The reactivity order of alkyl halide is `3^(@) gt 2^(@) gt 1^(@)` in

Options

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

Answer

To determine the reactivity order of alkyl halides (tertiary, secondary, and primary) in different types of reactions, we need to analyze the mechanisms involved in these reactions. The question specifically asks about the reactivity order of alkyl halides in the context of SN1 and SN2 reactions.

### Step-by-Step Solution:

1. **Understanding Alkyl Halides**: Alkyl halides are organic compounds containing carbon and halogen atoms. Their reactivity can vary based on the structure of the alkyl group (primary, secondary, or tertiary) and the type of nucleophilic substitution or elimination reaction (SN1, SN2, E1, E2).

2. **SN1 Mechanism**: The SN1 (unimolecular nucleophilic substitution) mechanism involves the formation of a carbocation intermediate. The rate-determining step is the formation of this carbocation, which is influenced by the stability of the carbocation.

3. **Carbocation Stability**:
- Tertiary carbocations (3°) are the most stable due to hyperconjugation and the inductive effect from surrounding alkyl groups.
- Secondary carbocations (2°) are less stable than tertiary but more stable than primary.
- Primary carbocations (1°) are the least stable and are rarely formed.

4. **Reactivity Order in SN1**: Since the stability of the carbocation dictates the rate of the reaction in SN1, the reactivity order for alkyl halides in SN1 reactions is:
\[ \text{Tertiary (3°) > Secondary (2°) > Primary (1°)} \]

5. **SN2 Mechanism**: The SN2 (bimolecular nucleophilic substitution) mechanism involves a direct attack of the nucleophile on the carbon atom, leading to a transition state where both the nucleophile and the leaving group are partially bonded to the carbon.

6. **Reactivity Order in SN2**: In SN2 reactions, steric hindrance plays a significant role. Tertiary alkyl halides are hindered and less reactive, while primary alkyl halides are less hindered and more reactive. Thus, the reactivity order for SN2 is:
\[ \text{Primary (1°) > Secondary (2°) > Tertiary (3°)} \]

7. **Conclusion**: The reactivity order of alkyl halides in SN1 reactions is tertiary > secondary > primary, while in SN2 reactions, it is primary > secondary > tertiary. Therefore, the reactivity order of alkyl halides is specifically applicable to SN1 reactions when considering the stability of carbocations.

### Final Answer:
The reactivity order of alkyl halides in SN1 reactions is:
\[ 3^{(@)} > 2^{(@)} > 1^{(@)} \]