Aromatic hydrocarbons: naming and isomers
Aromatic hydrocarbons (arenes) contain one or more benzene rings. The name came from the pleasant smell of some of them. Those with one ring are benzenoid (benzene, toluene); those with fused rings include naphthalene and anthracene.
- Toluene = methylbenzene C₆H₅CH₃. Xylene = dimethylbenzene.
- For two groups on a ring we use positions: ortho (o) = 1,2; meta (m) = 1,3; para (p) = 1,4. So there are three dimethylbenzenes: o-, m- and p-xylene.
- C₆H₅– is called phenyl.
Structure of benzene
Benzene is C₆H₆. It is very unsaturated on paper, yet it does not decolourise bromine water or Baeyer's reagent easily. Why?
Kekulé structure (1865)
A six-carbon ring with alternating single and double bonds. Problem: it predicts two different 1,2-dibromobenzenes (Br atoms across a single or a double bond), but only one exists. Kekulé said the double bonds keep switching places.
Resonance
Today we say the two Kekulé forms are resonance structures. Real benzene is a hybrid: it is not switching, it is always in between. We draw it as a hexagon with a circle inside.
Orbital picture
- All six carbons are sp². The ring is flat; all bond angles are 120°.
- Each carbon has one unhybridised p orbital standing at right angles to the ring.
- The six p orbitals overlap sideways all round the ring, forming a continuous π cloud above and below the ring. The six π electrons are delocalised.
- All C–C bonds are 139 pm, between C–C (154 pm) and C=C (134 pm).
- Delocalisation lowers the energy: benzene is about 150 kJ/mol more stable than a ring with three fixed double bonds would be. This is its resonance energy.
Aromaticity (Hückel rule)
A compound is aromatic if it is:
- Cyclic (a ring),
- Planar (flat),
- Fully conjugated: every ring atom has a p orbital, so the π cloud goes all round,
- Has (4n + 2) π electrons, where n = 0, 1, 2, … That means 2, 6, 10, 14 π electrons.
Examples: benzene (6 π, n = 1) ✓; naphthalene (10 π, n = 2) ✓; cyclopentadienyl anion (6 π) ✓; cyclopropenyl cation (2 π, n = 0) ✓. Cyclooctatetraene (8 π) is not aromatic; it bends into a tub shape. Cyclopentadiene is not aromatic because one carbon is sp³ and breaks the ring of p orbitals.
Preparation of benzene
- Cyclic polymerisation of ethyne: 3HC≡CH → C₆H₆ (red-hot iron tube, 873 K).
- Decarboxylation of sodium benzoate: C₆H₅COONa + NaOH → C₆H₆ + Na₂CO₃ (soda lime, heat).
- Reduction of phenol: heat phenol vapour with zinc dust: C₆H₅OH + Zn → C₆H₆ + ZnO.
Benzene is a colourless liquid with a typical smell, lighter than water, does not mix with water, and burns with a sooty flame (high carbon content).
Electrophilic substitution reactions
The π cloud is rich in electrons, so benzene attracts electrophiles (E⁺). But adding across a bond would destroy the stable aromatic ring. So benzene substitutes: E replaces an H and the ring stays aromatic.
Mechanism in three steps
- Making E⁺: a reagent with a catalyst forms the electrophile. Example: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.
- Attack (slow): E⁺ takes two π electrons and bonds to one ring carbon. That carbon becomes sp³. The positive charge spreads over the other five carbons. This ion is the σ-complex (arenium ion). It is not aromatic.
- Loss of H⁺ (fast): a base (HSO₄⁻, AlCl₄⁻) takes the H⁺ from the sp³ carbon. The π ring closes again and aromaticity comes back.
The five key reactions
- Nitration: conc. HNO₃ + conc. H₂SO₄ (nitrating mixture), about 323–333 K → nitrobenzene C₆H₅NO₂. E⁺ = NO₂⁺ (nitronium ion).
- Halogenation: Cl₂ with a Lewis acid (anhydrous FeCl₃, FeBr₃ or AlCl₃) in the dark → chlorobenzene. The Lewis acid makes Cl⁺.
- Sulphonation: fuming H₂SO₄ (oleum) → benzenesulphonic acid C₆H₅SO₃H. E = SO₃.
- Friedel–Crafts alkylation: CH₃Cl + anhydrous AlCl₃ → toluene C₆H₅CH₃. E⁺ = CH₃⁺ (R⁺).
- Friedel–Crafts acylation: CH₃COCl + anhydrous AlCl₃ → acetophenone C₆H₅COCH₃. E⁺ = CH₃CO⁺ (acylium ion).
Addition and combustion (harsh conditions only)
- H₂ with Ni at high temperature and pressure → cyclohexane C₆H₁₂.
- Cl₂ in UV light (no catalyst) adds three times → benzene hexachloride C₆H₆Cl₆ (BHC).
- Burning: 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O, with a sooty flame.
Directive influence of a group
When benzene already has one group, the second group does not go anywhere at random. The first group decides the position.
