Through `S_(N)2` reaction we cannot convert

Options

A. `CH_(3) -CH_(2) -CI " into " CH_(3) CH_(2) - CN`
B. `CH_(3) - underset(CH_(3))underset(|)(CH) - Br ` into
`CH_(3) -underset(CH_(3))underset(|)(CH) -O -overset(CH_(3))overset(|)underset(CH_(3))underset(|)(C ) -CH_(3)`
C. `CH_(3) -underset(CH_(3))underset(|)(CH) -Br` into
`CH_(3) -underset(CH_(3))underset(|)(CH ) - O - overset(O) overset(||)(C ) - CH_(3)`
D.

Answer

To determine which compound cannot be converted through an \( S_N2 \) reaction, we need to analyze each option provided in the question. The \( S_N2 \) reaction is a bimolecular nucleophilic substitution reaction where a nucleophile attacks a carbon atom, leading to the displacement of a leaving group.

### Step-by-Step Solution:

1. **Option A: CH₃CH₂Cl to CH₃CH₂CN**
- In this case, the nucleophile \( CN^- \) can attack the carbon atom bonded to \( Cl \) in \( CH₃CH₂Cl \).
- The \( Cl \) atom acts as a leaving group, and the reaction proceeds smoothly to form \( CH₃CH₂CN \).
- Therefore, this conversion **can** occur via an \( S_N2 \) reaction.

2. **Option B: CH₃C(CH₃)CH₂Br to a bulky product**
- Here, the structure indicates a tertiary carbon due to the bulky \( C(CH₃) \) group.
- The nucleophile (which is bulky, such as \( K^+ \) with \( t-BuO^- \)) will not effectively perform an \( S_N2 \) reaction because it is a poor nucleophile due to steric hindrance.
- Instead, it will likely undergo an elimination reaction (E2) rather than substitution.
- Therefore, this conversion **cannot** occur via an \( S_N2 \) reaction.

3. **Option C: CH₃CHBrCH₃ to CH₃CHCH₃OC(=O)CH₃**
- The nucleophile \( OCOCH₃^- \) can attack the carbon bonded to \( Br \).
- The \( Br \) will leave as a leaving group, allowing the formation of the desired product.
- This conversion **can** occur via an \( S_N2 \) reaction.

4. **Option D: A compound with bromine, phenyl, deuterium, and hydrogen**
- The nucleophile \( CH₃^- \) can attack the carbon bonded to \( Br \).
- The \( Br \) will leave, allowing for the formation of a new compound.
- This conversion **can** also occur via an \( S_N2 \) reaction.

### Conclusion:
The only compound that cannot be converted through an \( S_N2 \) reaction is **Option B**: CH₃C(CH₃)CH₂Br to the bulky product.