Which of the following statement is correct for ${S_{N}}^{i}$ mechanism?

Q: Which of the following statement is correct for ${S_{N}}^{i}$ mechanism?

Which of the following statement is correct for ${S_{N}}^{i}$ mechanism?

Question

Accepted Answer

The ${S_{N}}^{i}$ mechanism, also known as the Substitution Nucleophilic Internal mechanism, is a specific type of nucleophilic substitution reaction. Let's analyze why Option 1 is correct and why the other options are incorrect:

1. **Option 1: The first step is dissociation into an intimate ion pair.**
   - This statement is correct for the ${S_{N}}^{i}$ mechanism. In this mechanism, the reaction begins with the dissociation of the substrate into an intimate ion pair. This means that the leaving group departs, forming a cation and an anion that remain in close proximity to each other. This intimate ion pair is a key feature of the ${S_{N}}^{i}$ mechanism, as it allows for the subsequent steps of the reaction to occur.

2. **Option 2: In second step the leaving group attacks from back side.**
   - This statement is incorrect. In the ${S_{N}}^{i}$ mechanism, the leaving group does not attack from the back side. Instead, the leaving group departs in the first step, forming an ion pair. The concept of back-side attack is more relevant to the ${S_{N}}^{2}$ mechanism, where the nucleophile attacks from the opposite side of the leaving group, leading to inversion of configuration.

3. **Option 3: There is inversion of configuration.**
   - This statement is incorrect for the ${S_{N}}^{i}$ mechanism. Inversion of configuration is characteristic of the ${S_{N}}^{2}$ mechanism, where a nucleophile attacks from the back side, causing the stereochemistry to invert. In the ${S_{N}}^{i}$ mechanism, the intimate ion pair formation does not necessarily lead to inversion of configuration.

4. **Option 4: An external nucleophile attacks the substrate.**
   - This statement is incorrect for the ${S_{N}}^{i}$ mechanism. The ${S_{N}}^{i}$ mechanism involves the formation of an intimate ion pair, and the reaction proceeds without the involvement of an external nucleophile attacking the substrate. The reaction is more about the internal rearrangement and stabilization of the ion pair rather than an external nucleophilic attack.

In summary, Option 1 is correct because it accurately describes the initial step of the ${S_{N}}^{i}$ mechanism, which involves the dissociation into an intimate ion pair. The other options describe features that are not characteristic of the ${S_{N}}^{i}$ mechanism.