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

Options

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

Explanation

To prepare cyclohexene from cyclohexanol, we need to consider the dehydration of cyclohexanol, which involves the elimination of a water molecule. Here’s a step-by-step solution to the question:

### Step-by-Step Solution:

1. **Identify the Reactant and Product**:
- The reactant is cyclohexanol (C6H12O), which has a hydroxyl (-OH) group attached to a cyclohexane ring.
- The product we want to obtain is cyclohexene (C6H10), which contains a double bond.

2. **Understand the Dehydration Process**:
- Dehydration of alcohols involves the removal of a water molecule (H2O) from the alcohol. In this case, the -OH group from cyclohexanol will be removed along with a hydrogen atom from an adjacent carbon atom.

3. **Choose an Appropriate Dehydrating Agent**:
- To facilitate the dehydration of cyclohexanol to cyclohexene, we need a strong acid that can protonate the hydroxyl group, making it a better leaving group.
- Among the options given:
- **Option 1**: Concentrated HCl + ZnCl2 (Lucas reagent) - This is not suitable as it primarily distinguishes between different types of alcohols.
- **Option 2**: Concentrated H3PO4 - This is a strong acid and a good dehydrating agent that can effectively remove water from cyclohexanol to form cyclohexene.
- **Option 3**: Concentrated HCl - This would lead to substitution reactions rather than dehydration.
- **Option 4**: Concentrated HBr - Similar to HCl, this would also lead to substitution reactions.

4. **Conclusion**:
- The best method to prepare cyclohexene from cyclohexanol is by using **concentrated H3PO4** (Option 2). This acid will promote the dehydration reaction effectively, yielding cyclohexene without forming substituted products.

### Final Answer:
The best method to prepare cyclohexene from cyclohexanol is by using **concentrated H3PO4**.

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