What you'll learn
Aromatic compounds, amines and polymers complete the organic chemistry of CAPE. Benzene has a uniquely stable ring structure that gives it chemistry very different from alkenes. Amines are organic bases found in dyes and medicines. Polymers — from polyethene bags to nylon fishing nets and the proteins in our bodies — are giant molecules built from small repeating units. This guide covers the structure and stability of benzene (the delocalised model versus the Kekulé model), electrophilic substitution reactions (nitration, halogenation, Friedel–Crafts), the effect of substituents in phenol and methylbenzene, the basicity and reactions of amines, diazonium salts and azo dyes, amino acids and proteins, and addition and condensation polymerisation, including the environmental issues of plastic waste in the Caribbean.
CAPE questions ask you to explain the stability of benzene using evidence, draw electrophilic substitution mechanisms, compare basicity, identify monomers from polymer structures and discuss disposal of polymers.
Key terms and definitions
Aromatic compound (arene) — A compound containing a benzene ring.
Delocalisation — The spreading of π electrons over several atoms, rather than being located between two.
Electrophilic substitution — A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring.
Phenol — C₆H₅OH, with an –OH group directly attached to a benzene ring.
Amine — A compound derived from ammonia in which one or more hydrogen atoms are replaced by organic groups.
Diazonium salt — A compound containing the –N≡N⁺ group, formed from an aromatic amine and nitrous acid.
Azo dye — A coloured compound containing the –N=N– group linking two aromatic rings.
Monomer — A small molecule that joins with others to form a polymer.
Addition polymerisation — Joining of unsaturated monomers without loss of any small molecule.
Condensation polymerisation — Joining of monomers with the loss of a small molecule such as water or HCl.
Peptide (amide) link — –CONH–, joining amino acids in proteins and in polyamides.
Core concepts
The structure of benzene
Kekulé proposed a ring of six carbons with alternating single and double bonds. Evidence shows this is not accurate:
- Bond lengths: all six C–C bonds in benzene are the same length (0.139 nm), between a single (0.154 nm) and a double bond (0.134 nm).
- Enthalpy of hydrogenation: hydrogenating cyclohexene releases about −120 kJ mol⁻¹. A Kekulé structure with three double bonds would be expected to release about −360 kJ mol⁻¹, but benzene releases only about −208 kJ mol⁻¹. Benzene is about 150 kJ mol⁻¹ more stable than predicted — this is its delocalisation energy.
- Reactivity: benzene does not decolourise bromine water and resists addition reactions, unlike alkenes.
The delocalised model: each carbon is sp² hybridised and bonded to two carbons and one hydrogen in a planar ring (bond angles 120°). The remaining p orbital on each carbon overlaps sideways with its neighbours to form a ring of delocalised π electrons above and below the plane. This is shown as a hexagon with a circle inside.
Because delocalisation gives extra stability, benzene undergoes substitution (which keeps the delocalised ring intact) rather than addition.
Electrophilic substitution
General mechanism:
- A strong electrophile (E⁺) is generated.
- The delocalised π electrons attack E⁺, forming an unstable intermediate in which delocalisation is partly disrupted (a positively charged ring with a horseshoe of delocalisation).
- A C–H bond breaks; the electrons return to the ring, restoring the delocalised system and releasing H⁺.
Nitration: benzene + concentrated nitric acid and concentrated sulfuric acid, below 55 °C, → nitrobenzene. The electrophile is the nitronium ion, NO₂⁺: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻ Nitrobenzene is used to make aniline (phenylamine) for dyes and explosives such as TNT.
Halogenation: benzene + bromine with an iron(III) bromide (or iron) halogen carrier catalyst → bromobenzene + HBr. The catalyst polarises Br₂ to form the electrophile Br⁺ (as FeBr₄⁻).
Friedel–Crafts alkylation and acylation: benzene + a chloroalkane or acyl chloride with an AlCl₃ catalyst forms alkylbenzenes or aromatic ketones, creating new C–C bonds.
Phenol and methylbenzene
Phenol is more reactive than benzene: a lone pair on the oxygen is partly delocalised into the ring, increasing its electron density. Phenol decolourises bromine water at room temperature without a catalyst, forming a white precipitate of 2,4,6-tribromophenol. Phenol is weakly acidic — it reacts with sodium hydroxide but not with carbonates — and is used to make antiseptics, resins and pharmaceuticals.
Methylbenzene is also more reactive than benzene because the methyl group donates electron density. Its side chain can be oxidised by hot alkaline KMnO₄ to form benzoic acid.
