Question

Why is benzene not considered a cycloalkene?

Answer 1

Because if it truly was, its enthalpy of hydrogenation (the enthalpy due to a reaction in which a substance is reduced by adding hydrogen) would be very similar to that of the theoretical, localized 1,3,5-cyclohexatriene.

`DeltaH_(rxn, "1,3,5-cyclohexatriene")^@ ~~ "-359.2 kJ/mol"`

`DeltaH_(rxn, "Benzene")^@ ~~ "-208.5 kJ/mol"`

It's clear that reducing benzene releases less energy, which means that its `\mathbf(pi)` system was lower in energy to begin with.

As a result, an additional stabilization effect (due to resonance delocalization of the `\mathbf(pi)` electrons) is present in benzene that is NOT present in the theoretical, localized structure that looks like benzene but is in fact NOT benzene.

You can see that in the diagram here:

To reiterate, the left resonance structure here as-drawn is not the same as the right resonance hybrid structure here: