Arenes and their reactions
| English | Chinese | Pinyin |
|---|---|---|
| arene | 芳烃 | fāng tīng |
| electrophilic substitution | 亲电取代 | qīn diàn qǔ dài |
| halogenation | 卤化 | lǔ huà |
| nitration | 硝化 | xiāo huà |
| Friedel–Crafts | 傅-克反应 | fù - kè fǎn yìng |
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:
Substitution preserves the aromatic stabilisation; addition would break up the delocalised system.
Arenes such as benzene typically react by:
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:
The conc. acid mixture generates the NO₂⁺ electrophile, giving nitrobenzene.
A Friedel–Crafts reaction (with AlCl₃) is used to:
Friedel–Crafts alkylation/acylation adds a carbon-based group to benzene.
Match each term to its meaning.
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.
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:
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