Which of the following compound is most reactive with HCl in presence of anhydrous `ZnCl_2` (Lucas reagent) ?

Options

A.
B.
C.
D.

Explanation

To determine which compound is most reactive with HCl in the presence of anhydrous ZnCl2 (Lucas reagent), we need to analyze the stability of the carbocations formed during the reaction. The Lucas test is used to differentiate between primary, secondary, and tertiary alcohols based on their reactivity.

### Step-by-Step Solution:

1. **Understand the Reaction Mechanism**:
- The reaction involves the protonation of the alcohol by HCl, which converts the -OH group into a better leaving group (water). This leads to the formation of a carbocation.

2. **Identify the Types of Alcohols**:
- The reactivity of alcohols in the presence of Lucas reagent is based on their classification:
- Tertiary alcohols react immediately.
- Secondary alcohols react within a few minutes.
- Primary alcohols do not react significantly.

3. **Evaluate Carbocation Stability**:
- The stability of the carbocation formed is crucial. The order of stability is:
- Tertiary > Secondary > Primary
- Tertiary carbocations are stabilized by three alkyl groups, while secondary carbocations are stabilized by two, and primary carbocations are the least stable.

4. **Analyze the Given Compounds**:
- For each compound, determine whether it is primary, secondary, or tertiary:
- If a compound is tertiary, it will react the fastest.
- If it is secondary, it will react slower.
- If it is primary, it will not react.

5. **Consider Any Additional Effects**:
- If there are substituents on the benzene ring (like -Cl), consider their effects (inductive or resonance) on the stability of the carbocation.
- Electron-donating groups stabilize carbocations, while electron-withdrawing groups destabilize them.

6. **Determine the Most Reactive Compound**:
- Compare the stability of the carbocations formed from the given compounds. The compound that forms the most stable carbocation will be the most reactive with HCl in the presence of ZnCl2.

### Conclusion:
After analyzing the compounds and their carbocation stability, the compound that forms the most stable carbocation will be the most reactive with HCl in the presence of anhydrous ZnCl2.

In this case, the third compound, which forms a benzyl carbocation, is the most reactive due to the resonance stabilization provided by the benzene ring.

### Final Answer:
The most reactive compound with HCl in the presence of anhydrous ZnCl2 (Lucas reagent) is the third compound.

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