Arenes (Benzene and Phenol Chemistry)

Full Notes

Benzene is an unsaturated hydrocarbon with the molecular formula C₆H₆.

Bonding in Benzene

The Kekulé model suggests alternating double and single C–C bonds.

However, X-ray diffraction shows that all C–C bonds are the same length (intermediate between single and double), and enthalpy changes of hydrogenation show benzene is more stable than expected.

Because of this we now use another model, called the delocalised model.

There is a delocalised system of π electrons, formed by the sideways overlap of unbonded p orbitals from each carbon atom.

These six p electrons form a continuous cloud of electrons above and below the plane of the carbon atoms, leading to uniform bond lengths and a more stable structure.

Matt’s exam tip

Make sure you can explain why we now propose the delocalised electron model of benzene – evidence of equal bond lengths, hydrogenation enthalpies and a tendency for substitution rather than addition reactions. Remember the theoretical molecule cyclo-1,3,5-triene doesn’t actually exist.

Evidence for Delocalisation: Bond Lengths

All C–C bond lengths in benzene are equal (~0.140 nm). This is between the length of a single bond (0.154 nm) and a double bond (0.134 nm).

As a result, benzene’s structure cannot be represented accurately by alternating single and double bonds (as in the theoretical Kekule model of cyclohexa-1,3,5-triene.

Evidence for Delocalisation: Enthalpies of Hydrogenation

Enthalpies of hydrogenation show that benzene is more stable than expected – especially when compared to the theoretical cyclohexa-1,3,5-triene structure.

Less energy is released when benzene is hydrogenated and turned into cyclohexane than would be predicted for cyclohexa-1,3,5-triene.

Hydrogenation of cyclohexene (one C=C bond): ΔH = −120 kJ mol⁻¹

Expected hydrogenation of cyclohexa-1,3,5-triene (three C=C bonds): ΔH = −360 kJ mol⁻¹
Actual hydrogenation of benzene: ΔH = −208 kJ mol⁻¹

This less exothermic enthalpy change for benzene indicates it is more stable than expected, due to delocalisation energy.

Reactivity of Benzene

Benzene is less reactive than alkenes because of its electron delocalisation, which spreads the π-electron density evenly across the ring. This delocalisation makes it harder to attract electrophiles.

Reactions of Benzene

Combustion

Benzene burns with a smoky flame (incomplete combustion).

Bromination

• Forms bromobenzene
• Requires FeBr₃ or AlBr₃ as a catalyst to polarise the Br₂ molecule.

Nitration

• Forms nitrobenzene
• Uses a nitrating mixture of conc. HNO₃ and H₂SO₄ to form NO₂⁺ electrophile.

Friedel–Crafts Alkylation/Acylation

• Forms alkylbenzene or acylbenzene
• React with halogenoalkanes or acyl chlorides using AlCl₃.

Electrophilic Substitution Mechanisms

Benzene reacts with electrophiles by electrophilic substitution. In all these reactions the following standard mechanism occurs:

• Step 1 Electrophile is generated (e.g., NO₂⁺, Br⁺) and electrophile attacks benzene ring.
• Step 2 A proton is lost
• Step 3 The delocalised system is restored with the electrophile substituted into the benzene ring.

Phenol and Bromine

Phenol is an aromatic compound which consists of a benzene ring with a hydroxy (OH) group.

Unlike benzene, phenol reacts readily with bromine water without a catalyst, producing a white precipitate of 2,4,6-tribromophenol.

The increased reactivity is due to the lone pair on the O atom delocalising into the ring, activating it and making it more reactive with electrophiles.

In the case of bromine, the increased electron density in the ring is able to polarise the Br₂ molecule more than for benzene, generating the electrophile needed.

Summary

• Benzene has equal C–C bond lengths and extra stability due to a delocalised π system.
• Hydrogenation data support the delocalised model rather than alternating double bonds.
• Benzene undergoes electrophilic substitution rather than addition.
• Bromination forms bromobenzene, nitration forms nitrobenzene and Friedel–Crafts reactions need electrophile generation from a halogenoalkane or acyl chloride.
• Phenol activates the ring so it reacts with bromine without a catalyst.