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Why is benzene so stable and how does it react?

The structure and stability of benzene, the delocalised model, electrophilic substitution reactions, and the reactions of phenol.

A focused answer to WJEC A-Level Chemistry Unit 4, covering the delocalised structure and stability of benzene, the evidence from enthalpies of hydrogenation, electrophilic substitution mechanisms (nitration and halogenation), and the reactivity of phenol.

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What this dot point is asking

WJEC wants you to describe the delocalised structure and stability of benzene, give the evidence for it, explain electrophilic substitution mechanisms such as nitration and halogenation, and describe the reactions of phenol.

The answer

The structure of benzene

Evidence for delocalisation

Electrophilic substitution

Because addition would destroy the stable ring, benzene undergoes electrophilic substitution: a strong electrophile attacks the Ο€\pi system, then H+\text{H}^+ is lost to restore aromaticity.

Phenol

Phenol (C6H5OH\text{C}_6\text{H}_5\text{OH}) is more reactive than benzene because the oxygen lone pair feeds electron density into the ring, so it reacts with bromine water at room temperature (no catalyst) to give 2,4,6-tribromophenol (white precipitate). Phenol is also weakly acidic, dissolving in sodium hydroxide.

Comparing benzene and phenol reactivity

Both react by electrophilic substitution, but phenol is far more reactive. The oxygen lone pair on phenol overlaps with the ring, raising its electron density, so phenol reacts with bromine water at room temperature without a catalyst and substitutes at three positions (2, 4 and 6) to give the tribromo product. Benzene, with no activating group, needs a strong electrophile generated by a catalyst (a halogen carrier for halogenation, or concentrated sulfuric acid to make the nitronium ion for nitration) and substitutes only once under mild conditions. This contrast is a favourite comparison question.

The Kekule problem

The Kekule structure drew benzene as cyclohexatriene with three localised double bonds. Three pieces of evidence overturn it: all the carbon-carbon bond lengths are equal (between single and double), benzene is far less reactive than an alkene (it does not decolourise bromine water), and its enthalpy of hydrogenation is about 208208 kJ molβˆ’1^{-1} less exothermic than three times that of cyclohexene. Together these show the Ο€\pi electrons are delocalised over the whole ring, giving the extra stability that defines aromaticity.

Examples in context

Explosives and dyes. Nitration of aromatics produces nitro compounds used in explosives (such as TNT) and as intermediates for azo dyes, central to the chemical industry. Antiseptics from phenol. Phenol's reactivity and mild acidity made it the first surgical antiseptic, and substituted phenols remain in disinfectants today.

Try this

Q1. Name the electrophile in the nitration of benzene. [1 mark]

  • Cue. The nitronium ion, NO2+\text{NO}_2^+.

Q2. State the observation when bromine water is added to phenol. [1 mark]

  • Cue. It is decolourised and a white precipitate (2,4,6-tribromophenol) forms.

Q3. State why benzene is more stable than the Kekule (cyclohexatriene) model predicts. [1 mark]

  • Cue. Its Ο€\pi electrons are delocalised over the whole ring, giving extra stabilisation.

Q4. State one piece of evidence that benzene does not contain localised double bonds. [1 mark]

  • Cue. All the carbon-carbon bonds are the same length (or benzene does not decolourise bromine water).

Q5. Name the catalyst used in the bromination of benzene. [1 mark]

  • Cue. A halogen carrier such as iron(III) bromide, FeBr3\text{FeBr}_3.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20194 marksDescribe the mechanism for the nitration of benzene, including the generation of the electrophile and a relevant equation.
Show worked answer β†’

The electrophile is the nitronium ion NO2+\text{NO}_2^+, generated by concentrated nitric and sulfuric acids: HNO3+2H2SO4β†’NO2++2HSO4βˆ’+H3O+\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + 2\text{HSO}_4^- + \text{H}_3\text{O}^+.

The delocalised Ο€\pi electrons of benzene attack NO2+\text{NO}_2^+, forming an unstable intermediate (a partly delocalised cation).

A hydrogen ion is then lost, restoring the aromatic ring: C6H6+NO2+β†’C6H5NO2+H+\text{C}_6\text{H}_6 + \text{NO}_2^+ \rightarrow \text{C}_6\text{H}_5\text{NO}_2 + \text{H}^+.

Markers reward generating NO2+\text{NO}_2^+, the attack by the ring's Ο€\pi electrons, and the loss of H+\text{H}^+ to regenerate the aromatic system.

WJEC 20213 marksExplain why benzene undergoes substitution rather than addition reactions, referring to its stability.
Show worked answer β†’

Benzene has a ring of six delocalised Ο€\pi electrons spread above and below the carbon ring, which makes it unusually stable (its enthalpy of hydrogenation is less exothermic than expected for three double bonds).

Addition would break this delocalised system and lose the stabilisation, so it is unfavourable.

Substitution keeps the delocalised ring intact, so benzene reacts by electrophilic substitution rather than addition.

Markers reward the delocalised stable ring, the loss of stability on addition, and substitution preserving the aromatic system.

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