Tertiary alkyl halide are practially inert to substitution by `SN^(2)` mechanism because of-

Options

A. the carbocation formed is unstable
B. there is steric hindrace
C. there is inductive effect
D. the rate of reaction is faster is `S_(N^2)` mechanism.

Explanation

To understand why tertiary alkyl halides are practically inert to substitution by the SN2 mechanism, we can analyze the factors involved in the SN2 reaction.

### Step-by-Step Solution:

1. **Understanding the SN2 Mechanism**:
- The SN2 (Substitution Nucleophilic Bimolecular) mechanism involves a nucleophile attacking the carbon atom that is bonded to the leaving group (halide in this case) in a single concerted step. This results in the simultaneous bond formation and bond breaking.

2. **Steric Hindrance in Tertiary Alkyl Halides**:
- Tertiary alkyl halides have three alkyl groups attached to the carbon atom bonded to the halide. This creates significant steric hindrance, which means that the bulky groups surrounding the carbon make it difficult for the nucleophile to approach and attack the carbon atom.

3. **Impact of Steric Hindrance on Reaction Rate**:
- The rate of an SN2 reaction is inversely proportional to the steric hindrance around the carbon atom. As steric hindrance increases (as in the case of tertiary alkyl halides), the likelihood of the nucleophile successfully attacking the carbon decreases. Therefore, tertiary alkyl halides are less reactive in SN2 reactions.

4. **Comparison with Primary and Secondary Alkyl Halides**:
- In contrast, primary alkyl halides have less steric hindrance (only one alkyl group), making them more reactive in SN2 reactions. Secondary alkyl halides have moderate steric hindrance, leading to intermediate reactivity.

5. **Conclusion**:
- Since tertiary alkyl halides are surrounded by bulky groups that hinder the nucleophile's approach, they are practically inert to substitution by the SN2 mechanism. The correct answer to the question is that steric hindrance is the reason for this inertness.

### Final Answer:
Tertiary alkyl halides are practically inert to substitution by the SN2 mechanism because of steric hindrance.

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