An `S_(N^2)` reaction occurs through the formation of a

Options

A. carbocation
B. carbanion
C. free radical
D. transition state.

Explanation

To solve the question regarding the mechanism of an SN2 reaction, we need to understand the characteristics of this type of reaction. Here’s a step-by-step breakdown:

### Step 1: Understand the SN2 Mechanism
The SN2 (Substitution Nucleophilic Bimolecular) reaction is characterized by a single-step mechanism where the nucleophile attacks the substrate at the same time as the leaving group departs.

**Hint:** Recall that SN2 reactions involve a nucleophile and a substrate, and they occur in one concerted step.

### Step 2: Identify the Transition State
In an SN2 reaction, there is no formation of a stable intermediate like a carbocation. Instead, there is a transition state where the nucleophile is partially bonded to the substrate while the leaving group is partially leaving.

**Hint:** Remember that the transition state is a high-energy state that cannot be isolated; it represents the point of maximum energy along the reaction pathway.

### Step 3: Recognize the Role of Bonds
During the SN2 reaction, one bond is formed (between the nucleophile and the carbon) while another bond is broken (between the carbon and the leaving group). This simultaneous process is crucial to the mechanism.

**Hint:** Focus on the concept of bond formation and bond breaking occurring at the same time in the reaction.

### Step 4: Conclusion
Since the question asks about the formation during an SN2 reaction, the correct answer is that an SN2 reaction occurs through the formation of a transition state.

**Final Answer:** The correct answer is "transition state."