Question

Cyclohexanol is dehydrated to cyclohexene on heating with conc.${{H}_{2}}S{{O}_{4}}$. If the yield of this reaction is 75 %, how much cyclohexene will be obtained from 100 g of cyclohexanol? ${{C}_{6}}{{H}_{12}}{{O}_{6}}\xrightarrow{Con.{{H}_{2}}S{{O}_{4}}}{{C}_{6}}{{H}_{10}}$
(A) 61.5 g
(B) 65.7 g
(C) 69.3 g
(D) 70.9 g

Answer 1

Hint: We are given the percent yield of the reaction and we are asked to find the actual yield of cyclohexene. The percent yield of a particular reaction measures the reaction’s efficiency and is the ratio between the actual yield and the theoretical yield.

Complete answer:

> In the given question cyclohexene is being prepared by the dehydration of cyclohexanol using sulfuric acid as the acid catalyst. The reaction is given below
\[{{C}_{6}}{{H}_{12}}{{O}_{6}}\xrightarrow{Con.{{H}_{2}}S{{O}_{4}}}{{C}_{6}}{{H}_{10}}\]

> Let's look into the concept of percent yield, actual yield and theoretical yield . To calculate the efficiency of a given reaction, we need to compare the actual and theoretical yields by finding the percent yield of a reaction. The percentage yield of a reaction is the ratio between the actual yield and theoretical yield and this relation can be represented as follows
\[Percent\text{ }yield=\dfrac{Actual\text{ }yield}{Theoretical\text{ }yield}\times 100\]

> As we are aware, the molar masses of cyclohexanol and cyclohexene are 100${g}/{mol}\;$ and 82 ${g}/{mol}\;$ respectively. Therefore, when 100 gram of cyclohexanol is dehydrated, 82 gram of cyclohexene will be obtained. That is the theoretical yield of cyclohexene is 82 g. But the dehydration yield is given as 75% or in other words the percent yield is given as 75%. We need to find the actual yield(x).
Let’s substitute the given values in the above equation and we get,
$\Rightarrow$ \[75=\dfrac{x}{82}\times 100\]
$\Rightarrow$ \[x=\dfrac{75\times 82}{100}\]
$\Rightarrow$ \[x=61.5g\]

Therefore, 61.5 g of cyclohexene will be obtained from 100 g of cyclohexanol.

So, the correct answer is “Option A”.

Note: Keep in mind that alcohols can be dehydrated with concentrated acids, such as phosphoric or sulfuric acids. This reaction will take place rapidly with the formation of a more substituted alkene. Cyclic alkenes generally produce one product in high yield, while Straight chain alkenes can produce more than one product.