AQA A-Level Chemistry: Aromatic chemistry (7405) – Revision Notes

These notes condense section 3.3.10 Aromatic chemistry of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.10.1 Bonding and 3.3.10.2 Electrophilic substitution. The whole section is A-level only. It is examined on A-level Paper 2, and A-level Paper 3 can test any content.

For full explanations and worked examples, read the aromatic chemistry study guide first. Then test yourself with the aromatic chemistry practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. The free 10-minute diagnostics help you find weak spots. Electrophiles and curly arrows were first met in the alkenes revision notes, and enthalpy changes in the energetics revision notes.

Key definitions

Term	Meaning
Aromatic compound	Contains a benzene ring
Delocalised electrons	Electrons shared over more than two atoms; in benzene, six p electrons spread over all six carbons
Cyclohexa-1,3,5-triene	Theoretical C₆H₆ with three fixed C=C and three C–C bonds; not the real structure
Enthalpy of hydrogenation	Enthalpy change when H₂ adds across C=C bonds to give a saturated product
Electrophile	Electron pair acceptor
Electrophilic substitution	An electrophile replaces a hydrogen atom on the ring; the ring stays intact
Nitronium ion	NO₂⁺, the electrophile in nitration
Acylium ion	RCO⁺, the electrophile in Friedel–Crafts acylation
Monosubstitution	Only one hydrogen on the ring is replaced (the limit of the specification)

Bonding in benzene (3.3.10.1)

• C₆H₆: six carbons in a regular hexagon, one H on each carbon.
• Each C forms three σ bonds (two C, one H): bond angles 120°, molecule planar.
• Each C has one electron in a p orbital at right angles to the ring.
• The p orbitals overlap sideways all round the ring: the six p electrons are delocalised above and below the plane.
• All C–C bonds are the same length, intermediate between C–C single and C=C double.
• Delocalisation makes benzene more stable than cyclohexa-1,3,5-triene.

Method: use hydrogenation data to show extra stability

1. Take ΔH for one C=C (from cyclohexene).
2. Multiply by 3 for the theoretical triene.
3. Compare with the measured value for benzene: benzene’s is less exothermic.
4. Both give cyclohexane, so benzene must start lower in energy.
5. The difference is the extra stability; give the reason: delocalisation of p electrons.

Worked reminder with common data: 3 × (−120) = −360 kJ mol⁻¹ predicted; benzene −208 kJ mol⁻¹; benzene is 152 kJ mol⁻¹ more stable. (Use whatever values the question gives.)

Energy level diagram, in words. Put cyclohexane on the lowest line. Draw benzene + 3H₂ above it by the size of benzene’s measured value, and cyclohexa-1,3,5-triene + 3H₂ higher still by the predicted value. Label the gap between the two upper lines as the extra stability from delocalisation. All arrows for hydrogenation point down, because the reactions are exothermic. If a question gives a value for cyclohexa-1,3-diene, the same logic applies: predict 2 × one C=C and compare.

Substitution, not addition

• Addition would break the delocalised ring and lose the extra stability.
• Substitution replaces one H and keeps the delocalised ring.
• Benzene’s electron density is spread over six carbons, so it needs a strong electrophile (a positive ion) to react.

Electrophilic substitution (3.3.10.2)

Method: outline any electrophilic substitution mechanism

1. Write the equation that makes the electrophile.
2. Arrow 1: from the delocalised ring (circle) to the atom of the electrophile that bonds (N in NO₂⁺, C in RCO⁺).
3. Intermediate: attacked carbon carries H and the new group; horseshoe over the other five carbons, gap facing the attacked carbon, + inside.
4. Arrow 2: from the C–H bond into the ring.
5. Product with a full circle, plus H⁺.
6. Show the catalyst being regenerated.

Nitration

Item	Detail
Reagents	concentrated HNO₃ and concentrated H₂SO₄
Electrophile	NO₂⁺
Forming the electrophile	HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻ (or HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O)
Overall	C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Catalyst regenerated	H⁺ + HSO₄⁻ → H₂SO₄
Product	nitrobenzene
Importance	step in synthesis: manufacture of explosives; reduction gives aromatic amines (3.3.11.1) used to make dyes

Friedel–Crafts acylation

Item	Detail
Reagents	acyl chloride RCOCl, with AlCl₃ catalyst, dry conditions
Electrophile	RCO⁺ (e.g. CH₃CO⁺ from ethanoyl chloride)
Forming the electrophile	RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
Overall (ethanoyl chloride)	C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
Catalyst regenerated	H⁺ + AlCl₄⁻ → AlCl₃ + HCl
Product	phenyl ketone (phenylethanone from ethanoyl chloride)
Importance	forms a new C–C bond to the ring: an important step in synthesis

