Which is the most reactive alkyl halide for the following reaction ?
`RX +H_2OrarrR-OH+HX-`

Options

A.
B.
C.
D.

Answer

To determine the most reactive alkyl halide for the reaction \( RX + H_2O \rightarrow R-OH + HX \), we need to analyze the stability of the carbocations that would form during the reaction. The reactivity of alkyl halides in nucleophilic substitution reactions is influenced by the stability of the carbocation intermediate formed when the halide ion leaves.

### Step-by-Step Solution:

1. **Identify the Reaction Mechanism**:
The reaction involves an alkyl halide \( RX \) reacting with water \( H_2O \) to produce an alcohol \( R-OH \) and a halide ion \( HX \). The first step is the departure of the halide ion \( X^- \), forming a carbocation \( R^+ \).

2. **Evaluate Carbocation Stability**:
The stability of the carbocation formed after the halide leaves is crucial. The more stable the carbocation, the more reactive the alkyl halide will be. Carbocation stability increases in the following order:
- Methyl \( (CH_3^+) \) < Primary \( (1^\circ) \) < Secondary \( (2^\circ) \) < Tertiary \( (3^\circ) \) < Resonance-stabilized carbocations.

3. **Analyze Each Option**:
- **Option 1**: A secondary alkyl halide (e.g., \( R-Br \)). The carbocation formed is a secondary carbocation, which is moderately stable.
- **Option 2**: A tertiary alkyl halide (e.g., \( R-Br \) with a methyl group). The carbocation formed is a tertiary carbocation, which is more stable than a secondary one.
- **Option 3**: An alkyl halide with a vinyl group (e.g., \( R-Br \) with a double bond). The carbocation formed here is a resonance-stabilized carbocation, which is highly stable due to resonance.
- **Option 4**: Bromobenzene. The carbocation formed here is very unstable due to the sp² hybridization of the benzene ring, making it unsuitable for this reaction.

4. **Determine the Most Reactive Alkyl Halide**:
Among the options, the alkyl halide that forms the most stable carbocation is the one with resonance stabilization (Option 3). This stability allows for a faster reaction rate.

5. **Conclusion**:
Therefore, the most reactive alkyl halide for the given reaction is the one represented in Option 3.

### Final Answer:
**Option 3** is the most reactive alkyl halide for the reaction \( RX + H_2O \rightarrow R-OH + HX \).

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