1. Benzene Structure & The Disproof of the Kekule Model
Benzene has the molecular formula C₆H₆. In 1865, Friedrich Kekule proposed that benzene was a six-membered carbon ring containing alternating single and double carbon-carbon bonds (cyclohexa-1,3,5-triene). However, experimental evidence conclusively disproved this structure.
Three Critical Pieces of Evidence Disproving the Kekule Model
- Identical Carbon-Carbon Bond Lengths: X-ray diffraction shows that all six C-C bonds in benzene are of exactly equal length: 0.139 nm. This is strictly intermediate between a standard C-C single bond (0.154 nm) and a C=C double bond (0.134 nm). If Kekule were correct, the ring would be an irregular, alternating hexagon.
- Thermochemical Enthalpy of Hydrogenation:
- Hydrogenation of cyclohexene (one C=C double bond) has ΔH = -120 kJ mol⁻¹.
- Theoretical cyclohexa-1,3,5-triene with three isolated double bonds would have ΔH = 3 × (-120) = -360 kJ mol⁻¹.
- Experimental hydrogenation of benzene releases only -208 kJ mol⁻¹.
- Benzene is 152 kJ mol⁻¹ more stable than predicted by the Kekule structure. This 152 kJ mol⁻¹ energetic stabilisation is called the delocalisation resonance energy.
- Chemical Unreactivity towards Addition: Benzene does not decolourise bromine water at room temperature and does not undergo typical alkene electrophilic additions. It undergoes substitution reactions to preserve its energetically stable aromatic ring.
- Benzene is planar: a flat, regular hexagon of carbon atoms.
- All six C-C bonds are the same length, between a single and a double bond.
- Each carbon has one p electron. These overlap to form a ring of six delocalised π electrons above and below the plane.
- The delocalised ring makes benzene more stable than the Kekule structure (the hydrogenation evidence in Section 2).
2. Hydrogenation Enthalpy Energy Diagram
3. Why Benzene Undergoes Electrophilic Substitution
Each carbon atom in benzene forms three σ bonds in the plane of the ring (two to adjacent carbons, one to hydrogen), with 120 degree bond angles. The fourth electron is in a p orbital at right angles to the ring plane.
Beyond the OxfordAQA specification: these σ bonds are often described as sp² hybrid bonds.
The six p-orbitals overlap sideways in all directions above and below the plane of the carbon ring, forming a continuous delocalised pi electron system containing six electrons. This doughnut-shaped pi cloud has high electron density, attracting electrophiles.
However, addition of an electrophile would permanently disrupt the continuous pi ring, sacrificing the 152 kJ mol⁻¹ delocalisation stability. Therefore, benzene undergoes electrophilic substitution, where a hydrogen atom is replaced while the continuous delocalised ring is regenerated.
4. Nitration of Benzene
Benzene reacts with a nitrating mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at a temperature below 55 °C to form nitrobenzene:
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
Generation of the Nitronium Ion Electrophile (NO₂⁺)
Concentrated sulfuric acid acts as a catalyst and a Bronsted-Lowry acid by protonating nitric acid:
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
Mechanism: Electrophilic Aromatic Substitution
- The high electron density of the benzene pi ring attacks the positive nitronium ion (NO₂⁺). A curly arrow goes from the ring to the nitrogen of NO₂⁺.
- This breaks the aromatic ring, forming an unstable, positively charged horseshoe intermediate (arenium ion). The horseshoe open face must point directly towards the carbon carrying the -NO₂ group and -H atom.
- A curly arrow goes from the C-H bond into the ring, releasing H⁺ and reforming the stable delocalised aromatic pi electron ring.
- Catalyst regeneration: H⁺ + HSO₄⁻ → H₂SO₄.
The temperature is kept below 55 °C. Higher temperatures result in multiple substitutions, producing 1,3-dinitrobenzene and explosive 1,3,5-trinitrobenzene.
Uses of Nitration
- Explosives: for example TNT (2-methyl-1,3,5-trinitrobenzene), made by nitrating methylbenzene.
- Making amines: nitro compounds are reduced to aromatic amines such as phenylamine, which are used to make dyes. See Amines.
