The best method to prepare cyclohexene from cyclohexanol is by using:

Options

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

Explanation

To prepare cyclohexene from cyclohexanol, the best method is to use concentrated phosphoric acid (H₃PO₄) as a dehydrating agent. Here is a step-by-step breakdown of the process:

### Step-by-Step Solution:

1. **Starting Material**: Begin with cyclohexanol (C₆H₁₃OH). This is the alcohol that we want to convert into cyclohexene.

2. **Addition of Concentrated Phosphoric Acid**: Add concentrated phosphoric acid (H₃PO₄) to cyclohexanol. The phosphoric acid acts as a dehydrating agent.

3. **Protonation of Alcohol**: The oxygen atom in cyclohexanol has lone pairs that can attack the hydrogen ion (H⁺) from phosphoric acid, leading to the formation of an oxonium ion (C₆H₁₃OH₂⁺). This step makes the alcohol more reactive.

4. **Formation of Carbocation**: The oxonium ion is unstable and can lose a water molecule (H₂O) to form a carbocation (C₆H₁₃⁺). This is a key step in the dehydration process.

5. **Elimination of a Proton**: A hydrogen atom from a carbon adjacent to the carbocation is removed (deprotonation), leading to the formation of a double bond. This results in the formation of cyclohexene (C₆H₁₁).

6. **Final Product**: The final product of the reaction is cyclohexene, which is an alkene.

### Summary of Reaction:
Cyclohexanol + H₃PO₄ → Cyclohexene + H₂O

### Conclusion:
The best method to prepare cyclohexene from cyclohexanol is by using concentrated phosphoric acid (H₃PO₄).

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