Which of the following haloalkanes reacts with aqueous KOH most easily ? Explain giving reason.
(i). 1-Bromobutane
(ii) 2-Bromobutane
(iii) 2-Bromo-2-methylpropane
(iv). 2-Chlorobutane.

Options

A. 1-Bromobutane
B. 2-Bromobutane
C. 2-Bromo-2-methylpropane
D. 2-Chlorobutane

Answer

To determine which haloalkane reacts with aqueous KOH most easily, we need to analyze the stability of the carbocations formed during the nucleophilic substitution reaction (SN1 mechanism). The stability of carbocations is crucial because the reaction proceeds through the formation of these intermediates.

### Step-by-Step Solution:

1. **Identify the Haloalkanes**: We have four haloalkanes to consider:
- (i) 1-Bromobutane
- (ii) 2-Bromobutane
- (iii) 2-Bromo-2-methylpropane
- (iv) 2-Chlorobutane

2. **Understand the SN1 Mechanism**: The SN1 reaction mechanism involves two steps:
- Formation of a carbocation intermediate.
- Nucleophilic attack by the hydroxide ion (OH⁻) on the carbocation.

3. **Evaluate Carbocation Stability**:
- **1-Bromobutane**: When it reacts, it forms a primary carbocation (less stable).
- **2-Bromobutane**: This reaction forms a secondary carbocation (more stable than primary).
- **2-Bromo-2-methylpropane**: This forms a tertiary carbocation (most stable).
- **2-Chlorobutane**: This forms a primary carbocation (less stable).

4. **Compare Stability**:
- Tertiary carbocations are the most stable due to hyperconjugation and inductive effects.
- Secondary carbocations are moderately stable.
- Primary carbocations are the least stable.

5. **Conclusion**: Since the stability of the carbocation is the key factor in determining the reactivity of haloalkanes with aqueous KOH, the haloalkane that will react most easily is the one that forms the most stable carbocation.

6. **Final Answer**: Among the given options, **2-Bromo-2-methylpropane** (iii) will react with aqueous KOH most easily because it forms a tertiary carbocation, which is the most stable.