Question

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

Short answer

The correct matching is option (b).

Step-by-step solution

Understanding the Reaction Types

We are given different types of reaction mechanisms: \( \mathrm{SN}_{1} \), \( \mathrm{SN}^{i} \), \( \mathrm{E}_{2} \), and \( \mathrm{E}_{1 \mathrm{Cb}} \). Each of these reactions has specific intermediates or configurations associated with them.

Associating Reactions with Phenomena - \( \mathrm{SN}_{1} \) Reaction

The \( \mathrm{SN}_{1} \) reaction proceeds through a carbocation intermediate. This is because in \( \mathrm{SN}_{1} \), the rate-determining step involves the formation of a carbocation after the loss of a leaving group.

Associating Reactions with Phenomena - \( \mathrm{SN}^{i} \) Reaction

The \( \mathrm{SN}^{i} \) (nucleophilic substitution internal) reaction involves Walden inversion, where there is inversion of configuration at the chiral carbon where the substitution occurs.

Associating Reactions with Phenomena - \( \mathrm{E}_{2} \) Reaction

In the \( \mathrm{E}_{2} \) elimination reaction, the anti-periplanar configuration is important for the reaction to proceed. This configuration involves the leaving group and the hydrogen being in an anti arrangement to allow for elimination.

Associating Reactions with Phenomena - \( \mathrm{E}_{1 \mathrm{Cb}} \) Reaction

The \( \mathrm{E}_{1 \mathrm{Cb}} \) reaction goes through a carbanion intermediate. Here, abstraction of a proton forms a carbanion which then allows the leaving group to depart.

Verify the Correct Matching

We have identified:1. \( \mathrm{SN}_{1} \rightarrow \) Carbocation intermediate (4)2. \( \mathrm{SN}^{i} \rightarrow \) Walden inversion (1)3. \( \mathrm{E}_{2} \rightarrow \) Anti-periplanar configuration (3)4. \( \mathrm{E}_{1 \mathrm{Cb}} \rightarrow \) Carbanion intermediate (2)Therefore, the correct option is (b).

Key concepts

Reaction Mechanisms

In chemistry, reaction mechanisms describe the step-by-step sequence of elementary reactions by which overall chemical change occurs. Understanding reaction mechanisms helps us predict the outcome of chemical reactions and the specific intermediates that form during these processes.
For example:

• S_N1 Reaction: Involves a two-step mechanism where a carbocation forms as an intermediate after the loss of a leaving group, followed by the attack of a nucleophile.
• S_Ni Reaction: Known for the inversion at the chiral center, making it a single-step process.
• E₂ Reaction: A one-step mechanism that requires an anti-periplanar arrangement of the leaving group and proton for elimination to occur.
• E_{1 ext{Cb}} Reaction: Involves the formation of a carbanion intermediate due to proton abstraction.

By understanding these mechanisms, chemists can control reactions to yield the desired products with higher efficiency.

Nucleophilic Substitution

Nucleophilic substitution reactions are a class of reactions where a nucleophile replaces a leaving group in a molecular entity. They are a cornerstone of organic chemistry, playing a critical role in the synthesis of various compounds.
Two main types are:

• S_N1 Reaction: This type involves a two-step mechanism, beginning with the formation of a carbocation intermediate. The rate of the reaction depends on the stability of the carbocation, making tertiary carbocations more favorable.
• S_N2 Reaction: This is a single-step reaction where the nucleophile attacks the substrate as the leaving group departs. The rate of reaction depends on the concentration of both the nucleophile and the substrate, and it results in the inversion of configuration at the reaction center, known as Walden inversion.

Understanding these types of reactions is crucial for designing pathways in organic synthesis and in understanding biochemical processes.

Elimination Reactions

Elimination reactions involve the removal of two atoms or groups from a molecule, resulting in the formation of a double bond. They are integral to the synthesis of alkenes and are divided mainly into two types based on their mechanisms.

• E₂ Reaction: This bimolecular reaction requires the base-induced removal of a proton and simultaneous expulsion of a leaving group. The base and substrate concentration both influence the rate of this reaction, occurring in a concerted step with an anti-periplanar alignment between the leaving group and the hydrogen to be removed.
• E₁ Reaction: Occurs through a two-step mechanism, where the first step involves the formation of a carbocation intermediate upon the loss of the leaving group, followed by the removal of a proton to form a double bond. The reaction rate is dependent only on the concentration of the substrate.
• E_{1 ext{Cb}} Reaction: Uniquely, this reaction forms a carbanion intermediate before the expulsion of the leaving group. It is less common and occurs under specific conditions where the carbanion is stabilized by resonance or inductive effects.

Comprehending these reactions is essential for strategic planning in the synthesis and modification of organic compounds.

Carbanion Intermediate

A carbanion is an anion in which carbon bears a negative charge. It is a crucial intermediate in many organic reactions, particularly elimination and nucleophilic substitution reactions.
Carbanions are formed by:

• Deprotonation of carbon atoms often facilitated by a base.
• Stabilization through resonance, electronegative substituents, or inductive effects that delocalize the negative charge.

For instance, in the E_{1 ext{Cb}} mechanism, the removal of a proton leads to the formation of a carbanion, which subsequently allows the leaving group to leave, forming a double bond.
Understanding the stability and reactivity of carbanions allows chemists to predict and control their role in various reactions. They are critical in reactions where a carbanion acts as a nucleophile to form new C-C bonds or participate in elimination reactions.