Which of the reagents on reaction with cyclohexanol gives best yield of cyclohexene?

Options

A. conc. HCl
B. conc. HBr
C. conc. `H_3PO_4`
D. `HCl + ZnCl_2`

Answer

To determine which reagent gives the best yield of cyclohexene when reacting with cyclohexanol, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Identify the Reagents**: The question provides four reagents. We will analyze each one to see if it can convert cyclohexanol to cyclohexene.

2. **Option 1: Concentrated HCl**:
- Cyclohexanol reacts with concentrated HCl.
- The concentrated acid donates H⁺ to the hydroxyl group (-OH) of cyclohexanol, converting it into a better leaving group (water).
- A carbocation is formed.
- Chloride ion (Cl⁻) from HCl attacks the carbocation, resulting in the formation of chlorocyclohexane.
- **Conclusion**: This does not yield cyclohexene.

3. **Option 2: Concentrated HBr**:
- Cyclohexanol reacts with concentrated HBr.
- Similar to HCl, HBr donates H⁺ to the -OH group, forming a good leaving group.
- A carbocation is formed.
- Bromide ion (Br⁻) attacks the carbocation, leading to the formation of bromocyclohexane.
- **Conclusion**: This does not yield cyclohexene.

4. **Option 3: Concentrated Phosphoric Acid (H₃PO₄)**:
- Cyclohexanol reacts with concentrated phosphoric acid.
- H₃PO₄ dissociates to provide H⁺, which protonates the -OH group, forming a good leaving group.
- A carbocation is formed.
- Instead of a nucleophile attacking the carbocation, dehydration occurs, where water leaves, resulting in the formation of cyclohexene.
- **Conclusion**: This yields cyclohexene.

5. **Option 4: Lucas Reagent**:
- Lucas reagent is used to differentiate between alcohols based on their degree (1°, 2°, 3°).
- It does not promote dehydration to form alkenes.
- **Conclusion**: This is not the correct option for forming cyclohexene.

### Final Conclusion:
The best yield of cyclohexene is obtained when cyclohexanol reacts with **concentrated phosphoric acid (H₃PO₄)**.