Comparing enthalpies of hydrogenation of Kekulé and actual Benzene? Watch

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Report Thread starter 2 years ago
Hi, I just have a quick question about this. It's more of a reminder but I just can't for the life of me remember how to explain this:

If Kekulé's model of benzene were correct, you'd expect the enthalpy of hydrogenation to be 3 x (cyclohexene) = 3 x -120kJmol-1 = -360kJmol-1

However, in reality actual benzene has an enthalpy of hydrogenation of -208kJmol-1
how does this show actual benzene is more stable than Kekulé's model of benzene.

Basically, I've forgotten how to explain how something is more stable if it is less exothermic. Something to do with energy required for bond breaking I think, but I can't seem to remember properly.

Any explanation much appreciated!

Badges: 11
Report 2 years ago
It's to do with the fact that Kekule's structure has 3 double bonds, which would be very electron-dense and so require a higher amount of energy to break the bond. To break cyclohexene's single double bonds, it evolves -120 kJ mol -1, so you'd expect -360 kJ mol -1 for the Kekule model. The fact that the enthalpy is lower than expected suggests an alternative structure which would be an intermediate bond between a single and double bond - the delocalised model.

When the density of electrons is evenly spread, as in the delocalised model, you're going to have a lower enthalpy c:

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