Edexcel International A Level Chemistry Arenes Exam Questions

About Edexcel International A Level Chemistry Arenes

Arenes in Unit 5 of Edexcel International A Level Chemistry is about benzene and its derivatives. You compare the Kekule structure with the delocalised model, using evidence from enthalpies of hydrogenation, bond lengths and benzene's resistance to addition reactions. The main mechanism is electrophilic substitution, studied through nitration, halogenation and Friedel-Crafts alkylation and acylation, including how each electrophile is generated. Phenol is compared with benzene to show how the OH group activates the ring. The topic links to Organic nitrogen compounds, because nitrobenzene is reduced to phenylamine. Examiners test the evidence for delocalisation, mechanisms with electrophile generation and the reactivity of phenol.

What you need to know

• Delocalised structure Benzene is a planar ring of six carbons, each forming three sigma bonds. The remaining p orbitals overlap to form a ring of delocalised pi electrons above and below the plane.
• Evidence against Kekule All C to C bond lengths are equal, between single and double bond lengths. The enthalpy of hydrogenation of benzene (about -208 kJ mol-1) is much less exothermic than three times that of cyclohexene (3 x -120 = -360 kJ mol-1), so benzene is more stable. Benzene does not decolourise bromine water.
• Electrophilic substitution The electrophile attacks the delocalised electrons, forming an intermediate in which delocalisation is partly broken; loss of H+ restores the stable ring. Substitution keeps the delocalised system, which is why benzene does not undergo addition.
• Nitration Concentrated nitric and sulfuric acids at about 55 C produce the nitronium ion: HNO3 + 2H2SO4 gives NO2+ + H3O+ + 2HSO4-. Nitrobenzene forms.
• Halogenation and Friedel-Crafts Bromine with an FeBr3 (or iron) halogen carrier gives bromobenzene. Friedel-Crafts reactions use a halogenoalkane or acyl chloride with AlCl3 to add an alkyl or acyl group, for example forming phenylethanone from ethanoyl chloride.
• Phenol Phenol reacts with bromine water at room temperature without a catalyst, giving a white precipitate of 2,4,6-tribromophenol. A lone pair on oxygen is partly delocalised into the ring, increasing electron density and activating it towards electrophiles.

Common mistakes in Arenes

• Drawing the curly arrow to the electrophile from a single carbon instead of from the delocalised ring.
• Drawing the intermediate with a full circle. The intermediate has a broken horseshoe of delocalisation covering five carbons, with a positive charge, open towards the carbon bonded to the electrophile.
• Forgetting to show how the catalyst is regenerated, for example H+ + AlCl4- gives AlCl3 + HCl.
• Explaining the stability of benzene only as 'it has delocalised electrons'. Use the enthalpy of hydrogenation data to quantify the extra stability.
• Saying phenol is more reactive because OH is electron withdrawing. The oxygen lone pair donates electron density into the ring.

Try three Arenes exam-style questions