Question

Cyclohexanol is dehydrated to form cyclohexene on heating with conc. ${{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$. If the yield of this reaction is 75%, how much cyclohexene will be obtained from 100 g of cyclohexanol?
A.$61.5$ g
B.82 g
C.$109.3$ g
D.75 g

Answer 1

:Hint: The alcohols undergo dehydration reaction with conc. Sulphuric acid or conc. Phosphoric acid by undergoing elimination reaction where one mole of the alcohol gives one mole of the alkene as the product.

Complete step by step answer:
The reaction of cyclohexanol with conc. Sulphuric acid can be given as follows:
${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{11}}}\text{OH }\xrightarrow{\text{Conc}\text{.}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}}\text{ }{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{10}}}+{{\text{H}}_{\text{2}}}\text{O}$
The molecular weight of cyclohexanol = \left[ \left( 12\times 6 \right)+\left( 12\times 1 \right)+16 \right]=100$$$\text{g/mol}$$ Hence, 100 g of cyclohexanol = \dfrac{100}{100}$= 1 mole of cyclohexanol. As per the stoichiometry of the reaction, one mole of cyclohexanol gives one mole of cyclohexene. The molecular weight of cyclohexene =$\left[ \left( 12\times 6 \right)+\left( 10\times 1 \right) \right]=82$$$\text{g/mol}$$
Hence one mole of cyclohexanol would get dehydrated to form a mole of cyclohexene.
The percentage yield = $\dfrac{\text{experimental yield}}{\text{theoretical yield}}\times 100$
As the percentage yield of the reaction = 75 %, therefore the experimental yield of cyclohexene is,
$\text{experimental yield}=\dfrac{\text{percentage yield}}{100}\times \text{theoretical yield}$ $\Rightarrow \text{experimental yield}=\dfrac{75}{100}\times 82$= $61.5$g.
Therefore, the amount of cyclohexene obtained from 100 g of cyclohexanol, experimentally =$61.5$g .

Hence option A is correct.
Note:
Concentrated acids such as the Sulphuric acid and the Phosphoric acid release the hydrogen ion in the reaction medium which attacks the lone pair of electrons on the oxygen atom of the hydroxyl group and thus the oxygen atom gets the positive formal charge. Due to this, a beta-hydrogen atom from the molecule, in the beta position of the hydroxyl group is released in the form of hydrogen ion, leaving behind a pair of electrons that forms the double bond. So this reaction is a beta elimination process.