5. Sulfonation of Benzene
- Reagent: fuming sulfuric acid (concentrated H₂SO₄ that contains dissolved SO₃).
- Conditions: warm.
- Electrophile: SO₃. Three oxygen atoms pull electrons away from sulfur, so the S atom is δ+.
C₆H₆ + SO₃ → C₆H₅SO₃H
The product is benzenesulfonic acid.
Mechanism
- The delocalised ring attacks the S atom of SO₃.
- This gives the horseshoe intermediate, with an SO₃⁻ group.
- The C-H bond breaks and its electrons go back into the ring. The H⁺ lost joins the O⁻ to give -SO₃H.
Uses of Sulfonation
- Surfactants: alkylbenzenesulfonates (as sodium salts) are used in detergents.
- Sulfonamide drugs: antibacterial medicines made from sulfonic acid derivatives.
6. Friedel-Crafts Acylation of Benzene
Acylation introduces an acyl group (R-CO-) into the benzene ring to yield an aromatic ketone. Reagents: an acyl chloride and an anhydrous aluminium chloride (AlCl₃) catalyst. Conditions: anhydrous, heat under reflux.
Generation of the Acylium Ion Electrophile
AlCl₃ acts as a Lewis acid halogen carrier, accepting a chloride ion from the acyl chloride:
RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
RCO⁺ is the acylium ion. It can also be drawn as R-C⁺=O.
Reaction with Benzene
The strongly electrophilic acylium ion (R-C⁺=O) attacks benzene via electrophilic substitution. The intermediate loses a proton to form an aromatic ketone (e.g. phenylethanone from ethanoyl chloride), and the catalyst is regenerated:
C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
H⁺ + AlCl₄⁻ → AlCl₃ + HCl
7. Friedel-Crafts Alkylation of Benzene
Alkylation puts an alkyl group onto the ring. Reagents: a halogenoalkane (RCl) and an anhydrous AlCl₃ catalyst.
Making the Electrophile
RCl + AlCl₃ → R⁺ + AlCl₄⁻
Reaction with Benzene
C₆H₆ + CH₃CH₂Cl → C₆H₅CH₂CH₃ + HCl
The product is ethylbenzene. The mechanism has the same pattern as nitration. The catalyst is regenerated: H⁺ + AlCl₄⁻ → AlCl₃ + HCl.
The OxfordAQA specification only covers monosubstitution: one alkyl group added to the ring.
8. Methylbenzene
Methylbenzene, C₆H₅CH₃, has a CH₃ group on the ring.
- The CH₃ group releases electrons into the ring.
- So the ring reacts faster than benzene with electrophiles.
- Substitution happens mainly at positions 2 and 4.
| Reaction | Reagents | Main products |
|---|---|---|
| Nitration | Conc. HNO₃ + conc. H₂SO₄ | 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene, CH₃C₆H₄NO₂ |
| Sulfonation | Fuming sulfuric acid | Mainly 4-methylbenzenesulfonic acid, CH₃C₆H₄SO₃H |
| Acylation | RCOCl + AlCl₃ | Mainly the 4-substituted ketone, for example CH₃C₆H₄COCH₃ with ethanoyl chloride |
Example: C₆H₅CH₃ + HNO₃ → CH₃C₆H₄NO₂ + H₂O
9. Chlorine and UV Light: Free-Radical Reactions
In UV light, Cl₂ splits into chlorine free radicals. These react in a different way from electrophiles.
Benzene + Cl₂ in UV light: addition to the ring
C₆H₆ + 3Cl₂ → C₆H₆Cl₆
- The product is 1,2,3,4,5,6-hexachlorocyclohexane.
- Six Cl atoms add to the ring. The ring loses its delocalisation.
Methylbenzene + Cl₂ in UV light: substitution in the side chain
Conditions: UV light, boiling, no catalyst.
C₆H₅CH₃ + Cl₂ → C₆H₅CH₂Cl + HCl
- The product is (chloromethyl)benzene. The ring is not changed.
