Chemistry of Hydrocarbons

Benzene, $\text{C}_6\text{H}_6$, is the simplest and parent member of the aromatic class. Kekulé described it as a six-membered ring with alternating single and double bonds, the structure we write as 1,3,5-cyclohexatriene. That picture explains the formula and is also wrong, because a molecule with three fixed double bonds would behave like a triene. Benzene is planar, a regular hexagon, with six $sp^2$ hybridised carbons in a trigonal planar arrangement. Each carbon has one unhybridised $p$ orbital at right angles to the ring plane, and the six $p$ orbitals overlap sideways to form a continuous $\pi$ cloud above and below it. The molecular orbital treatment replaces three localised $\pi$ bonds with one delocalised system over all six carbons, satisfying Hückel's rule and making the ring aromatic rather than merely unsaturated.

Two measurements settle the argument. The carbon to carbon bond length is $1.39\ \text{Å}$, between a single and a double bond, so there are no alternating long and short bonds to find. More telling is the hydrogenation energy. Cyclohexene releases $-119.5\ \text{kJ/mol}$, so a hypothetical cyclohexatriene should release about three times that, $-358.5\ \text{kJ/mol}$; benzene releases only $-208\ \text{kJ/mol}$. The difference, $150.5\ \text{kJ/mol}$, is the resonance energy, the extra stability the delocalised arrangement has over the alternating structure. The reference compound used to quantify it is the hypothetical 1,3,5-cyclohexatriene, not the real cyclohexene and not cyclohexane.

The consequence is the reactivity rule that governs the rest of the chapter. Delocalised $\pi$ electrons are held more tightly than an alkene's, so a weak electrophile cannot open the ring, and a strong one finds it cheaper to replace a hydrogen and restore the aromatic system than to add across a bond and destroy it. Ranked by willingness to react, the hydrocarbons run alkanes, then alkynes, then alkenes, with benzene below the alkenes despite being far more stable.