Amines
Amines are bases because the nitrogen has a lone pair that can accept a proton: CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻
Basicity order: primary aliphatic amines (e.g. ethylamine) > ammonia > aromatic amines (e.g. phenylamine).
- Alkyl groups donate electrons to the nitrogen, making its lone pair more available.
- In phenylamine, the lone pair is delocalised into the benzene ring, making it less available.
Preparation: aliphatic amines from halogenoalkanes and excess ammonia; phenylamine by reducing nitrobenzene with tin and concentrated HCl, then adding NaOH.
Diazonium salts and azo dyes
Phenylamine reacts with nitrous acid (from NaNO₂ and HCl) below 10 °C to form benzenediazonium chloride. Above this temperature, it decomposes to phenol and nitrogen gas.
The diazonium ion then reacts with phenol in alkaline solution (a coupling reaction) to form a yellow-orange azo dye. The –N=N– group links two aromatic rings, extending the delocalised system so that the compound absorbs visible light. Azo dyes are used in textiles, food colourings and indicators such as methyl orange.
Amino acids and proteins
Amino acids contain both an amine (–NH₂) and a carboxylic acid (–COOH) group. In solution, they exist as zwitterions (+H₃N–CHR–COO⁻) and are amphoteric. Except glycine, they are chiral.
Amino acids join by condensation, forming peptide links (–CONH–) and releasing water. Many amino acids joined together form polypeptides and proteins. Proteins can be hydrolysed back to amino acids by heating with acid or by enzymes; the amino acids can then be identified by chromatography.
Addition polymerisation
Alkenes join together when the double bonds open, with no by-products:
n CH₂=CH₂ → –[CH₂–CH₂]ₙ– (polyethene)
Examples: poly(ethene) (bags, bottles), poly(propene) (ropes, containers), poly(chloroethene) / PVC (pipes, window frames), poly(phenylethene) / polystyrene (packaging, food containers). To find the monomer, take the repeat unit and put a double bond between the two main-chain carbons.
Condensation polymerisation
Monomers with two functional groups join, releasing a small molecule.
- Polyesters (e.g. Terylene/PET): a diol + a dicarboxylic acid, joined by ester links (–COO–), releasing water. PET is used for drinks bottles and fibres.
- Polyamides (e.g. nylon-6,6): a diamine + a dicarboxylic acid (or diacyl chloride), joined by amide links (–CONH–). Nylon is used in fishing nets and ropes — important in Caribbean fisheries.
- Proteins are natural polyamides.
Polymers and the environment
Addition polymers are non-biodegradable because their strong, non-polar C–C and C–H bonds are not attacked by microorganisms. In the Caribbean, plastic waste pollutes beaches and coral reefs, harms turtles and seabirds, and blocks drains, worsening urban flooding; several Caribbean countries have introduced bans on single-use plastic bags and polystyrene food containers.
Disposal options:
- Landfill — wastes land and the plastics persist for centuries.
- Incineration — reduces volume and recovers energy but produces CO₂, and burning PVC releases toxic HCl and dioxins.
- Recycling — saves resources but requires sorting and is costly.
- Biodegradable and photodegradable polymers — condensation polymers such as polyesters and polyamides can be hydrolysed, and new bioplastics are designed to break down.
Worked examples
Example 1: Evidence for delocalisation
Use enthalpy data to show that benzene is more stable than the Kekulé structure predicts. (3 marks)
ΔH hydrogenation of cyclohexene = −120 kJ mol⁻¹, so three C=C bonds would give −360 kJ mol⁻¹ (1). Benzene's actual value is about −208 kJ mol⁻¹ (1). Benzene is therefore about 152 kJ mol⁻¹ more stable, due to delocalisation of its π electrons (1).
Example 2: Basicity
Explain why ethylamine is a stronger base than phenylamine.
In ethylamine, the ethyl group donates electron density to the nitrogen, making its lone pair more available to accept a proton. In phenylamine, the lone pair is partly delocalised into the benzene ring, so it is less available.
Example 3: Identifying a monomer
A polymer has the repeat unit –[CH₂–CH(CH₃)]–. Name the monomer.
Propene, CH₂=CHCH₃.
Example 4: Condensation polymer
Identify the type of polymer and the small molecule lost when hexane-1,6-diamine reacts with hexanedioic acid.
A polyamide (nylon-6,6), formed by condensation polymerisation with loss of water.
Example 5: Describing the nitration mechanism
Describe the mechanism for the nitration of benzene.
