in determining reaction speed. As the energy barrier is lower, the reaction proceeds faster.
• The transition state marks the transformation point where bonds are simultaneously being formed and broken.

Understanding this concept helps clarify why \(\text{SN}^2\) reactions are characterized by simultaneous bond-making and bond-breaking events, setting them apart from the step-wise \(\text{SN}^1\) reactions.

Configuration Inversion

In the fascinating world of chemistry, configuration inversion is a hallmark of the \(\text{SN}^2\) mechanism. During such reactions, the configuration of the molecule at the carbon atom in question gets flipped. Imagine an umbrella turning inside out during a windy day!

This occurs due to the backside attack of the nucleophile. The incoming nucleophile approaches from the side opposite the leaving group. Here's how it unfolds:

• The nucleophile pushes its way in, causing the bonds around the central carbon to rearrange.
• As the leaving group departs from the opposite side, it results in inversion of the original configuration.

This inversion process is contrary to retention of configuration. It's a distinguishing property of \(\text{SN}^2\) reactions and contributes to the unique characteristics of products formed through this mechanism.