Arenes and their reactions
Introduced| English | Meaning |
|---|---|
| arene/ˈæren/ | An aromatic hydrocarbon containing one or more aromatic ring systems. |
| electrophilic substitution/ɪˌlektrəʊˈfɪlɪk ˌsʌbstɪˈtjuːʃn/ | A reaction in which an electrophile replaces an atom or group in a molecule. |
| halogenation/həˌlɒdʒəˈneɪʃn/ | halogenation |
| nitration/naɪˈtreɪʃn/ | A reaction that introduces a nitro group into a molecule. |
| Friedel–Crafts/ˈfriːdl kræfts/ | Friedel–Crafts |
The chemistry of the ring
- Arenes 芳烃 are aromatic hydrocarbons, built on the benzene ring.
- The stable, electron-rich ring reacts mostly by electrophilic substitution 亲电取代.
- This keeps the ring, rather than destroying it by addition.
Arene substitution route
Follow electrophilic substitution on benzene.
Benzene reacts mostly by electrophilic substitution because:
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Substitution preserves the aromatic stabilisation; addition would break up the delocalised system.
Arenes such as benzene typically react by:
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Substitution keeps the stable ring intact.
Reactions of benzene
| Reaction | Reagents | Product |
|---|---|---|
| halogenation 卤化 | $\text{Cl}_2$/$\text{Br}_2$ + $\text{AlCl}_3$ | a halogenoarene |
| nitration 硝化 | conc. $\text{HNO}_3$ + conc. $\text{H}_2\text{SO}_4$ | nitrobenzene |
| Friedel–Crafts 傅-克反应 | $\text{RCl}$ or $\text{RCOCl}$ + $\text{AlCl}_3$ | adds an alkyl/acyl group |
| side-chain oxidation | hot alkaline $\text{KMnO}_4$ | benzoic acid |
| hydrogenation | $\text{H}_2$, Pt/Ni | cyclohexane |
The main reactions of benzene
Nitration of benzene uses:
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The conc. acid mixture generates the NO₂⁺ electrophile, giving nitrobenzene.
A Friedel–Crafts reaction (with AlCl₃) is used to:
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Friedel–Crafts alkylation/acylation adds a carbon-based group to benzene.
Match each term to its meaning.
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Each item links the term to its correct meaning.
Benzene reacts mainly by electrophilic substitution, not addition, because that keeps its stable delocalised ring intact.
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Addition would break the delocalised ring and lose its stability, so substitution (swapping an H) is favoured.
The mechanism (nitration)
- The acid mix makes the electrophile $\text{NO}_2^+$.
- the ring's delocalised electrons bond to the electrophile → an unstable intermediate.
- an $\text{H}^+$ is lost, restoring the stable ring.
- Substitution wins because it keeps the delocalisation; addition would destroy it.
Naphthalene, used in mothballs, is a simple arene of two fused benzene rings.
In electrophilic substitution, after the ring bonds to the electrophile:
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Losing H⁺ from that carbon restores the delocalised ring, completing the substitution.
Why substitution, not addition
- Benzene reacts by substitution to preserve its stable delocalised ring.
- Addition would destroy the extra stability, so it is not favoured.
Friedel-Crafts: alkylation adds an alkyl group, acylation an acyl group
You've got it
- arenes react by electrophilic substitution (keeping the ring)
- key reactions: halogenation, nitration, Friedel–Crafts (add alkyl/acyl), side-chain oxidation (→ benzoic acid), hydrogenation
- mechanism: ring attacks the electrophile → intermediate → loses H⁺ to restore the ring