- With excess Cl₂, substitution continues to C₆H₅CHCl₂ and then C₆H₅CCl₃.
Contrast: Cl₂ with an AlCl₃ catalyst, in the dark
Now Cl₂ substitutes the ring by electrophilic substitution. Methylbenzene gives 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene.
| Reactant | Conditions | Where Cl goes | Product |
|---|---|---|---|
| Benzene | Cl₂, UV light | Adds to the ring | C₆H₆Cl₆ |
| Methylbenzene | Cl₂, UV light, boiling | Substitutes the side chain (CH₃) | C₆H₅CH₂Cl |
| Methylbenzene | Cl₂, AlCl₃, dark | Substitutes the ring | 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene |
10. Ring Cl vs Side-Chain Cl: Relative Reactivity
(Chloromethyl)benzene: Cl on the side chain
It reacts like a halogenoalkane. Warm it with NaOH(aq) and nucleophilic substitution gives phenylmethanol:
C₆H₅CH₂Cl + NaOH → C₆H₅CH₂OH + NaCl
Chlorobenzene: Cl on the ring
It is very unreactive to nucleophiles:
- A lone pair on Cl overlaps with the delocalised ring. The C-Cl bond becomes stronger and shorter (partial double-bond character).
- The electron-rich ring repels nucleophiles.
Test to Tell Them Apart
- Warm each compound with NaOH(aq).
- Acidify with dilute HNO₃.
- Add AgNO₃(aq).
- (Chloromethyl)benzene: white precipitate of AgCl.
- Chlorobenzene: no precipitate.
11. Worked Mechanism & Thermochemical Calculation
Calculate the expected enthalpy of hydrogenation for the non-delocalised Kekule model of benzene, and determine the delocalisation energy given the experimental value of benzene is -208 kJ mol⁻¹.
Step 1: Calculate theoretical hydrogenation for 3 double bonds:
Expected for 3 C=C bonds: 3 × (-120) = -360 kJ mol⁻¹.
Step 2: Compare with the actual value:
Actual value for benzene: -208 kJ mol⁻¹.
Delocalisation energy = -208 - (-360) = 152 kJ mol⁻¹.
Benzene is 152 kJ mol⁻¹ lower in energy (more stable) than the theoretical non-delocalised triene.
Reactants: Benzene + Ethanoyl chloride (CH₃COCl)
Catalyst: Anhydrous aluminium chloride (AlCl₃)
Electrophile: CH₃-C⁺=O (ethanoyl cation / acylium ion)
Equation: C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl
Product: Phenylethanone (an aromatic ketone) and steamy fumes of hydrogen chloride.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
AQA does not cover sulfonation, alkylation or chlorine with methylbenzene.
12. Practice Questions
Try each question, then open the answer to check your mark points.
Question 1 (2 marks): Name the products of methylbenzene with Cl₂ (a) in UV light and (b) with AlCl₃ in the dark.
Show Answer & Mark Points
- (a) (chloromethyl)benzene, C₆H₅CH₂Cl (1)
- (b) 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene (1)
Question 2 (2 marks): Write equations to show how AlCl₃ forms the electrophile in Friedel-Crafts alkylation with chloroethane, and how it is regenerated.
Show Answer & Mark Points
- CH₃CH₂Cl + AlCl₃ → CH₃CH₂⁺ + AlCl₄⁻ (1)
- H⁺ + AlCl₄⁻ → AlCl₃ + HCl (1)
Question 3 (3 marks): Explain why chlorobenzene does not react with warm NaOH(aq) but (chloromethyl)benzene does.
Show Answer & Mark Points
- In chlorobenzene the C-Cl bond is strengthened by overlap of a Cl lone pair with the delocalised ring (1)
- The electron density of the ring repels nucleophiles (1)
- The side-chain C-Cl in (chloromethyl)benzene behaves like a halogenoalkane, with a δ+ carbon open to attack by OH⁻ (1)
Question 4 (2 marks): Give the electrophile and one use of the product in the sulfonation of benzene.
Show Answer & Mark Points
- SO₃ (1)
- Detergents (surfactants) or sulfonamide drugs (1)