Which of the following reaction follows `S_N1` mechanism ?

Options

A. `(CH_3)_3C - CH_2Cl+CH_3OK`
B. `(CH_3)_2CHCH_2Cl+KCN`
C. `(CH_3)_3C-Cl+NaOH(aq).`
D. `(CH_3)_2CHI+H_2O`

Answer

To determine which of the following reactions follows the SN1 mechanism, we need to understand the characteristics of the SN1 mechanism:

### Step-by-Step Solution:

1. **Understanding SN1 Mechanism**:
- The SN1 mechanism is a unimolecular nucleophilic substitution reaction. It involves two steps:
- The first step is the formation of a carbocation intermediate after the leaving group departs.
- The second step is the nucleophile attacking the carbocation.

2. **Rate of Reaction**:
- The rate of an SN1 reaction depends only on the concentration of the substrate (the compound undergoing substitution), not on the concentration of the nucleophile.

3. **Carbocation Stability**:
- The stability of the carbocation formed is crucial. The order of carbocation stability is:
- Tertiary (most stable) > Secondary > Primary (least stable).
- A more stable carbocation is more likely to form in an SN1 reaction.

4. **Polar Protic Solvent**:
- SN1 reactions typically occur in polar protic solvents, which can stabilize the carbocation and the leaving group.

5. **Analyzing the Options**:
- **Option 1**: If the substrate forms a primary carbocation and there is no polar protic solvent, this option cannot follow the SN1 mechanism.
- **Option 2**: If the substrate forms a secondary carbocation and there is no polar protic solvent, this option also cannot follow the SN1 mechanism.
- **Option 3**: If the substrate forms a tertiary carbocation but lacks a polar protic solvent, this option is not correct for SN1.
- **Option 4**: If the substrate is CH3CH2CHI, it can form a tertiary carbocation, and if a polar protic solvent is present, this option can follow the SN1 mechanism.

6. **Conclusion**:
- Based on the analysis, **Option 4** is the correct answer as it follows the SN1 mechanism due to the presence of a polar protic solvent and the formation of a stable carbocation.

### Final Answer:
**Option 4** (CH3CH2CHI) follows the SN1 mechanism.