The Student Room Group

Resonance/ Stabilisation Energies of Aromatic Compounds

When books quote stabilisation energies they always use positive values, but I would have thought they would have been negative since the compound is more stable. Or am I thinking about this the wrong way?
I have to calculate the stabilisation energy of naphthalene and have been told to get the sign right, so I'm not sure as most quote positive but on a diagram it appears negative.

Anyone have any ideas?

Thankyou
NicolaJayne
When books quote stabilisation energies they always use positive values, but I would have thought they would have been negative since the compound is more stable. Or am I thinking about this the wrong way?
I have to calculate the stabilisation energy of naphthalene and have been told to get the sign right, so I'm not sure as most quote positive but on a diagram it appears negative.

Anyone have any ideas?

Thankyou


Resonance stabilisation energy is just a number. It is the energy saved by having an actual structure with resonance instead of a theoretical non-delocalised structure. As the non-delocalised structure doesn't exist it is pretty semantic to give a sign for the resonance energy.
You are quite correct in saying that the energy level of the resonance forms is lower than that of the theoretical non-existent molecule.
However, normally this is assessed by considering a reaction of the molecule and comparing it with simpler non-delocalised forms.

example
benzene is hydrogenated using H2 and a nickel catalyst. delta H = 207 kJ mol-1
cyclohexene is similarly hydrogenated. delta H = -119 kJ mol-1

If there were no resonance delocalisation you would expect the value for benzene to be three times the value for ethene. But it's much less.

This 'missing' energy is the resonance delocalisation energy.
missing energy = (3 x 119) - 207 = 150 kJ mol-1