The enthalpy of hydrogenation of cyclohexene is $-119.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$. If resoance energy…
The enthalpy of hydrogenation of cyclohexene is $-119.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$. If resoance energy of benzene is -150 $\mathrm{kJ} \mathrm{mol}^{-1}$, its enthalpy of hydrogenation would be:
$-358.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$-508.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$-208.1 \mathrm{~kJ} \mathrm{~mol}^{-1}$
$-269.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Solution
$\therefore$ Heat of hydrogenation of cyclohaxene $=-119.5 \mathrm{~kJ} / \mathrm{mol}$
$\therefore$ Heat of hydrogenation of benzene $=3 \times-119.5=-358.5 \mathrm{~kJ} / \mathrm{mol}$
Resonance energy
$=$ Observed $\Delta \mathrm{H}-$ Calculated $\Delta \mathrm{H}$
$=-150.4=-358.5-x$
$x=-208.1 \mathrm{~kJ}$
Related Theory
The energy calculated for a resonance hybrid is lower than the energies of any of the alternative structures; the molecule is then said to be stabilized by resonance.