- Formation of the electrophile: concentrated sulfuric acid protonates nitric acid, which loses water to form the nitronium ion: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.
- Attack: a curly arrow from the delocalised ring to NO₂⁺ forms a bond between a ring carbon and the nitrogen. The intermediate is a positively charged ion in which the delocalised system is partly broken (drawn as a horseshoe over five carbons with a + sign).
- Loss of H⁺: a curly arrow from the C–H bond back into the ring restores the delocalised system, releasing H⁺, which reacts with HSO₄⁻ to regenerate sulfuric acid (the catalyst).
The product is nitrobenzene. The temperature is kept below about 55 °C to reduce further nitration.
Example 6: Repeat units from monomers
Draw the repeat unit of the polymer formed from (a) chloroethene, (b) ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
(a) Addition polymer: –[CH₂–CHCl]– (poly(chloroethene), PVC). (b) Condensation polymer (a polyester, PET): –[O–CH₂–CH₂–O–CO–C₆H₄–CO]–, formed with loss of water at each ester link.
Example 7: Why polyamides can biodegrade
Explain why nylon can eventually be broken down but poly(ethene) persists in the environment.
Nylon contains amide links (–CONH–), which can be hydrolysed by acids, bases or enzymes, breaking the chains into smaller molecules over time. Poly(ethene) contains only strong, non-polar C–C and C–H bonds, which are not attacked by water or by the enzymes of most microorganisms, so it can persist for hundreds of years.
Example 8: Zwitterions and amphoteric behaviour
Write equations to show how glycine acts as a buffer by reacting with both acid and alkali.
Glycine exists as the zwitterion ⁺H₃NCH₂COO⁻. With acid: ⁺H₃NCH₂COO⁻ + H⁺ → ⁺H₃NCH₂COOH With alkali: ⁺H₃NCH₂COO⁻ + OH⁻ → H₂NCH₂COO⁻ + H₂O Because it can remove both added H⁺ and added OH⁻, glycine solution resists changes in pH.
Comparing alkenes and arenes
| Ethene (alkene) | Benzene (arene) | |
|---|---|---|
| π electrons | localised between two carbons | delocalised around the ring |
| Typical reaction | electrophilic addition | electrophilic substitution |
| Reaction with bromine water | decolourises rapidly | no reaction without a catalyst |
| Reason | high electron density in one double bond | addition would destroy stable delocalisation |
This comparison is a frequent source of exam questions on the special stability of benzene.
Common mistakes and how to avoid them
- Saying benzene undergoes addition like alkenes.
- Forgetting the halogen carrier in bromination of benzene (but not of phenol).
- Drawing the intermediate with a full circle — the delocalisation is broken in the intermediate.
- Getting basicity order wrong — aliphatic > ammonia > aromatic.
- Forgetting to keep diazotisation below 10 °C.
- Confusing addition and condensation polymers — condensation always loses a small molecule.
How this links to your Internal Assessment
Possible SBAs include preparing an azo dye and testing it on fabrics, investigating the biodegradability of different plastics buried in soil over several weeks, hydrolysing a protein (such as egg albumen) and identifying the amino acids by paper chromatography, or making nylon by the "nylon rope trick" at the interface of two solutions. A study of plastic waste collected from a local beach, classified by polymer type, can make an excellent environmental investigation. Use safety precautions for corrosive and toxic reagents.
Exam technique for Aromatic compounds, amines and polymers
- For benzene's structure, give bond-length, enthalpy and reactivity evidence.
- Draw electrophilic substitution with the electrophile formation, the intermediate and restoration of the ring.
- Explain basicity using availability of the nitrogen lone pair.
- For polymers, draw repeat units with continuation bonds and identify monomers and linkages.
Quick revision summary
- Benzene: delocalised π ring, equal bond lengths, ~150 kJ mol⁻¹ extra stability; undergoes electrophilic substitution.
- Nitration (NO₂⁺ from conc. HNO₃/H₂SO₄, < 55 °C); bromination (FeBr₃ catalyst); Friedel–Crafts (AlCl₃).
- Phenol and methylbenzene are more reactive than benzene; phenol + Br₂(aq) → white 2,4,6-tribromophenol.
- Amines are bases; aliphatic > NH₃ > aromatic.
- Diazotisation below 10 °C; coupling with phenol forms azo dyes.
- Amino acids are zwitterions; peptide links form proteins.
- Addition polymers (polyethene, PVC) vs condensation polymers (polyesters, nylon); plastic waste and disposal issues.