Ortho and para directors
–OH, –OCH₃, –NH₂, –NHR, –CH₃ (alkyl), –Cl, –Br, –I. Most have a lone pair next to the ring (or push electrons, like CH₃). Resonance puts extra electron density on the ortho and para carbons, so E⁺ goes there. Example: toluene + nitration → o-nitrotoluene + p-nitrotoluene.
- –OH, –NH₂, –CH₃ also make the ring react faster than benzene: they are activating.
- Halogens are special: they pull electrons through the σ bond (−I effect), so the ring is slower (deactivating), but their lone pairs still direct to ortho/para.
Meta directors
–NO₂, –CN, –CHO, –COR, –COOH, –SO₃H. These pull electrons out of the ring (the atom next to the ring has a multiple bond to an electronegative atom). Resonance leaves ortho and para most positive, so E⁺ goes to the meta position, which is less poor. They are all deactivating. Example: nitrobenzene + nitration (harder conditions) → m-dinitrobenzene.
Carcinogenicity and toxicity
Benzene and polynuclear hydrocarbons (more than two fused benzene rings) are toxic, and many are carcinogenic (cancer-causing).
- They form when organic matter such as coal, wood, petrol or tobacco burns incompletely. They are found in soot, coal tar, vehicle exhaust and cigarette smoke.
- Examples: 1,2-benzanthracene, 3-methylcholanthrene, 1,2,5,6-dibenzanthracene and benzo[a]pyrene.
- Inside the body, enzymes convert them into reactive molecules that bind to DNA. This damages genes and can start cancer.
- Long exposure to benzene vapour can damage the bone marrow and cause leukaemia (blood cancer).
Staying safe: avoid smoking and second-hand smoke, keep kitchens with wood or coal fires ventilated, do not breathe petrol vapour, and in the lab use benzene only in a fume hood (toluene is a safer solvent).
Key formulas and definitions
- Hückel rule: aromatic if cyclic, planar, conjugated and π electrons = 4n + 2
- Benzene C–C = 139 pm (C–C 154, C=C 134)
- C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (conc. H₂SO₄)
- C₆H₆ + Cl₂ → C₆H₅Cl + HCl (anhyd. FeCl₃)
- C₆H₆ + H₂SO₄(fuming) → C₆H₅SO₃H + H₂O
- C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl (anhyd. AlCl₃)
- C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (anhyd. AlCl₃)
- o/p: –OH, –NH₂, –OCH₃, –CH₃, –X | m: –NO₂, –CN, –CHO, –COOH, –SO₃H
Worked examples
1. Is the cyclopentadienyl anion (C₅H₅⁻) aromatic?
Check all four rules. Cyclic ✓. Planar ✓. The CH⁻ carbon has a lone pair in a p orbital, so every atom has a p orbital: conjugated ✓. π electrons: 2 double bonds × 2 = 4, plus the lone pair 2 = 6. 6 = 4(1) + 2 ✓. So it is aromatic.
2. How many π electrons does naphthalene (C₁₀H₈) have? Is it aromatic?
It has 5 double bonds in the usual drawing → 10 π electrons. 10 = 4(2) + 2, n = 2. It is flat and fully conjugated, so it is aromatic.
3. Write the electrophile and product in the Friedel–Crafts acylation of benzene with ethanoyl chloride.
CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻. The acylium ion CH₃CO⁺ attacks the ring; H⁺ is lost. Product: acetophenone C₆H₅COCH₃ (+ HCl).
4. Predict the main products when toluene is nitrated.
–CH₃ is an o/p director (it pushes electrons into the ring). So the NO₂ goes to ortho or para: o-nitrotoluene and p-nitrotoluene.
5. Predict the product when nitrobenzene is brominated (Br₂/FeBr₃).
–NO₂ pulls electrons and is a meta director. Br goes to the meta position: m-bromonitrobenzene (1-bromo-3-nitrobenzene).
6. Starting from benzene, which order of steps gives m-chloronitrobenzene: nitrate first or chlorinate first?
Nitrate first. –NO₂ is meta-directing, so the Cl then goes meta. If you chlorinated first, –Cl (o/p director) would send NO₂ to ortho and para instead.
7. How many grams of nitrobenzene can be made from 39 g of benzene if the yield is 100%?
C₆H₆ = 78 g/mol, so 39 g = 0.5 mol. 1 mol benzene → 1 mol nitrobenzene C₆H₅NO₂ (72 + 5 + 14 + 32 = 123 g/mol). Mass = 0.5 × 123 = 61.5 g.
Common mistakes
- Thinking benzene switches between the two Kekulé forms. It is always the hybrid; all bonds are equal at every moment.
- Calling every ring with double bonds aromatic. Check all four rules, especially planar and 4n + 2 (cyclooctatetraene with 8 π is not).
- Treating halogens as meta directors because they deactivate the ring. They deactivate but still direct ortho/para.
- Writing addition for benzene + Br₂ with FeBr₃. With a Lewis acid, benzene substitutes (bromobenzene), keeping the ring.