The structure and bonding of benzene, evidence for the delocalised model, the stability of the aromatic ring, and electrophilic substitution reactions such as nitration and halogenation.

What this topic is asking

Eduqas topic OA1.2 covers aromaticity: the structure and bonding of benzene, the evidence for the delocalised model over the older Kekule structure, the stability that delocalisation confers, and the characteristic electrophilic substitution reactions (such as nitration and halogenation). The contrast between benzene's stability and the reactivity of alkenes is central.

The structure and bonding of benzene

Evidence for delocalisation

Three observations show benzene is not the Kekule structure of alternating single and double bonds:

• Bond lengths. All six carbon-carbon bonds are identical and intermediate in length between a single and a double bond.
• Enthalpy of hydrogenation. Hydrogenating benzene releases less energy than three times that of cyclohexene, so benzene is more stable than the hypothetical "cyclohexatriene" by the delocalisation (resonance) energy.
• Lack of reactivity. Benzene does not decolourise bromine water, whereas an alkene with real double bonds would.

Electrophilic substitution

Halogenation works similarly, using a halogen carrier (such as $\text{AlCl}_3$ or $\text{FeBr}_3$) to generate the electrophile.

Examples in context

Example 1. Making explosives and dyes. Nitration is the first step in producing TNT (from methylbenzene) and many azo dyes; the stability of the aromatic ring lets multiple substitutions build complex molecules.

Example 2. Why aromatic pollutants persist. The stability of the benzene ring makes polycyclic aromatic hydrocarbons resistant to breakdown, which is why some are persistent environmental pollutants and carcinogens.

Try this

Q1. State two pieces of evidence that benzene has a delocalised structure rather than three localised double bonds. [2 marks]

• Cue. All six C-C bond lengths are equal (between single and double); and the enthalpy of hydrogenation is less exothermic than expected for three double bonds, showing extra stability.

Q2. Name the catalyst and the electrophile used to chlorinate benzene. [2 marks]

• Cue. A halogen carrier such as $\text{AlCl}_3$ (or $\text{FeCl}_3$); the electrophile is $\text{Cl}^+$ (generated from $\text{Cl}_2$ by the carrier).