Why is benzene not considered a cycloalkene?

Question

Brooklyn Barber · Accepted Answer

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:

Brooklyn Barber · Answer

undefined Benzene is not considered a cycloalkene because it does not exhibit the typical chemical reactivity of cycloalkenes. Cycloalkenes have double bonds between carbons in the ring structure, while benzene has alternating single and double bonds, known as delocalized pi-electron system. This delocalization results in a planar, hexagonal ring structure with bond lengths intermediate between single and double bonds. As a result, benzene does not undergo typical addition reactions characteristic of cycloalkenes.