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

Question

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

Step-by-Step Solution

Answer

The correct matching is option (a): 1-i, 2-iv, 3-iii, 4-ii.

Step 1: Understanding SN2 Reaction

The SN2 reaction is a bimolecular nucleophilic substitution reaction. It is known for its mechanism that results in a Walden inversion, where the configuration of the chiral center is inverted.

Step 2: Understanding SN1 Reaction

The SN1 reaction is a unimolecular nucleophilic substitution reaction. It involves the formation of a carbocation intermediate, which is a key feature of its mechanism.

Step 3: Understanding E2 Reaction

The E2 reaction is a bimolecular elimination reaction. It requires an anti-periplanar configuration between the leaving group and the hydrogen atom that is removed to form a double bond.

Step 4: Understanding E1cs Reaction

The E1cs reaction is an elimination reaction that involves a carbanion intermediate, which is formed during the course of the reaction.

Step 5: Matching List I with List II

Using the information from the previous steps: - SN2 (2): Walden inversion (i) - SN1: Carbocation intermediate (iv) - E2: Anti-periplanar configuration (iii) - E1cs: Carbanion intermediate (ii) The correct matching is (a).

Key Concepts

SN1 Reaction

The SN1 reaction, also known as a unimolecular nucleophilic substitution reaction, involves a two-step mechanism. This type of reaction typically occurs in polar protic solvents which stabilize the carbocation intermediate. The term "unimolecular" indicates that the rate of the reaction depends only on the concentration of the substrate, not the nucleophile. During an SN1 reaction, the bond between the carbon atom and the leaving group breaks, forming a positively charged carbocation. This step is the slowest and therefore the rate-determining step. Once the carbocation is formed, it rapidly reacts with a nucleophile to form the final product.

SN2 Reaction

The SN2 reaction, short for bimolecular nucleophilic substitution, is a one-step reaction mechanism. Here, the nucleophile attacks the substrate from the opposite side of the leaving group, resulting in a Walden inversion. This inversion leads to a stereochemical change in the molecule, effectively flipping the configuration at the chiral center. Unlike SN1 reactions, SN2 reactions depend on the concentration of both the substrate and the nucleophile, making it a second-order reaction. Such reactions generally occur in polar aprotic solvents, which enhance the nucleophile's strength and its ability to attack the substrate effectively.

E2 Reaction

The E2 reaction, or bimolecular elimination reaction, is a one-step process involving the removal of a proton and a leaving group from the same molecule to form a double bond. This process requires a particular arrangement known as anti-periplanar configuration, where the hydrogen atom and the leaving group are on opposite sides of the molecule. This reaction is concerted, meaning the proton removal and leaving-group expulsion happen simultaneously. E2 reactions are often favored in strong bases and occur with substrates that can form stable alkenes. The stereochemistry of the starting material can influence the final product due to the anti-periplanar requirement.

E1cs Reaction

The E1cs reaction stands for the Elimination 1-conjugate substitution reaction. It is an elimination process characterized by the formation of a transient carbanion intermediate. Unlike its counterparts, E2 reactions, which are concerted, the E1cs mechanism occurs in steps. The initial step involves the loss of a leaving group, which generates a carbanion. This intermediate can then deprotonate to form a double bond. Such reactions are typically facilitated by the presence of a strong base, which helps stabilize the negatively charged carbanion.

Nucleophilic Substitution

Nucleophilic substitution reactions are fundamental in organic chemistry, involving the replacement of a leaving group by a nucleophile. These reactions contrast with elimination reactions that remove elements from a compound without replacement. The two primary types are SN1 and SN2. In SN1, the leaving group departs before the nucleophile approaches, leading to a carbocation intermediate. In SN2, the nucleophile attacks as the leaving group departs, creating a direct inversion of configuration at the carbon center. Understanding these mechanisms helps in predicting reaction outcomes and in synthesizing desired products.

Elimination Reactions

Elimination reactions are processes where elements are removed from a molecule, resulting in the formation of double or triple bonds. These reactions can follow different mechanisms, primarily E1 and E2. In E1 reactions, the process occurs in two steps and involves carbocation intermediates. E2 reactions, conversely, are one-step processes where the leaving group and a proton are removed in a concerted fashion, often requiring an anti-periplanar configuration. Recognizing which pathway a reaction will follow depends on various factors including the nature of the substrate and the solvent.

Reaction Mechanisms

Understanding the reaction mechanisms is crucial for grasping how and why chemical reactions occur. A reaction mechanism maps out the step-by-step molecular events leading from reactants to products. It includes all intermediate structures and transition states involved in the process. For nucleophilic substitution and elimination reactions, mechanistic details help predict product configuration and stereochemistry. In SN1 and E1 reactions, carbocation intermediates play a pivotal role. SN2 and E2 reactions, on the other hand, involve simultaneous bond-breaking and making, showing different stereochemical outcomes.

Carbocation Intermediate

A carbocation intermediate is a positively charged species, critical in many organic reactions such as SN1 and E1 mechanisms. Formed when a bond to a leaving group is broken, carbocations are often trigonal planar, allowing nucleophiles to attack from either side, leading to a mixture of stereoisomers. Stability of carbocations follows the order: tertiary > secondary > primary, because of the ability of neighboring atoms to share electron density or hyperconjugation. The stability of these intermediates can significantly influence both the reaction pathway and the speed at which the reaction proceeds.

Carbanion Intermediate

Carbanions are negatively charged intermediates formed during certain organic reactions, particularly those involving deprotonation. In contexts like the E1cs mechanism, carbanions are crucial for understanding the stability and reactivity of the reacting species. Carbanion stability depends on various factors, including the electronegativity of the atoms involved and resonance stabilization. Typically, carbanions tend to be more stable when adjacent to electron-withdrawing groups, which help delocalize the negative charge.

Walden Inversion

Walden inversion is a phenomenon observed in SN2 reactions, where the configuration of a chiral center is inverted as a result of a nucleophilic attack from the opposite side of the leaving group. This inversion creates a mirror image of the initial configuration, significantly changing the compound's stereochemistry. Understanding Walden inversion is essential for predicting the outcome of SN2 reactions, especially when dealing with chiral molecules. This concept helps chemists manipulate stereoisomerism in synthesis, ensuring the desired product configuration is achieved.

Anti-Periplanar Configuration

The anti-periplanar configuration is a spatial arrangement crucial for E2 elimination reactions. It requires that the hydrogen atom and the leaving group are on opposite sides of the molecule, allowing for the most effective orbital overlap during the reaction. This configuration ensures that the elimination proceeds smoothly, forming a stable double bond through synchronous removal of the hydrogen atom and the leaving group. This stereochemical requirement of being "anti" helps in predicting E2 reaction outcomes, promoting the formation of the desired alkene.