Question

The enthalpy of hydrogenation of benzene is -207 kJ/mol and its resonance energy is -150 kJ/mol. Calculate the enthalpy of hydrogenation $-△H$ in kJ/mol for 1,4-cyclohexadiene.

Answer 1

Resonance energy of benzene $=△H_{exp}-△H_{cal}$
$\Rightarrow -150=△H_{exp}-(-207)$
$\Rightarrow △H_{exp}=-357kJ/mol$
This is the enthalpy of hydrogenation of 3 $C=C$ bonds.
Hence for 1,4-cyclohexadiene,
$△H=-357\times \frac{2}{3}=-238kJ/mol$