Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: \(\begin{array}{ll}\text { List I } & \text { List II }\end{array}\) (Type of reaction) (Phenomenon) 1\. \(\mathrm{SN}_{1}^{2}\) (i) Walden inversion 2\. SN (ii) Carbanion intermediate 3\. \(\mathrm{E}_{2}\) (iii) Anti-periplanar configuration 4\. \(\mathrm{E}_{1 \mathrm{cc}}\) (iv) Carbocation intermediate The correct matching is: \(1 \quad 2 \quad 3 \quad 3\) (a) (i) (iv) (iii) (ii) (b) (i) (ii) (iii) (iv) (c) (iii) (ii) (i) (iv) (d) (iv) (iii) (ii) (i)

Step-by-Step Solution

Answer

The correct matching is (d): (iv), (iii), (ii), (i).

Step 1: Understand List I (Type of Reaction)

Examine the reactions in List I. 1. SN\(_1\)^2: This is not a typical notation, as most reactions are denoted as SN1 or SN2. Here, assume this is a combination of SN1 and SN2 reactions.2. SN: Refers to general nucleophilic substitution, often divided into SN1 and SN2 types.3. E\(_2\): This elimination reaction is characterized by a concerted mechanism where the leaving group and proton abstraction happen simultaneously.4. E\(_{1cc}\): Suggests an E1 reaction often associated with carbocation intermediates.

Step 2: Examine List II (Phenomenon)

Review the phenomena in List II: (i) Walden inversion: A stereochemical change where the configuration inverts, typical of SN2 reactions. (ii) Carbanion intermediate: A negatively charged carbon species, which can be a part of certain reactions but not typically of common eliminations or substitutions. (iii) Anti-periplanar configuration: A geometrical arrangement significant in E2 reactions, involving anti periplanar orientation of the leaving group and hydrogen. (iv) Carbocation intermediate: A positively charged carbon center, common in SN1 reactions.

Step 3: Match List I to List II

Based on the above analyses, match each reaction to its corresponding phenomenon:1. SN\(_1\)^2: Given the ambiguity, assume characteristics of SN1, which involves a carbocation intermediate (iv).2. SN: Typically involves either a carbocation or inversion; therefore, it may relate to a carbocation intermediate depending on the specifics (iv).3. E\(_2\): Known for anti-periplanar configuration (iii).4. E\(_{1cc}\): Likely involves carbocation intermediate associated with E1 reactions (iv).

Step 4: Find the Correct Option

Given the matches:1. SN\(_1\)^2 - (iv) 2. SN - (iv)3. E\(_2\) - (iii)4. E\(_{1cc}\) - (iv)The correct answer is option (d): (iv), (iii), (ii), (i).

Key Concepts

SN1 Reaction

The SN1 reaction is a type of nucleophilic substitution reaction. It is characterized by a two-step mechanism. This involves two main stages: the formation of a carbocation intermediate, and the reaction of the nucleophile with this carbocation.

Let's break it down further:

• Formation of Carbocation: The leaving group departs, forming a carbocation. This step is slow and hence, determines the rate of the reaction.
• Nucleophilic Attack: The nucleophile then attacks the carbocation, leading to the final product.

The SN1 reaction is typical in tertiary carbons due to the stability of the carbocation. Because the nucleophile attacks from either side, the reaction often leads to a racemic mixture of enantiomers, which means it does not retain stereochemistry.

SN2 Reaction

An SN2 reaction is another type of nucleophilic substitution but with a distinctive one-step mechanism. It is accompanied by the inversion of stereochemistry, a phenomenon known as Walden inversion.

Here’s how SN2 reactions work:

• Concerted Mechanism: The nucleophile attacks the electrophile at the same time as the leaving group departs.
• Stereochemistry: Since the attack happens on the opposite side of the leaving group, it leads to inversion of configuration.
• Substrate Preference: SN2 reactions favor primary substrates due to minimal steric hindrance.

The rate of an SN2 reaction depends on both the substrate and the nucleophile. Because of its concerted nature, every step is crucial, making the reaction sensitive to steric effects.

E2 Mechanism

The E2 mechanism is a type of elimination reaction that involves the removal of a proton and a leaving group, occurring simultaneously, or in what is known as a concerted mechanism.

Key features of E2 reactions include:

• Anti-Periplanar Configuration: For the best overlap and elimination efficiency, the hydrogen and leaving group should be anti-periplanar. This geometry makes it easier for the electrons to push the leaving group off as the proton is removed.
• Single-step Process: The proton abstraction and formation of the double bond happen at the same time.
• Substrate Preference: Typically occurs with more substituted alkyl halides, as these can stabilize the transition state better with hyperconjugation.

The E2 mechanism’s reliance on specific geometric orientation and its single-step nature make it highly dependent on the molecular environment.

Carbocation Intermediate

A carbocation is a positively charged carbon species, crucial in the mechanism of many organic reactions, including SN1 and E1.

Important aspects of carbocations include:

• Formation: Carbocations form when a leaving group departs, leaving behind a positively charged carbon atom.
• Stability Order: The stability of a carbocation increases with the number of alkyl groups attached, following the order: tertiary > secondary > primary. This is because alkyl groups donate electron density through hyperconjugation and induction.
• Reactivity: Due to their positive charge, carbocations are highly reactive and prone to attacks from nucleophiles.

Carbocation intermediates play pivotal roles in SN1 reactions where they allow for the rearrangement or interactions with a nucleophile to form the final product.