Cyclohexene is best prepared from cyclohexanol by which of the following

Options

A. conc. `H_(3) PO_(4)`
B. conc. HCl/`ZnCl_(2)`
C. conc. HCl
D. Conc. HBr

Answer

To prepare cyclohexene from cyclohexanol, we need to consider the dehydration process, which involves the removal of a water molecule from the alcohol to form an alkene. Let's go through the steps to determine the best reagent for this transformation.

### Step-by-Step Solution:

1. **Identify the Starting Material**:
- We start with cyclohexanol, which has the structure:
\[ \text{C}_6\text{H}_{12}\text{O} \]

2. **Consider the Dehydration Reaction**:
- The goal is to remove a water molecule (H₂O) from cyclohexanol to form cyclohexene (C₆H₁₄).

3. **Choose the Reagents**:
- We have four options to consider:
1. Concentrated phosphoric acid (H₃PO₄)
2. Concentrated HCl in the presence of ZnCl₂ (Lukas reagent)
3. Concentrated HCl
4. Concentrated HBr

4. **Evaluate Each Reagent**:
- **Option 1: Concentrated Phosphoric Acid (H₃PO₄)**:
- Phosphoric acid acts as a dehydrating agent. It protonates the hydroxyl group (-OH) of cyclohexanol, converting it into a better leaving group (water). The reaction proceeds through the formation of a carbocation, followed by the elimination of water, leading to the formation of cyclohexene.

- **Option 2: Concentrated HCl in presence of ZnCl₂**:
- This reagent will convert the -OH group into a -Cl group (substitution reaction) rather than promoting dehydration. The product will be cyclohexyl chloride, not cyclohexene.

- **Option 3: Concentrated HCl**:
- Similar to option 2, concentrated HCl will protonate the -OH group, but it will also lead to substitution, forming cyclohexyl chloride instead of cyclohexene.

- **Option 4: Concentrated HBr**:
- Like HCl, concentrated HBr will also protonate the -OH group and lead to substitution, resulting in cyclohexyl bromide rather than cyclohexene.

5. **Conclusion**:
- The best reagent for dehydrating cyclohexanol to form cyclohexene is **concentrated phosphoric acid (H₃PO₄)**, as it promotes the elimination of water and the formation of the alkene.

### Final Answer:
The best reagent to prepare cyclohexene from cyclohexanol is **concentrated phosphoric acid (H₃PO₄)**.