Solveeit Logo
Login

Question

Question: The heat of hydrogenation of cyclohexene is \(28.6\;Kcal\) and that of cyclohexadiene is about twice...

The heat of hydrogenation of cyclohexene is 28.6  Kcal28.6\;Kcal and that of cyclohexadiene is about twice as much of cyclohexene(i.e.,  55.4Kcal)\left( {i.e.,\;55.4Kcal} \right) . Then what would be the heat of hydrogenation of benzene, which has three double bonds?
A. Thrice that of cyclohexene (28.63Kcal)\left( {28.63Kcal} \right)
B. The same as that of cyclohexene
C. The same as that of cyclohexadiene
D. 49.8Kcal49.8Kcal

Explanation

Solution

We are going to write down the heat of hydrogenation(△Hhyd)\left( {\vartriangle {H_{hyd}}} \right) of each compound given along with their structure for better understanding. The compound with single double bond is cyclohexene , cyclohexadiene has two double bonds and benzene has three double bonds.

Complete step-by-step solution: In order to solve this question we are going to draw the structure of cyclohexene ,cyclohexadiene and benzene.

In this question we have been given the heat of hydrogenation(i.e.,  55.4Kcal)\left( {i.e.,\;55.4Kcal} \right) of cyclohexene is 28.6  Kcal28.6\;Kcal and heat of hydrogenation of cyclohexadiene is twice of cyclohexene (i.e.,  55.4Kcal)\left( {i.e.,\;55.4Kcal} \right) .
We are asked to find the heat of hydrogenation(i.e.,  55.4Kcal)\left( {i.e.,\;55.4Kcal} \right) of benzene ,which has three double bonds.
So to proceed further, we know that,
C6H10+C6H8→C6H6{C_6}{H_{10}} + {C_6}{H_8} \to {C_6}{H_6}
Here cyclohexene has one double bond so, counting the number of carbon and hydrogen , the molecular formula for cyclohexene is C6H10{C_6}{H_{10}}
Cyclohexadiene has two double bonds , counting the number of carbon and hydrogen , the molecular formula for cyclohexane is C6H8{C_6}{H_8}
Benzene has three double bonds, counting the number of carbon and hydrogen , the molecular formula for Benzene is C6H6{C_6}{H_6}
So adding up the amount of heat of hydrogenation we get,
28.6+55.428.6 + 55.4
Which gives =84 = 84
Which is approximately thrice of 28.628.6 i.e., Thrice of cyclohexene.

So among all the options, option A. is correct.

Note: Students can make mistakes in writing the correct structure of the compounds . Knowing the correct molecular formula and forming the equation of the compounds is important . Also an important concept here to keep in mind is the concept of heat of hydrogenation.