Which of the following is not true for `S_(N^(1))` reaction ?

Options

A. They occur through a single step concerted reaction
B. They are favoured by polar solvents
C. `3^(@)` alkyl halides generally react through this mechanism
D. Concentration of nucleophile does not affect the rate of such reaction

Answer

To determine which statement is not true for the \( S_N1 \) reaction, we need to analyze each of the provided options.

### Step-by-Step Solution:

1. **Understanding \( S_N1 \) Mechanism**:
- The \( S_N1 \) (Substitution Nucleophilic Unimolecular) reaction is a two-step mechanism. The first step involves the formation of a carbocation, and the second step involves the nucleophile attacking the carbocation.

2. **Analyzing the Options**:
- **Option A**: "They occur through a single step concerted reaction."
- This statement is **not true**. The \( S_N1 \) reaction occurs through two steps: the formation of a carbocation and the subsequent attack by the nucleophile.

- **Option B**: "They are favored by polar solvents."
- This statement is **true**. Polar solvents stabilize the carbocation intermediate formed during the reaction, making the \( S_N1 \) reaction more favorable.

- **Option C**: "Tertiary alkyl halides generally react through this mechanism."
- This statement is **true**. Tertiary alkyl halides form stable tertiary carbocations, which favor the \( S_N1 \) mechanism.

- **Option D**: "Concentration of nucleophile does not affect the rate of such reaction."
- This statement is **true**. In \( S_N1 \) reactions, the rate is determined only by the concentration of the alkyl halide, as the formation of the carbocation is the rate-determining step.

3. **Conclusion**:
- The only statement that is not true for the \( S_N1 \) reaction is **Option A**: "They occur through a single step concerted reaction."

### Final Answer:
**Option A** is the statement that is not true for \( S_N1 \) reactions.

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