Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity

This guide covers subtopic 30.1, Arenes, from Topic 30, Hydrocarbons, and subtopic 31.1, Halogen compounds, from Topic 31, Halogen compounds, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Both are A Level content, combined here because halogenoarenes are a direct product of one of benzene’s own substitution reactions.

Before studying this

This resource assumes benzene’s sp²/delocalised structure from A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape, electrophiles and curly-arrow notation from Organic Mechanisms: An Introduction, and SN1/SN2 substitution from Halogenoalkanes — the comparison with halogenoalkane reactivity at the end of this page depends directly on that AS resource.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topics 30 and 31

30.1 Arenes — describing reactions of benzene/methylbenzene: halogenation with Cl₂/Br₂ and an AlCl₃/AlBr₃ catalyst; nitration with concentrated HNO₃/H₂SO₄ at 25–60 °C; Friedel-Crafts alkylation (CH₃Cl, AlCl₃, heat) and acylation (CH₃COCl, AlCl₃, heat); side-chain oxidation with hot alkaline KMnO₄ then dilute acid to give benzoic acid; hydrogenation with H₂/Pt or Ni catalyst and heat; describing the electrophilic substitution mechanism (nitration and bromination examples) and explaining, via delocalisation, why substitution predominates over addition; predicting side-chain vs ring halogenation from conditions; describing the directing effects of –NH₂, –OH, –R, –NO₂, –COOH and –COR.

31.1 Halogen compounds — recalling that halogenoarenes are produced by substitution of an arene with Cl₂/Br₂ and an AlCl₃/AlBr₃ catalyst; explaining the difference in reactivity between a halogenoalkane and a halogenoarene, exemplified by chloroethane and chlorobenzene.

Reactions of benzene

Benzene’s delocalised ring makes it resistant to the addition reactions typical alkenes undergo, and instead it reacts by substitution, preserving the aromatic system:

Reaction	Reagents and conditions	Product
Halogenation	Cl₂ or Br₂, AlCl₃ or AlBr₃ catalyst	halogenoarene (e.g. chlorobenzene)
Nitration	concentrated HNO₃ + concentrated H₂SO₄, 25–60 °C	nitrobenzene
Friedel-Crafts alkylation	CH₃Cl, AlCl₃, heat	methylbenzene
Friedel-Crafts acylation	CH₃COCl, AlCl₃, heat	phenylethanone
Side-chain oxidation	hot alkaline KMnO₄, then dilute acid	benzoic acid (from methylbenzene)
Hydrogenation	H₂, Pt or Ni catalyst, heat	cyclohexane

Hydrogenation is the one reaction here that does add across the ring (fully saturating it to cyclohexane) — it needs a strong catalyst and heat precisely because disrupting the delocalised system is energetically costly, consistent with the ring’s aromatic stability.

The electrophilic substitution mechanism

Nitration of benzene. Concentrated H₂SO₄ protonates HNO₃ and generates the electrophile, the nitronium ion, NO₂⁺:

HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

The mechanism then proceeds in two steps:

Step 1 — the electrophile NO₂⁺ is attacked by a pair of electrons from the delocalised ring system (a curly arrow from the ring’s π system to NO₂⁺). This breaks the delocalisation at that carbon, forming a positively charged, non-aromatic intermediate (an “arenium ion”) in which the remaining four electrons are delocalised over the other five carbons.

Step 2 — a curly arrow from the C–H bond at that same carbon reforms a bond within the ring’s π system, and H⁺ is lost (typically to HSO₄⁻, regenerating H₂SO₄) — restoring full delocalisation and aromaticity, giving nitrobenzene.

Bromination of benzene follows the same two-step pattern: AlBr₃ polarises Br₂ enough to generate an electrophilic bromine species (Br⁺, associated with AlBr₄⁻), which the ring attacks in step 1 to form the arenium-ion intermediate, followed by loss of H⁺ in step 2 to restore aromaticity.

Why substitution, not addition. Addition (as with an alkene) would permanently destroy the delocalised π system, converting it into isolated, higher-energy double bonds — energetically far less favourable than substitution, which only briefly disrupts delocalisation in the intermediate before immediately restoring it in step 2. The aromatic stabilisation gained by keeping the ring intact is the reason electrophiles substitute rather than add.

Side-chain vs ring reaction

Whether halogenation occurs in the ring or in the side-chain of an alkylbenzene depends entirely on the conditions, not the reagent alone:

• Ring substitution: Cl₂ or Br₂ with an AlCl₃/AlBr₃ catalyst, in the dark/cold — electrophilic substitution as above.
• Side-chain substitution: Cl₂ or Br₂ with UV light, no catalyst — a free-radical substitution mechanism (initiation, propagation, termination) identical in kind to the alkane halogenation mechanism met in Hydrocarbons: Alkanes and Alkenes, just happening on the side-chain carbon instead of an isolated alkane carbon.

