The enthalpy of hydrogenation of cyclohexene is `-119.5kJ mol^(-1)`. If resonance energy of benzene is `-150.4kJ mol^(-1)`, its enthalpy of hydrogenation would be `:`

Options

A. `- 269.9 kJ mol^(-1)`
B. `- 358.5 kJ mol^(-1)`
C. `- 508.9 kJ mol^(-1)`
D. `- 208.1 kJ mol^(-1)`

Answer

D

Explanation

Resonance energy
= (Theoretical enthalpy of hydrogenation)
- (Actual enthalpy of hydrogenation)
`Delta_(h) H^(@) = - 119.5 kJ mol^(-1)`
If benzene contained three noninteracting double bonds, its
`Delta_(h) H^(@)` would be `3 xx (- 119.5 kJ mol^(-1)) = - 358.5 kJ mol^(-1)`
`Delta_(h) H^(@) = - 358.5 kJ mol^(-1)`
Thus,
Actual enthalpy of hydrogenation
= (Theoretical enthalpy of hydrogenation)
- (Resonace energy)
`= (- 358.5 kJ mol^(-1) - (150.4 kJ mol^(-1))`
`= - 208kJ mol^(-1)`
Note that the actual enthalpy of hydrogenation is lower by an amount equal to resonance energy,