Benzene, electrophilic substitution and phenol

What you’ll learn

• Use evidence for benzene’s delocalised structure.
• Explain the stages of electrophilic aromatic substitution.
• Compare the reactivity of phenol with benzene.

Understand the idea

Benzene is planar, with six overlapping p orbitals forming a delocalised pi system above and below the ring. All carbon–carbon bonds have the same length. Its hydrogenation is less exothermic than a model with three isolated carbon–carbon double bonds predicts, showing extra stabilisation. Infrared and diffraction evidence also conflict with a simple alternating set of distinct single and double bonds.

Electrophilic substitution preserves the aromatic ring overall. The pi system attacks an electrophile, temporarily losing delocalisation and forming a positively charged intermediate. A base then removes the hydrogen on the substituted carbon; the carbon–hydrogen bond pair restores the delocalised system. Halogenation, nitration, sulfonation and Friedel–Crafts alkylation or acylation use different electrophiles; a Lewis acid can help generate a stronger electrophile in halogenation and Friedel–Crafts chemistry.

The nitronium ion is the electrophile in nitration. In phenol, an oxygen lone pair interacts with the ring and increases its electron density, so reaction with bromine water is much easier than for benzene and produces a white precipitate of 2,4,6-tribromophenol. This comparison concerns delocalisation and electron donation; it does not mean that benzene behaves like an ordinary alkene.

Key formulae

$$\mathrm{C_6H_6+3H_2\longrightarrow C_6H_{12}}$$

$$\mathrm{ArH+E^+\longrightarrow ArE+H^+}\quad\text{(overall substitution)}$$

$$\mathrm{NO_2^+}\quad\text{(nitronium electrophile)}$$

$$\mathrm{C_6H_5OH+3Br_2\longrightarrow C_6H_2Br_3OH+3HBr}$$

Worked example

Using the supplied data, hydrogenating one isolated C=C bond releases $120\,\mathrm{kJ\,mol^{-1}}$, while hydrogenating benzene releases $208\,\mathrm{kJ\,mol^{-1}}$. Estimate the extra stabilisation relative to three isolated double bonds.

1. The isolated-bond model predicts $3(-120)=-360\,\mathrm{kJ\,mol^{-1}}$.
2. The actual benzene hydrogenation value is $-208\,\mathrm{kJ\,mol^{-1}}$.
3. The difference is $360-208=152\,\mathrm{kJ\,mol^{-1}}$.

Answer: Benzene is lower in energy by approximately $152\,\mathrm{kJ\,mol^{-1}}$ relative to that model.