Directing effects

Once a substituent is already on the ring, it influences where the next electrophile attacks — different substituents direct to different positions:

Substituent	Directs to	Effect on reactivity
–NH₂, –OH	2- and 4- (and 6-) positions	activates (more reactive than benzene)
–R (alkyl)	2- and 4- (and 6-) positions	activates
–NO₂, –COOH, –COR	3- (and 5-) position	deactivates (less reactive than benzene)

–NH₂ and –OH direct 2,4 (and 6) because their lone pair donates into the ring by delocalisation, increasing electron density most strongly at those positions; –NO₂, –COOH and –COR direct 3 (and 5) because they withdraw electron density from the ring (by induction and/or their own delocalisation pulling electron density away), leaving the 3/5 positions comparatively most electron-rich of the available options. Phenol: Reactions and Acidity applies the –OH directing effect in full detail.

Halogenoarenes vs halogenoalkanes

Producing a halogenoarene: substituting an arene with Cl₂ or Br₂ and an AlCl₃/AlBr₃ catalyst — the same ring-substitution reaction above, e.g. benzene → chlorobenzene, or methylbenzene → a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene (the methyl group directing 2,4- as expected).

Halogenoarenes are far less reactive than halogenoalkanes towards nucleophilic substitution — chlorobenzene does not undergo the SN1/SN2 hydrolysis that chloroethane readily does with, for example, aqueous silver nitrate. Two reinforcing reasons, both traceable to the ring’s delocalised system:

• The C–Cl bond in chlorobenzene is shorter and stronger than in chloroethane, because one of chlorine’s lone pairs partially delocalises into the ring’s π system, giving the C–Cl bond partial double-bond character.
• The carbon bonded to Cl is part of the delocalised ring itself, so attack by a nucleophile at that carbon (as in nucleophilic substitution) would have to disrupt the ring’s aromatic stabilisation — energetically costly in a way that simply doesn’t apply to the isolated, non-aromatic carbon skeleton of chloroethane.

Both effects point the same way, which is why the reactivity gap between the two is so large — halogenoarenes are essentially inert to the conditions that hydrolyse halogenoalkanes readily.

Common mistakes

Drawing benzene reacting by addition (e.g. with Br₂ alone, no catalyst, as if it were an alkene). Benzene needs a catalyst (AlCl₃/AlBr₃) to generate a sufficiently electrophilic species and reacts by substitution, not addition — plain Br₂ doesn’t react with benzene under normal conditions at all.

Forgetting the second step of the electrophilic substitution mechanism. Losing H⁺ to restore aromaticity is not optional bookkeeping — it’s the whole reason the mechanism is substitution rather than addition, and markers expect to see it drawn explicitly.

Choosing ring conditions when side-chain substitution is asked for, or vice versa. The catalyst (AlCl₃/AlBr₃, dark) gives ring substitution; UV light with no catalyst gives side-chain (free-radical) substitution — these are two entirely different mechanisms triggered by different conditions on the same starting material.

Assuming halogenoarenes and halogenoalkanes react the same way just because they both contain C–X. Chlorobenzene’s resistance to nucleophilic substitution is a direct, testable consequence of the ring’s delocalisation — not a minor detail, but a core comparison the syllabus specifically requires.

Quick revision checklist

• Six benzene reactions: halogenation, nitration, Friedel-Crafts alkylation and acylation, side-chain oxidation, hydrogenation
• Electrophilic substitution: electrophile attacks (breaks delocalisation) → arenium ion intermediate → H⁺ lost (restores delocalisation)
• Substitution beats addition because it preserves aromatic stabilisation
• Catalyst + dark = ring substitution; UV + no catalyst = side-chain free-radical substitution
• –NH₂/–OH/–R direct 2,4-; –NO₂/–COOH/–COR direct 3-
• Halogenoarenes far less reactive than halogenoalkanes: stronger, shorter C–Cl bond (lone-pair delocalisation) + attack would disrupt ring aromaticity

Related resources

• A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape — benzene’s structure this mechanism depends on
• Halogenoalkanes — the SN1/SN2 reactivity halogenoarenes are compared against
• Phenol: Reactions and Acidity — the –OH directing effect applied in full
• Hydrocarbons: Alkanes and Alkenes — the free-radical substitution mechanism reused for side-chain halogenation
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.