Reactions of arenes

Chemistry· 5 min read

1. Electrophilic Substitution Overview★★☆☆☆⏱ 15 min

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📘 Definition

Electrophilic substitution

A reaction where an electrophile replaces a hydrogen atom on the benzene ring, preserving the stable delocalized π-system of the aromatic ring.

Example:

Chlorination of benzene to produce chlorobenzene

Arenes are far less reactive than alkenes towards addition reactions because addition would break the stable delocalized π-system, resulting in a very high activation energy. Substitution retains aromatic stability, so it is the favoured reaction pathway for arenes.

📐 Worked Example

Write the two-step mechanism for electrophilic substitution of benzene with a general electrophile $E^{+}$ .

1
Step 1 (rate-determining step): The electron-rich delocalized π-system attacks the electrophile, forming a positively charged non-aromatic intermediate called a Wheland intermediate.
2
$\ce{C6H6 + E+ -> [C6H6E]+}$
3
Step 2: A proton is lost from the carbon bonded to the electrophile, restoring the delocalized π-system and aromatic stability.
4
$\ce{[C6H6E]+ -> C6H5E + H+}$

Exam tip:

Always draw a partial broken circle in the Wheland intermediate to get full marks in mechanism questions.

2. Core Reactions of Benzene★★☆☆☆⏱ 20 min

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Four core electrophilic substitution reactions of benzene are routinely tested in CIE exams, each requiring specific reagents and conditions that must be recalled exactly.

Reaction	Reagents	Conditions	Main Product
Nitration	Conc $\ce H N O 3$ + conc $\ce H 2 S O 4$	50-60°C	Nitrobenzene
Chlorination	$\ce C l 2$ + anhydrous $\ce A l C l 3$	Room temperature	Chlorobenzene
Friedel-Crafts Alkylation	Chloroalkane + anhydrous $\ce A l C l 3$	Anhydrous, RT	Alkylbenzene
Friedel-Crafts Acylation	Acyl chloride + anhydrous $\ce A l C l 3$	Anhydrous, reflux	Phenylketone

📐 Worked Example

Identify the reagents and conditions required to convert benzene to ethylbenzene, and name the reaction type.

1
1. Ethylbenzene has a 2-carbon alkyl group added to benzene, so this is a Friedel-Crafts alkylation reaction.
2
1. Reagents are chloroethane and anhydrous aluminium chloride ( $\ce A l C l 3$ ), which acts as a Lewis acid catalyst to generate the electrophilic ethyl carbocation.
3
1. Required conditions are strictly anhydrous, at room temperature.

3. In-situ Electrophile Generation★★★☆☆⏱ 15 min

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CIE exams regularly ask for equations showing how electrophiles are generated in the reaction mixture before substitution occurs. The catalyst (a strong acid or Lewis acid) is responsible for generating the electrophile.

📐 Worked Example

Write the equations for generation of the nitronium ion ( $\ce N O 2 +$ ) electrophile for benzene nitration.

1
Step 1: Concentrated sulfuric acid (stronger acid) protonates nitric acid:
2
$\ce{HNO3 + H2SO4 <=> H2NO3+ + HSO4-}$
3
Step 2: Protonated nitric acid loses water to form the electrophile:
4
$\ce{H2NO3+ -> NO2+ + H2O}$

For halogenation, the Lewis acid catalyst polarizes the halogen molecule to generate the electrophile: $\ce B r 2 + F e B r 3 - > B r + + F e B r 4 -$ .

Exam tip:

Always show the correct positive charge on the generated electrophile to avoid losing marks.

4. Substituent Directing Effects★★★★☆⏱ 20 min

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📘 Definition

Directing effect

The influence of an existing substituent on the benzene ring that determines the position of the next incoming electrophilic substitution.

Substituents are grouped by their directing effect: most activating groups and halogens (deactivated) are ortho/para directors, while most other deactivating groups are meta directors.

📐 Worked Example

Predict the major products of mononitration of methylbenzene (toluene) and explain your answer.

1
1. The methyl group on methylbenzene is an activating, ortho/para directing group.
2
1. The methyl group donates electron density to the ring, activating the positions 2 (ortho) and 4 (para) relative to itself for electrophilic attack.
3
1. The major products are therefore 2-nitromethylbenzene (ortho) and 4-nitromethylbenzene (para), with very little meta product formed.

Exam tip:

Remember the key exception: halogens are deactivating but still ortho/para directors.

5. Common Pitfalls

Wrong move:

Drawing a full delocalized circle in the Wheland intermediate

Why:

The intermediate is non-aromatic, with delocalization broken at the substituted carbon

Correct move:

Draw a partial broken circle in the intermediate, only across the 5 unsubstituted carbons

Wrong move:

Confusing electrophilic substitution with addition for benzene

Why:

Addition breaks aromatic stability, so it is not favoured under normal conditions

Correct move:

Recall that arenes undergo substitution to retain their stable aromatic structure

Wrong move:

Writing $\ce A l C l 3$ as a consumed reactant

Why:

$\ce A l C l 3$ is a catalyst that is regenerated at the end of the reaction

Correct move:

Write $\ce A l C l 3$ above the reaction arrow, not as a reactant

Wrong move:

Claiming all deactivating groups are meta directors

Why:

Halogens are a common exception tested regularly in exams

Correct move:

Memorise that halogens are deactivating but ortho/para directing

Wrong move:

Using >60°C for mononitration of benzene

Why:

Higher temperatures lead to multiple substitution (dinitration) instead of mononitration

Correct move:

Recall that mononitration requires 50-60°C

6. Quick Reference Cheatsheet

Substituent	Type	Directing
Alkyl (-CH₃)	Activating	Ortho/para
-OH, -NH₂	Activating	Ortho/para
-NO₂	Deactivating	Meta
-COOH, -CN	Deactivating	Meta
-Cl, -Br	Deactivating	Ortho/para
Reaction	Reagents	Conditions
Nitration	Conc HNO₃ + conc H₂SO₄	50-60°C
Chlorination	Cl₂ + AlCl₃	RT, anhydrous
Friedel-Crafts	RCl/AcylCl + AlCl₃	Anhydrous, RT

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Electrophilic substitution mechanism
2023 · 4
Directing effect of substituents
2021 · 1
Reaction conditions identification

What's Next

Reactions of arenes form the foundation of aromatic organic synthesis, a high-weight topic for Paper 4 of CIE 9701. Understanding directing effects allows you to plan multi-step syntheses of complex substituted aromatic compounds, where the order of adding substituents directly determines the final product. This knowledge is also required to understand reactions of aromatic amines, dyes, and pharmaceutical intermediates that are regularly tested in exams.