Naming the products

Acyl chloride	Electrophile	Product
CH₃COCl, ethanoyl chloride	CH₃CO⁺	C₆H₅COCH₃, phenylethanone
CH₃CH₂COCl, propanoyl chloride	CH₃CH₂CO⁺	C₆H₅COCH₂CH₃, 1-phenylpropan-1-one
CH₃CH₂CH₂COCl, butanoyl chloride	CH₃CH₂CH₂CO⁺	C₆H₅COCH₂CH₂CH₃, 1-phenylbutan-1-one

Practical link

• Section 3.3.10 lists no required practical.
• The specification suggests preparing methyl 3-nitrobenzoate by nitrating methyl benzoate, then recrystallising it and measuring its melting point.
• Key steps: keep the mixture cold, add the nitrating mixture slowly, pour onto ice, filter under reduced pressure, recrystallise from the minimum volume of hot solvent, dry, then take the melting point.
• Pure: sharp melting point close to the data value. Impure: lower and over a range.
• These are the techniques of Required practical 10, listed under 3.3.9.2.

Must-know distinctions

• Alkene vs benzene: alkenes do electrophilic addition; benzene does electrophilic substitution.
• Localised vs delocalised: an alkene’s C=C electron density sits between two carbons; benzene’s p electrons are spread over six.
• Bond length vs bond type: benzene’s C–C bonds are all equal, between single and double.
• Theoretical vs measured hydrogenation: the triene value is predicted (3 × one C=C); benzene’s is measured and less exothermic.
• NO₂⁺ vs NO₂: the electrophile is the nitronium ion; the group in the product is a neutral nitro group.
• Acylation catalyst vs reagent: AlCl₃ is the catalyst; the acyl chloride supplies the RCO group.
• Ketone vs aldehyde: acylation of benzene gives a ketone.

Quick self-test

1. State the bond angle in benzene and the shape of the molecule.
2. Describe how the C–C bond lengths in benzene compare with C–C and C=C bonds.
3. ΔH hydrogenation of cyclohexene is −120 kJ mol⁻¹. Predict ΔH for hydrogenating cyclohexa-1,3,5-triene to cyclohexane.
4. Benzene’s measured value is −208 kJ mol⁻¹. By how much is benzene more stable than the triene?
5. Give one reason why benzene does substitution, not addition.
6. Write an equation to form the electrophile in nitration.
7. Which atom of NO₂⁺ does arrow 1 point to?
8. Write an equation to show AlCl₃ being regenerated in acylation.
9. Name the product of benzene with ethanoyl chloride and AlCl₃.
10. Calculate the atom economy for making nitrobenzene: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O. (Ar: H 1.0, C 12.0, N 14.0, O 16.0)
11. Calculate the atom economy for C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl. (Ar: Cl 35.5)
12. Give two uses of nitration in synthesis.

Answers

1. 120°; planar (flat hexagon).
2. All six are the same length, intermediate between a C–C single bond and a C=C double bond.
3. 3 × (−120) = −360 kJ mol⁻¹.
4. −208 − (−360) = +152, so benzene is 152 kJ mol⁻¹ more stable.
5. Addition would destroy the delocalised ring and its extra stability; substitution keeps it.
6. HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻ (or HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O).
7. The nitrogen atom.
8. H⁺ + AlCl₄⁻ → AlCl₃ + HCl.
9. Phenylethanone, C₆H₅COCH₃.
10. Mr: C₆H₅NO₂ 123.0, C₆H₆ 78.0, HNO₃ 63.0. 123.0 / 141.0 × 100 = 87.2%.
11. Mr: C₆H₅COCH₃ 120.0, CH₃COCl 78.5. 120.0 / 156.5 × 100 = 76.7%.
12. Manufacture of explosives; making aromatic amines (by reducing the nitro compound), which are used to make dyes.

Where marks are usually lost

• Writing “bonds between single and double” without saying length, or drawing Kekulé double bonds when the question is about delocalisation.
• Giving the stability difference without saying that benzene is the more stable one and why (delocalisation).
• Saying the triene value is “measured” — it is a prediction.
• Explaining “no addition” by saying benzene has no double bonds, instead of loss of delocalisation.
• Arrow 1 starting from a carbon atom or a C–H bond rather than the ring.
• The intermediate drawn with a full circle, no +, or the horseshoe gap on the wrong side.
• Arrow 1 pointing at an O of NO₂⁺, or at the O of RCO⁺.
• Leaving out the equation that makes the electrophile, or the catalyst regeneration step.
• Forgetting HCl as the by-product of acylation.
• Naming the acylation product as an aldehyde or an acyl chloride.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.10 Aromatic chemistry (A-level only), covering 3.3.10.1 Bonding and 3.3.10.2 Electrophilic substitution.