Among the choices of alkyl bromide , the least reactive bromide in a `S_(N^(2))` reaction is :

Options

A. 1-bromopentane
B. 2-bromo-2-methylbutane
C. 1-bromo-3-methylbutane
D. 1-bromo-2-methylbutane.

Explanation

To determine which alkyl bromide is the least reactive in an \( S_N2 \) reaction, we need to consider the reactivity order of alkyl halides. The reactivity of alkyl halides in \( S_N2 \) reactions is influenced by steric hindrance, which affects how easily the nucleophile can approach and attack the carbon atom bonded to the halogen.

### Step-by-Step Solution:

1. **Understand the \( S_N2 \) Mechanism**:
- The \( S_N2 \) (Substitution Nucleophilic Bimolecular) reaction involves a nucleophile attacking the carbon atom that is bonded to the halogen (bromine in this case) and displacing the halogen atom.

2. **Know the Reactivity Order**:
- The general order of reactivity for alkyl halides in \( S_N2 \) reactions is:
- 1° (primary) > 2° (secondary) > 3° (tertiary)
- This means that primary alkyl halides react faster than secondary, which in turn react faster than tertiary alkyl halides.

3. **Identify Steric Hindrance**:
- Steric hindrance refers to the crowding around the reactive site (the carbon atom bonded to the halogen). The more crowded the carbon atom, the less reactive it is in an \( S_N2 \) reaction.
- Tertiary alkyl halides are the most hindered and therefore the least reactive in \( S_N2 \) reactions.

4. **Evaluate the Given Alkyl Bromides**:
- If the options include different types of alkyl bromides (1°, 2°, and 3°), identify which one is tertiary.
- For example, if the options include:
- 1-bromopropane (1°)
- 2-bromobutane (2°)
- 2-bromo-2-methylpropane (3°)
- The 2-bromo-2-methylpropane is a tertiary alkyl bromide and will be the least reactive in \( S_N2 \) reactions due to significant steric hindrance.

5. **Conclusion**:
- Based on the reactivity order and steric hindrance, the least reactive alkyl bromide in an \( S_N2 \) reaction is the one that is tertiary. In this case, it is 2-bromo-2-methylpropane.

### Final Answer:
The least reactive bromide in an \( S_N2 \) reaction is **2-bromo-2-methylpropane**.