Most reactive halide towards `S_(N^(1))` reactions is

Options

A. n-butyl chloride
B. sec-butyl chloride
C. tert -butyl chloride
D. Allyl chloride

Answer

To determine the most reactive halide towards SN1 reactions, we need to analyze the stability of the carbocations formed during the reaction. The stability of the carbocation is crucial because the more stable the carbocation, the more reactive the halide will be in an SN1 reaction.

### Step-by-Step Solution:

1. **Understanding SN1 Mechanism**:
- The SN1 (Substitution Nucleophilic Unimolecular) reaction involves the formation of a carbocation intermediate after the leaving group (in this case, a halide) departs. The stability of this carbocation is key to determining the reactivity of the halide.

**Hint**: Recall that the stability of carbocations increases with the degree of substitution (tertiary > secondary > primary).

2. **Analyzing the Given Halides**:
- **N-butyl chloride (1-chlorobutane)**:
- Structure: CH3-CH2-CH2-CH2-Cl
- Upon losing Cl, it forms a primary carbocation (CH3-CH2-CH2-CH2+), which is the least stable.

**Hint**: Identify the type of carbocation formed from each halide.

- **Secondary butyl chloride (2-chlorobutane)**:
- Structure: CH3-CH2-CH(Cl)-CH3
- Upon losing Cl, it forms a secondary carbocation (CH3-CH2-CH+-CH3), which is more stable than a primary carbocation but less stable than a tertiary one.

**Hint**: Compare the stability of primary vs. secondary carbocations.

- **Tertiary butyl chloride (t-butyl chloride)**:
- Structure: (CH3)3C-Cl
- Upon losing Cl, it forms a tertiary carbocation ((CH3)3C+), which is quite stable due to hyperconjugation and inductive effects from the three methyl groups.

**Hint**: Remember that tertiary carbocations are the most stable among the three types.

- **Allyl chloride**:
- Structure: CH2=CH-CH2-Cl
- Upon losing Cl, it forms an allylic carbocation (CH2=CH-CH2+). This carbocation is stabilized by resonance, making it even more stable than a tertiary carbocation.

**Hint**: Consider the effect of resonance on carbocation stability.

3. **Comparing Stability**:
- **Stability Order**:
- Allylic carbocation > Tertiary carbocation > Secondary carbocation > Primary carbocation.
- Therefore, allylic chloride is the most reactive halide towards SN1 reactions because it forms the most stable carbocation.

4. **Conclusion**:
- Based on the analysis, the most reactive halide towards SN1 reactions is **allyl chloride**.

### Final Answer:
**Allyl chloride is the most reactive halide towards SN1 reactions.**