Topic 18 · page 10 of 10

Uses and exam technique

Know one clear use for each compound in the topic, how to choose a reaction to make a given product, and how to word the two explanations examiners ask for most, benzene against alkenes and phenol against benzene.

12 min read18.318.418.6

Read first: Friedel-Crafts reactions, Phenol and bromine water

Uses of the compounds in this topic 18.418.6

The specification says that knowledge of the common uses of the organic compounds in this topic is expected. One clear, specific use for each is enough.

Compound Made by Common use
nitrobenzene nitration of benzene starting material for making dyes; nitration of arenes is also used to make explosives
benzenesulfonic acids sulfonation detergents and dyes
ethylbenzene Friedel-Crafts alkylation converted to phenylethene, the monomer for poly(phenylethene), also called polystyrene
phenylethanone and other aromatic ketones Friedel-Crafts acylation intermediates in making medicines and perfumes
phenol (not made in this topic) antiseptics and disinfectants; making resins and plastics

Choosing the reaction 18.4

Exam questions often give a product and ask how to make it from benzene. Look at what has replaced the hydrogen.

To make… Replace H with… Reagents and conditions
bromobenzene –Br Br₂, FeBr₃ catalyst, room temperature, dry
nitrobenzene –NO₂ concentrated HNO₃ and concentrated H₂SO₄, about 50 °C
benzenesulfonic acid –SO₃H fuming sulfuric acid, warm
an alkylbenzene –R halogenoalkane RCl, AlCl₃, reflux, dry
an aromatic ketone –COR acyl chloride RCOCl, AlCl₃, reflux, dry
2,4,6-tribromophenol three –Br on phenol bromine water, room temperature

Explaining reactivity 18.318.6

Two questions are asked more than any others. Learn both as chains of three links.

MethodBenzene against an alkene

  • Alkene: the π electrons are localised between two carbons, giving high electron density; this polarises Br₂, so it reacts without a catalyst (addition).
  • Benzene: the π electrons are delocalised over six carbons, giving lower electron density between any two carbons; this does not polarise Br₂ enough, so it needs a catalyst (substitution).

MethodPhenol against benzene

  • A lone pair on oxygen is in a p-orbital that overlaps with the π system of the ring.
  • The lone pair is partly delocalised into the ring, so the electron density increases.
  • The ring polarises Br₂ more easily, so phenol reacts with bromine water, at room temperature, with no catalyst.

Check yourself

  1. Give one use of each of nitrobenzene, ethylbenzene and phenol.
  2. State the reagents and conditions needed to make (a) nitrobenzene, (b) phenylethanone, from benzene.
  3. Explain why benzene is more resistant to bromination than cyclohexene.
  4. Explain why phenol reacts with bromine water but benzene does not.
  5. Give two pieces of evidence for the delocalised structure of benzene, one from data and one from reactivity.
  6. Benzene is treated with chloromethane and aluminium chloride. Name the organic product, write the equation for the formation of the electrophile and name the type of mechanism.

Answers cover these until you have tried the questions

  1. Nitrobenzene: dyes (or explosives). Ethylbenzene: converted to phenylethene for making poly(phenylethene). Phenol: antiseptics and disinfectants (or resins).
  2. (a) Concentrated HNO₃ and concentrated H₂SO₄, about 50 °C. (b) Ethanoyl chloride, CH₃COCl, and anhydrous AlCl₃, heat under reflux, dry.
  3. In cyclohexene the π electrons are localised between two carbons: high electron density, which polarises bromine so it reacts. In benzene the π electrons are delocalised over six carbons, giving lower electron density between any two carbons, which cannot polarise bromine enough.
  4. A lone pair on the oxygen of phenol is partly delocalised into the ring, increasing the electron density of the ring, so the ring polarises bromine molecules more easily and no catalyst is needed. Benzene's ring has lower electron density.
  5. Data: enthalpy of hydrogenation −208 kJ mol⁻¹ is 152 kJ mol⁻¹ less exothermic than the −360 kJ mol⁻¹ predicted for the Kekulé structure (or all C–C bonds are 0.139 nm). Reactivity: benzene does not decolourise bromine water like an alkene, and undergoes substitution rather than addition.
  6. Methylbenzene. CHX3Cl+AlClX3→CHX3X++AlClX4X−\ce{CH3Cl + AlCl3 -> CH3^+ + AlCl4^-}. Electrophilic substitution.