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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

30 Hydrocarbons

30.1 Arenes

Arenes

Specification Reference Organic Chemistry, Hydrocarbons 30.1

Quick Notes

  • Arenes are aromatic compounds with delocalised π-electrons in a ring.
    Benzene is an example of an arene.
  • Benzene undergoes electrophilic substitution reactions, not addition.
  • Key reactions include:
    • Halogenation: Cl2 or Br2 with AlCl3/AlBr3 catalyst.
    • Nitration: conc. HNO3 + conc. H2SO4 at 25–60°C.
    • Friedel–Crafts alkylation/acylation: CH3Cl or CH3COCl + AlCl3, heat.
    • Side-chain oxidation: hot KMnO4 oxidises alkyl side chains to –COOH.
    • Hydrogenation: H2 with Ni/Pt catalyst forms cyclohexane.
  • Substituents affect reactivity and position of further substitution.
    • Activating groups (such as –OH, –NH2, –R groups) make substitutions more likely at carbons 2, 4 and 6.
    • Deactivating groups (such as –NO2, –COOH, –COR) make substitutions more likely at carbons 3 and 5.
  • Side-chain halogenation occurs with UV light (whereas ring halogenation requires a catalyst).

Full Notes

Aromatic compounds like benzene and methylbenzene are stable due to their delocalised π-electron system.

Instead of undergoing addition, they favour electrophilic substitution, where a hydrogen atom is replaced, allowing the reforming of the aromatic ring and maintaining stability. See benzene for more detail.

Halogenation

Reagents: Cl2 or Br2 with AlCl3 or AlBr3 catalyst.

CIE A-Level Chemistry halogenation of benzene with chlorine or bromine using AlCl3 catalyst.

Nitration

Reagents: Concentrated HNO3 and concentrated H2SO4.

CIE A-Level Chemistry nitration of benzene mechanism.

Friedel–Crafts Alkylation

Reagents: Haloalkane (e.g. CH3Cl) with AlCl3 catalyst.

CIE A-Level Chemistry alkylation of benzene mechanism using haloalkane and AlCl3.

Friedel–Crafts Acylation

Reagents: Acyl chloride (e.g. CH3COCl) with AlCl3 catalyst.

CIE A-Level Chemistry acylation of benzene mechanism using acyl chloride and AlCl3.

Side-chain oxidation

Reagents: Hot, alkaline KMnO4 followed by acid.

CIE A-Level Chemistry oxidation of alkylbenzenes with KMnO4 to carboxylic acids.

Hydrogenation

Reagents: H2 with Ni or Pt catalyst.

CIE A-Level Chemistry hydrogenation of benzene to cyclohexane with nickel or platinum catalyst.

Electrophilic Substitution Mechanism

Unlike alkenes, benzene undergoes substitution rather than addition, as the delocalised ring is preserved.

CIE A-Level Chemistry electrophilic substitution mechanism for benzene.
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The general mechanism for electrophilic substitution shown above is the only one you need to know for benzene. Focus on how the electrophile is generated as that is different for different reactions. The rest is the always the same.

Side-Chain vs Ring Halogenation

For alkyl-benzenes, whether the side chain or ring undergoes substitution depends on reaction conditions.

CIE A-Level Chemistry difference between side chain and ring halogenation in methylbenzene.

Substituent Effects on Further Substitution

Groups already on the ring affect both reactivity and substitution positions.

Activating Groups: such as –OH, –NH2, –R groups

CIE A-Level Chemistry activating groups directing positions 2,4,6 on benzene ring.

These increase electron density in the ring, directing new groups to positions 2 and 4 (ortho/para).

Deactivating Groups: such as –NO2, –COOH, –COR groups

CIE A-Level Chemistry deactivating groups directing substitution to meta positions.

These withdraw electron density from the ring, reducing reactivity and directing to position 3 (meta).

Summary