Which of the following can’t be an example of an `S_(N)`2 reaction?

Options

A. `CH_(3)- CH_(2) - OH overset(-H_(2)O)(rarr) CH_(2) = CH_(2)`
B. `(CH_(3))_(2) "CHBr" overset(OH^(-))(rarr) (CH_(3))_(2) CH - OH + Br^(-)`
C. `CH_(3) Br overset(OH^(-))(rarr) CH_(3)OH + Br^(-)`
D. `(CH_(3))_(3) C - Br overset(OH^(-))(rarr)(CH_(3))_(3) C-OH + OH^(-)`

Answer

To determine which of the following cannot be an example of an SN2 reaction, we need to analyze each option provided.

### Step-by-Step Solution:

1. **Understanding SN2 Reactions**:
- SN2 (Substitution Nucleophilic Bimolecular) reactions involve a nucleophile attacking an electrophile (usually a carbon atom bonded to a leaving group) in a single concerted step. This results in the simultaneous bond formation and bond breaking.

2. **Analyzing the Options**:
- **Option A: Dehydration of Alcohol**:
- Dehydration of alcohol typically involves the elimination of water (H2O) and is not a nucleophilic substitution reaction. Instead, it usually follows an E1 or E2 mechanism, where a double bond is formed. Therefore, this cannot be an example of an SN2 reaction.

- **Option B: Isopropyl Bromide Reaction with OH⁻**:
- In this case, isopropyl bromide (a secondary alkyl halide) reacts with hydroxide ion (OH⁻). This reaction can proceed via an SN2 mechanism, where the nucleophile attacks the carbon atom and displaces the bromine atom. Thus, this is a valid SN2 reaction.

- **Option C: Methyl Bromide Reaction with OH⁻**:
- Methyl bromide (a primary alkyl halide) reacting with hydroxide ion (OH⁻) is a classic example of an SN2 reaction. The nucleophile attacks the carbon atom, leading to the substitution of bromine with hydroxide. This is also a valid SN2 reaction.

3. **Conclusion**:
- After analyzing all options, we can conclude that the dehydration of alcohol (Option A) cannot be classified as an SN2 reaction.

### Final Answer:
The correct answer is **Option A: Dehydration of alcohol** cannot be an example of an SN2 reaction.