- describe the chemistry of arenes as exemplified by the following reactions of benzene and methylbenzene: (a) substitution reactions with $\text{Cl}_2$ and with $\text{Br}_2$ in the presence of a catalyst, $\text{AlCl}_3$ or $\text{AlBr}_3$, to form halogenoarenes (aryl halides) (b) nitration with a mixture of concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$ at a temperature between $25\text{ }^{\circ}\text{C}$ and $60\text{ }^{\circ}\text{C}$ (c) Friedel–Crafts alkylation by $\text{CH}_3\text{Cl}$ and $\text{AlCl}_3$ and heat (d) Friedel–Crafts acylation by $\text{CH}_3\text{COCl}$ and $\text{AlCl}_3$ and heat (e) complete oxidation of the side-chain using hot alkaline $\text{KMnO}_4$ and then dilute acid to give a benzoic acid (f) hydrogenation of the benzene ring using $\text{H}_2$ and $\text{Pt/Ni}$ catalyst and heat to form a cyclohexane ring
- describe the mechanism of electrophilic substitution in arenes: (a) as exemplified by the formation of nitrobenzene and bromobenzene (b) with regards to the effect of delocalisation (aromatic stabilisation) of electrons in arenes to explain the predomination of substitution over addition
- predict whether halogenation will occur in the side-chain or in the aromatic ring in arenes depending on reaction conditions
- describe that in the electrophilic substitution of arenes, different substituents direct to different ring positions (limited to the directing effects of $-\text{NH}_2$, $-\text{OH}$, $-\text{R}$, $-\text{NO}_2$, $-\text{COOH}$ and $-\text{COR}$)
Hydrocarbons
A-Level Chemistry · Topic 30
30.1
Arenes
Syllabus
Source: Cambridge International syllabus
Arenes 芳烃 are aromatic hydrocarbons, built on the benzene 苯 ring. The delocalised ring of electrons is stable and electron-rich, so benzene mostly reacts by electrophilic substitution 亲电取代 — keeping the ring — rather than by addition.
Naphthalene, used in mothballs, is a simple arene — two benzene rings fused together
Reactions of benzene and methylbenzene
| Reaction | Reagents and conditions | Product |
|---|---|---|
| halogenation | $\text{Cl}_2$ or $\text{Br}_2$, with $\text{AlCl}_3$ or $\text{AlBr}_3$ as a catalyst 催化剂 | a halogenoarene 卤代芳烃 (aryl halide) |
| nitration 硝化 | concentrated $\text{HNO}_3$ + concentrated $\text{H}_2\text{SO}_4$, $25$–$60\,°\text{C}$ | nitrobenzene |
| Friedel–Crafts alkylation 傅克烷基化 | $\text{CH}_3\text{Cl}$ + $\text{AlCl}_3$, heat | methylbenzene (adds an alkyl group) |
| Friedel–Crafts acylation 傅克酰基化 | $\text{CH}_3\text{COCl}$ + $\text{AlCl}_3$, heat | a phenyl ketone (adds an acyl group) |
| side-chain oxidation | hot alkaline $\text{KMnO}_4$, then dilute acid | benzoic acid 苯甲酸 |
| hydrogenation 氢化 | $\text{H}_2$, $\text{Pt/Ni}$ catalyst, heat | cyclohexane |
Friedel-Crafts: alkylation adds an alkyl group, acylation an acyl group
Benzene keeps its stable ring, reacting mostly by electrophilic substitution (and by addition only with hydrogen)
In the side-chain oxidation, the whole side-chain 侧链 (such as the $\text{–CH}_3$ on methylbenzene) is turned into a $\text{–COOH}$ group, giving benzoic acid. In the hydrogenation, three molecules of $\text{H}_2$ add to the benzene ring 苯环 to make a saturated cyclohexane ring.
The mechanism: electrophilic substitution
Take nitration as the example. The acid mix makes the electrophile $\text{NO}_2^+$. Then:
- the delocalised electrons of the ring form a bond to the electrophile, giving an unstable intermediate.
- an $\text{H}^+$ is lost from that carbon, which restores the stable ring.
Substitution wins over addition because the delocalisation 离域 (aromatic stabilisation) of the ring is kept. Addition would destroy this stable system, so it is not favoured.
Electrophilic substitution (nitration): the ring attacks the electrophile NO$_2^+$ to give an unstable intermediate, then loses H$^+$ to restore the ring
Side-chain or ring?
Where a halogen reacts depends on the conditions:
- with a halogen-carrier catalyst (such as $\text{AlCl}_3$) and no UV light → substitution in the ring.
- with UV light and no catalyst → free-radical substitution in the side-chain.
The conditions decide: a halogen carrier (AlCl₃) substitutes the ring; UV light substitutes the side-chain
Directing effects
A group already on the ring decides where the next group goes — its directing effect 定位效应:
A group already on the ring directs the next one: –NH$_2$/–OH/–R send it to positions 2 and 4; –NO$_2$/–COOH/–COR send it to position 3
| Group already present | Directs the new group to |
|---|---|
| $\text{–NH}_2$, $\text{–OH}$, $\text{–R}$ | positions 2 and 4 |
| $\text{–NO}_2$, $\text{–COOH}$, $\text{–COR}$ | position 3 |
Worked example. Methylbenzene and nitrobenzene are each nitrated. Predict where the new $\text{NO}_2$ group goes, and which compound reacts faster. Look at the group already on the ring. In methylbenzene the $\text{–CH}_3$ is an alkyl group: it directs the new group to positions 2 and 4, and it releases electrons into the ring, making the ring more attractive to an electrophile - so methylbenzene nitrates faster than benzene. In nitrobenzene the $\text{–NO}_2$ directs to position 3, and it withdraws electrons from the ring - so nitrobenzene nitrates more slowly than benzene. The group already present controls both the position and the rate, and the two always travel together: 2,4-directors activate the ring, 3-directors deactivate it.
Explore
Arene substitution route
Follow electrophilic substitution on benzene.
Explore
Directing effects lab
Classify substituents by how they affect the benzene ring.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| arene | 芳烃 | fāng tīng |
| benzene | 苯 | běn |
| electrophilic substitution | 亲电取代 | qīn diàn qǔ dài |
| catalyst | 催化剂 | cuī huà jì |
| halogenoarene | 卤代芳烃 | lǔ dài fāng tīng |
| nitration | 硝化 | xiāo huà |
| Friedel–Crafts alkylation | 傅克烷基化 | fù kè wán jī huà |
| Friedel–Crafts acylation | 傅克酰基化 | fù kè xiān jī huà |
| benzoic acid | 苯甲酸 | běn jiǎ suān |
| hydrogenation | 氢化 | qīng huà |
| side-chain | 侧链 | cè liàn |
| benzene ring | 苯环 | běn huán |
| delocalisation | 离域 | lí yù |
| directing effect | 定位效应 | dìng wèi xiào yìng |
30.1
Exam tips
- Benzene undergoes electrophilic substitution, not addition, because addition would destroy the stable delocalised ring.
- Learn nitration (concentrated $\text{HNO}_3$/$\text{H}_2\text{SO}_4$, $50-60\ ^\circ\text{C}$) and halogenation (halogen + $\text{AlCl}_3$) with the electrophile-generating step.
- Compare reactivity: benzene resists addition far more than an alkene because of delocalisation.