30.1 Arenes
- Syllabus
- 9701–2028–2029
- Topic
- 30.1
- Level
- A2
Learning objectives
30.1.1Chemistry of arenes• Describe the chemistry of arenes as exemplified by the following reactions of benzene and methylbenzene:- (a) substitution reactions with Cl ₂ and with Br₂ with a catalyst, Al Cl ₃ or Al Br₃, to form halogenoarenes (aryl halides)- (b) nitration with a mixture of conc. HNO₃ and conc. H₂SO₄ at a temperature between- 25 °C and 60 °C- (c) Friedel–Crafts alkylation by CH₃Cl and Al Cl ₃ and heat- (d) Friedel–Crafts acylation by CH₃COCl and Al Cl ₃ and heat- (e) complete oxidation of the side-chain using hot alkaline KMnO₄ and then dil. acid to give a benzoic acid- (f) hydrogenation of the benzene ring using H₂ and Pt/Ni catalyst and heat to form a cyclohexane ring30.1.2Mechanism: electrophilic substitution in arenes• 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 addition30.1.3Whether halogenation will occur• Predict whether halogenation will occur in the side-chain or in the aromatic ring in arenes depending on reaction conditions30.1.4In the electrophilic substitution of arenes,• Describe that in the electrophilic substitution of arenes, different substituents direct to different ring positions (limited to the directing effects of –NH₂, –OH, –R, –NO₂, –COOH and –COR)
Choose arene reagents and conditions by identifying what changes
Arenes usually preserve the delocalised ring by substitution, but the syllabus also includes oxidation of an alkyl side-chain and hydrogenation of the ring. First identify whether the required change is on the ring, on the side-chain, or across the whole ring.
| Required change | Reagents and conditions | Main product from the named example | Reaction type and boundary |
|---|---|---|---|
| replace ring H by Cl or Br | Cl2/anhydrous AlCl3 or Br2/anhydrous AlBr3 | chlorobenzene or bromobenzene + HX | electrophilic substitution; the aromatic ring is restored |
| replace ring H by NO2 | concentrated HNO3 + concentrated H2SO4, 25-60 °C | nitrobenzene + H2O | electrophilic substitution |
| add CH3 to the ring | CH3Cl, anhydrous AlCl3, heat | methylbenzene + HCl | Friedel-Crafts alkylation, an electrophilic substitution |
| add COCH3 to the ring | CH3COCl, anhydrous AlCl3, heat | phenylethanone + HCl | Friedel-Crafts acylation, an electrophilic substitution |
| convert an alkyl side-chain to COOH | hot alkaline KMnO4, then dilute acid | methylbenzene gives benzoic acid | complete side-chain oxidation; the ring remains aromatic |
| saturate the aromatic ring | H2, Pt or Ni catalyst, heat | benzene gives cyclohexane | addition; aromaticity is lost |
Match every product to all parts of its reagent set. AlCl3 or AlBr3 is the halogen carrier for ring halogenation; concentrated H2SO4 belongs to the nitrating mixture; oxidation requires the alkaline heating stage followed by acidification; hydrogenation consumes three H2 molecules per benzene ring.
Methylbenzene follows the same ring reactions, with its methyl group directing new ring substitution mainly to positions 2 and 4. Its methyl side-chain can instead be oxidised completely to COOH, or halogenated by radical conditions described in the separate condition card.
Do not treat every arene reaction as substitution. Ring hydrogenation is addition and removes aromatic stabilisation; side-chain oxidation changes the substituent rather than replacing a ring hydrogen.
Electrophilic substitution temporarily breaks, then restores, aromaticity
Nitration and bromination use the same two-stage ring logic: generate a strong electrophile, let the benzene pi system form a bond to it, then remove H+ from that carbon so the C-H electrons restore the delocalised pi system.
| Reaction | Electrophile generation | Species that later removes H+ |
|---|---|---|
| nitration | HNO3 + H2SO4 -> NO2+ + HSO4- + H2O | HSO4- |
| bromination | Br2 + AlBr3 -> Br+ + AlBr4- | AlBr4- |
Step 1: draw a curly arrow from the benzene ring pi electrons to N in NO2+ or to Br+. This forms a C-E bond and a positively charged sigma complex in which the ring is temporarily not fully delocalised. Step 2: draw a curly arrow from the C-H bond on the substituted carbon back into the ring while H+ is removed. The delocalised pi system and aromaticity are restored.
For nitration, HSO4- accepts H+ and regenerates H2SO4. For bromination, AlBr4- accepts H+ to form HBr and regenerate AlBr3. The overall products are nitrobenzene + H2O or bromobenzene + HBr.
The first attack temporarily loses some aromatic stabilisation. Substitution restores it when H+ leaves; an addition product would retain extra groups and permanently disrupt the delocalised ring. This energetic advantage explains why substitution predominates over addition under these conditions.
The ring attacks the electrophile with an electron pair; the electrophile does not donate electrons to benzene. The positive sigma complex is an intermediate, not the final substituted arene.
Catalyst conditions halogenate the ring; UV and heat halogenate the side-chain
| Conditions with an alkylarene | Site and mechanism | Example with methylbenzene |
|---|---|---|
| Cl2 + anhydrous AlCl3, or Br2 + anhydrous AlBr3; no UV radical initiation | aromatic ring; electrophilic substitution | mainly 2- and 4-halogeno methylbenzene + HX |
| Cl2 or Br2, UV light, with the alkylarene heated/boiling; no Lewis-acid halogen carrier | alkyl side-chain; free-radical substitution | C6H5CH3 + Cl2 -> C6H5CH2Cl + HCl |
Read the conditions before choosing the product: AlCl3/AlBr3 signals electrophile generation and ring substitution; UV light signals radical formation and replacement of a side-chain hydrogen. The same starting alkylarene can therefore give constitutional isomers under different conditions.
With excess halogen under radical conditions, further side-chain hydrogens can also be replaced. For a single equivalent, show the first substitution product unless the question specifies excess reagent.
This ring-versus-side-chain choice applies to an arene that has an alkyl side-chain. Benzene itself has no side-chain hydrogen, so it cannot form a side-chain substitution product.
An existing substituent predicts the favoured positions for the next substitution
Call the carbon bearing the existing substituent carbon 1. Positions 2 and 6 are equivalent, positions 3 and 5 are equivalent, and position 4 is opposite carbon 1. Directing describes where an incoming electrophile is favoured during electrophilic substitution.
| Existing group | Electronic effect | Favoured positions for the incoming group |
|---|---|---|
| -NH2 | electron donating | 2, 4 and/or 6 |
| -OH | electron donating | 2, 4 and/or 6 |
| -R (alkyl) | electron donating | 2, 4 and/or 6 |
| -NO2 | electron withdrawing | 3 and/or 5 |
| -COOH | electron withdrawing | 3 and/or 5 |
| -COR | electron withdrawing | 3 and/or 5 |
Electron-donating groups increase ring electron density and favour attack at positions 2, 4 and 6. Electron-withdrawing groups remove ring electron density and favour positions 3 and 5. Use the exact group already attached to the ring; do not classify from a vaguely similar formula.
A methyl group is -R, so nitration or bromination of methylbenzene gives mainly the 2- and 4-substituted products. A nitro group is -NO2, so a further substitution on nitrobenzene is directed to position 3 (position 5 is equivalent in a monosubstituted ring).
A directing effect predicts favoured positions, not one guaranteed exclusive product. Activation/deactivation describes reaction rate, whereas directing describes position; keep the two ideas distinct.