The energy difference between the formula proposed by Kekulé and the real bonding situation can be e — Physical Chemistry — Thermodynamics Chemistry Question
Benzene and Cyclohexane
The energy difference between the formula proposed by Kekulé and the real bonding situation can be estimated by comparing the theoretically estimated and experimentally found enthalpies of hydrogenation for benzene. The enthalpy of hydrogenation of cyclohexene is 120 kJ mol–1. This value is the energy of hydrogenation of a double bond.
How can the enthalpy of the hydrogenation of benzene be calculated from its enthalpy of combustion and the enthalpies of combustion of cyclohexane and hydrogen? Make use of Hess’s law.
C6H6 + 7.5 O2 6 CO2 + 3 H2O ΔrH = – 3268 kJ mol–1
C6H12 + 9 O2 6 CO2 + 6 H2O ΔrH = – 3920 kJ mol–1
H2 + 0.5 O2 H2O ΔrH = – 289 kJ mol–1
Model Answer
C6H6 + 7.5 O2 6 CO2 + 3 H2O ΔrH = – 3268 kJ mol–1
6 H2O + 6 CO2 C6H12 + 9 O2 ΔrH = 3920 kJ mol–1
3 H2 + 1.5 O2 3 H2O (ΔrH = –3 × 289 kJ mol–1) ΔrH = – 867 kJ mol–1
C6H6 + 3 H2 C6H12 ΔrH = – 215 kJ mol–1
Calculate the expected enthalpy of hydrogenation of a six–membered ring with three double bonds and compare it with the value obtained in 74.1. What is the reason for this difference?
Model Answer
Making use of the enthalpy of hydrogenation of cyclohexane, the approximate value for a six–membered ring with three double bonds is
3 × (–120 kJ mol–1) = – 360 kJ mol–1
Thus aromatic benzene is more stable compared to a compound with three double bounds by a difference in enthalpy of hydrogenation of
–(360 – 215) kJ mol–1 = –145 kJ mol–1, resonance energy or delocalization energy.