Question

The enthalpy of hydrogenation of cyclohexene is $-119.5kJ/mol$. If resonance energy of benzene is $-150.4kJ/mol$ then its enthalpy of hydrogenation would be:

• A. $-269.9kJ/mol$
• B. $-358.5kJ/mol$
• C. $-508.9kJ/mol$
• D. $-208.1kJ/mol$

Answer 1

The correct option is D $-208.1kJ/mol$

As, Enthalpy of hydrogenation of cyclohexene is $-119.5kJ/mol$, i.e, Hydrogenation of one $C=C$ is $-119.5kJ/mol$.
$\therefore \Delta H_{cal}\text{for benzene is}3\times -119.5=-357kJ/mol$
So,
$\Delta H_{obs}=\Delta H_{cal}+\text{Energy needed to disturb resonance}$
$=(3\times -119.5)+150.4=-208.1kJ/mol$
Thus, enthalpy of hydrogenation of benzene is $-208.1kJ/mol$