Which of the following reactions follows concerted mechanism ?

Options

A. `S_(N)1`
B. `S_(N)2`
C. `E_(1)`
D. `E_(2)`

Answer

To determine which of the following reactions follows a concerted mechanism, we need to analyze each option based on the definition of a concerted mechanism. A concerted mechanism involves bond breaking and bond formation occurring in a single step.

### Step-by-Step Solution:

1. **Understanding the Mechanisms**:
- **SN1 Mechanism**: This is a unimolecular nucleophilic substitution reaction. It involves two steps: first, the leaving group departs, forming a carbocation intermediate, and then the nucleophile attacks the carbocation. Since bond breaking and bond formation occur in different steps, SN1 does not follow a concerted mechanism.
- **SN2 Mechanism**: This is a bimolecular nucleophilic substitution reaction. In this case, the nucleophile attacks the substrate from the opposite side of the leaving group, leading to a transition state where the bond to the leaving group is partially broken and the bond to the nucleophile is partially formed. Since both bond breaking and bond formation occur in a single step, SN2 follows a concerted mechanism.
- **E1 Mechanism**: This is a unimolecular elimination reaction. Similar to SN1, it involves two steps: first, the leaving group departs to form a carbocation, and then a base abstracts a proton to form a double bond. Because bond breaking and bond formation occur in separate steps, E1 does not follow a concerted mechanism.
- **E2 Mechanism**: This is a bimolecular elimination reaction. In this case, a base abstracts a proton while the leaving group departs simultaneously, leading to the formation of a double bond in a single step. Therefore, E2 follows a concerted mechanism.

2. **Conclusion**:
- From the analysis, we find that:
- SN1: Not concerted (Option A)
- SN2: Concerted (Option B)
- E1: Not concerted (Option C)
- E2: Concerted (Option D)
- The reactions that follow a concerted mechanism are **SN2 (Option B)** and **E2 (Option D)**.

### Final Answer:
The reactions that follow a concerted mechanism are **B (SN2)** and **D (E2)**.