Arenes and their reactions
Arenes and their reactions
- Arenes are aromatic hydrocarbons, built on the benzene ring.
- The stable, electron-rich ring reacts mostly by electrophilic substitution.
- This keeps the ring, rather than destroying it by addition.
Practice
Benzene reacts mostly by electrophilic substitution because:
Substitution preserves the aromatic stabilisation; addition would break up the delocalised system.
Reactions of benzene
| Reaction | Reagents | Product |
|---|---|---|
| halogenation | $\text{Cl}_2$/$\text{Br}_2$ + $\text{AlCl}_3$ | a halogenoarene |
| nitration | conc. $\text{HNO}_3$ + conc. $\text{H}_2\text{SO}_4$ | nitrobenzene |
| Friedel–Crafts | $\text{RCl}$ 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 |
Practice
Nitration of benzene uses:
The conc. acid mixture generates the NO₂⁺ electrophile, giving nitrobenzene.
Practice
A Friedel–Crafts reaction (with AlCl₃) is used to:
Friedel–Crafts alkylation/acylation adds a carbon-based group to benzene.
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.
Practice
In electrophilic substitution, after the ring bonds to the electrophile:
Losing H⁺ from that carbon restores the delocalised ring, completing the substitution.
You've got it
Key idea
- 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