Which mechanism has different reactivity order of alkyl halides `(1^@,2^@,3^@)` than others :

Options

A. `S_N1`
B. `S_N2`
C. E1
D. E2

Answer

To determine which mechanism has a different reactivity order of alkyl halides (1°, 2°, 3°) than others, we need to analyze the reactivity orders for the different mechanisms: SN1, SN2, E1, and E2.

### Step-by-Step Solution:

1. **Understanding the Mechanisms**:
- **SN1 (Substitution Nucleophilic Unimolecular)**: This mechanism involves a two-step process where the first step is the formation of a carbocation intermediate. The reactivity order is based on the stability of the carbocation formed.
- **SN2 (Substitution Nucleophilic Bimolecular)**: This is a one-step mechanism where the nucleophile attacks the substrate as the leaving group departs. The reactivity order is influenced by steric hindrance.
- **E1 (Elimination Unimolecular)**: Similar to SN1, it involves the formation of a carbocation and then the elimination of a proton to form a double bond. The reactivity order is the same as SN1.
- **E2 (Elimination Bimolecular)**: This is a concerted mechanism where the base removes a proton while the leaving group departs. The reactivity order is similar to SN2.

2. **Reactivity Orders**:
- **SN1**: Tertiary > Secondary > Primary
- **SN2**: Primary > Secondary > Tertiary
- **E1**: Tertiary > Secondary > Primary
- **E2**: Tertiary > Secondary > Primary

3. **Identifying the Different Order**:
- From the reactivity orders listed, we see that:
- SN1 and E1 have the same order: Tertiary > Secondary > Primary.
- E2 and SN1 also share the same order: Tertiary > Secondary > Primary.
- However, SN2 has a different order: Primary > Secondary > Tertiary.

4. **Conclusion**:
- The mechanism that has a different reactivity order of alkyl halides compared to the others is **SN2**.

### Final Answer:
The mechanism that has a different reactivity order of alkyl halides (1°, 2°, 3°) than others is **SN2**.