Structure, Resonance and Aromaticity of Benzene

Benzene, $\text{C}_6\text{H}_6$, is a perfectly flat, hexagonal molecule in which all six carbon

atoms are equivalent -- a fact that early structural theories struggled to explain simply by

drawing three alternating single and double bonds (the classical Kekule structure), because

that picture predicts two different carbon-carbon bond lengths (a longer single bond,

$\approx 154$ pm, alternating with a shorter double bond, $\approx 134$ pm) around the ring.

Experimentally, however, every one of benzene's six carbon-carbon bonds is found to have exactly

the same length, $\approx 139$ pm -- intermediate between a pure single and a pure double bond,

and the same for every bond around the ring, not alternating.

Resonance. This is explained by recognising that a single Kekule structure, with its double

bonds fixed at particular positions, cannot represent benzene's true electronic structure. Two

Kekule structures can in fact be drawn for benzene, differing only in which three of the six

carbon-carbon bonds are drawn as double bonds; since the two structures are equal in energy and

differ only in the placement of electrons (not of atoms), the real molecule is not either

structure but a resonance hybrid of the two -- the actual pi electron density is delocalised

evenly around the whole six-membered ring rather than being localised into three fixed double

bonds, giving all six bonds their observed, identical, intermediate bond length. This delocalised

electron cloud also makes benzene substantially more stable than a hypothetical molecule with three

genuinely separate, non-interacting double bonds ("cyclohexatriene") would be: comparing the actual

heat released when benzene is hydrogenated to cyclohexane against three times the heat released

hydrogenating a single, isolated double bond (as in cyclohexene) shows benzene releases roughly

$150\ \text{kJ mol}^{-1}$ less heat than expected -- this deficit is benzene's resonance (delocalisation) energy, a direct experimental measure of how much extra stability the

delocalised pi system provides.

Huckel's rule and aromaticity. More generally, a ring system is classified as aromatic --

and enjoys this same extra resonance stabilisation -- if it satisfies four structural conditions:

the ring must be (1) cyclic, (2) planar (so that every ring atom's $p$ orbital can align parallel

for continuous overlap), (3) fully conjugated (an unbroken cycle of overlapping $p$ orbitals, one

on every ring atom, usually meaning every ring atom is $sp^2$ hybridised), and (4) must contain

exactly $(4n+2)$ pi electrons in that delocalised system, where $n$ is a non-negative integer …