The delocalised model of benzene and the evidence for it, the stability of the ring, and the electrophilic substitution reactions of benzene (nitration, halogenation and Friedel-Crafts acylation).

What this topic is asking

OCR specification point 6.1.1 wants you to describe the delocalised model of benzene and the evidence for it, explain the stability of the ring, and give the electrophilic substitution reactions of benzene: nitration, halogenation and Friedel-Crafts acylation, with the mechanisms and conditions. Benzene is the foundation of aromatic chemistry.

The delocalised model

The evidence for delocalisation:

• All carbon-carbon bond lengths are equal (about $0.139\ \text{nm}$), between a single ($0.154\ \text{nm}$) and a double bond ($0.134\ \text{nm}$); the Kekule structure with alternating bonds would have two different lengths.
• The enthalpy of hydrogenation is less exothermic than three times that of cyclohexene, showing benzene is more stable than the Kekule model predicts.

Why benzene substitutes

Nitration

Halogenation and Friedel-Crafts

Examples in context

Example 1. Explosives and dyes from nitration. Repeated nitration of aromatic compounds makes explosives such as TNT (trinitrotoluene) and provides the nitro groups later reduced to amines for azo dyes, a direct industrial use of electrophilic substitution.

Example 2. Building drug molecules with Friedel-Crafts. Friedel-Crafts acylation attaches carbon chains to aromatic rings, a standard step in synthesising pharmaceuticals and fragrance molecules around a benzene core.

Try this

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

• Cue. All carbon-carbon bonds are the same length (intermediate between single and double); the enthalpy of hydrogenation is less exothermic than three times that of cyclohexene.

Q2. Name the electrophile in the nitration of benzene. [1 mark]

• Cue. The nitronium ion, $\text{NO}_2^+$.