Question

Cyclohexene is best prepared from cyclohexanol by which of the following?
(A) Conc. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$
(B) Conc. $\text{HCl}/\text{ZnC}{{\text{l}}_{\text{2}}}$
(C) Conc. $\text{HCl}$
(D) Conc. $\text{HBr}$

Answer 1

The formation of cyclohexene from cyclohexanol involves the removal of a water molecule from the compound and the formation of a cyclic alkene. But the presence of any nucleophile will form a substitution product.

Complete step by step solution: Cyclohexanol is alicyclic alcohol and cyclohexene is an unsaturated cyclic compound. The only difference in these two compounds in terms of structure is the difference of an HOH molecule or more specifically a water molecule.
So, the cyclohexene can be obtained from cyclohexanol by a dehydration process with the help of a concentrated acid catalyst. The acids which are usually incorporated in this process are conc. ${{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}$ and conc. ${{\text{H}}_{3}}\text{P}{{\text{O}}_{4}}$.
Now, the formation of cyclohexene involves the following mechanism:

We can see that the mechanism of dehydration reaction involves the formation of an intermediate carbocation, so other acid catalysts such as conc. $\text{HCl}/\text{ZnC}{{\text{l}}_{\text{2}}}$, conc. $\text{HBr}$or conc. $\text{HCl}$ cannot be used because all of these contain strong nucleophiles such as $\text{B}{{\text{r}}^{-}}$ and $\text{C}{{\text{l}}^{-}}$ which form a substitution product. The mechanism followed in this process is shown below:

Additional information: The dehydration mechanism for the formation of cyclohexene follows the E1-mechanism while the substitution reaction follows the ${{\text{S}}_{\text{N}}}1$ mechanism.

Hence, the correct answer is (A) Conc. ${{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}$.

Note: Out of the conc. ${{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}$ and conc. ${{\text{H}}_{3}}\text{P}{{\text{O}}_{4}}$, the latter is preferred over the former catalyst because of the strong oxidizing nature of sulphuric acid, it oxidizes to form some amount of carbon dioxide and sulphur dioxide along with alkene which needs to be removed from the reaction mixture.