Question

1. Although all bond lengths in benzene are identical (due to resonance stabilization), that is not always true for all aromatic compounds. In naphthalene, the C1-C2 bond is shorter than the C2-C3 bond. Explain why? And explain using the resonance theory.

2. Use the "polygon-in-circle" method to draw an MO diagram showing the pi-electron distribution pattern in the cyclopentadienyl anion. How is this diagram useful in explaining the aromatic properties of this anion? And what else can be said about the compound itself.

3. The heat of hydrogenation for cyclohexene and benzene are given below. Calculate the resonance stabilization energy of benzene.

C6H10 + H2 --> C6H12     Delta H = -120 kJ/mole

C6H6   + 3H2 --> C6H12   Delta H = -208 kJ/mole

Answer 1

If we assume benzene as a "normal" alkene, then the heat of hydrogenation should be:

but the heat of hydrogenation of benzene is:

Then, the resonance stabilization energy of benzene is: