Question

Heat of hydrogenation of cyclohexene is $x_1$ and that of benzene is $x_2$. Hence, resonance energy is:

• A. $x_1-x_2$
• B. $x_1-x_3$
• C. $3x_1-x_2$
• D. $x_1-3x_2$

Answer 1

The correct option is C $3x_1-x_2$

Heat of hydrogenation of cyclohexene $=x_1$
Benzene has the same carbon skeleton as cyclohexene with three double bonds. So one can expect the heat of hydrogenation of benzene to be $3x_1$. Since all the three double bonds in benzene are in conjugation which further stabilises this system with energy equal to the resonance energy.
Hence resonance energy of benzene $=3x_1-x_2$