Eduqas Chemistry for A Level Year 2: Student Bk

122 Eduqas A Level Chemistry Year 2: Component 2 Halogenation Benzene and bromine do not react together unless a catalyst such as iron filings, iron(III) bromide or aluminium bromide is present. + Br 2 + HBr Fe/FeBr 3 room temperature Br The π-electron system is not sufficiently nucleophilic to polarise the bromine molecule to any extent to give Br δ + – Br δ – . In the presence of iron(III) bromide the polarisation of the bromine molecule is more pronounced. Br Br Fe δ + δ – δ + Br Br Br If iron filings are used then these react with bromine to give iron(III) bromide. One representation of this electrophilic substitution of bromine into the ring is Br Br Fe + FeBr 4 – δ + δ – δ + Br Br Br Br Br Fe Br Br H Br H Br + HBr + FeBr 3 Br – + + The role of the iron(III) bromide is catalytic – encouraging greater polarisation of the Br–Br bond so that attack by the ring electrons can occur but being regenerated at the end of the reaction. Chlorination of benzene can be carried out in a similar way, often using anhydrous aluminium chloride or iron(III) chloride as the catalyst. If an excess of chlorine is used a mixture of 1,2-dichlorobenzene and 1,4-dichlorobenzene is produced. + xs Cl 2 + + HCl In industry the chlorination of benzene is carried out as a continuous process, so that any polychlorination is kept to a minimum. Chlorobenzene is an important industrial chemical and is nitrated and the nitro- derivatives converted to 2-nitrophenol and 2-nitrophenylamine. The insecticide DDT can be manufactured by the reaction of chlorobenzene with trichloroethanal. 2 C 6 H 5 Cl + Cl 3 CCHO Cl 3 C C H The use of DDT is now heavily restricted because of problems connected with its toxicity, its persistence in the environment and its presence in the food chain. & S C Stretch & Challenge Benzene reacts with sulfuric(VI) acid by electrophilic substitution to give benzenesulfonic acid, C 6 H 6 O 3 S, where the sulfur atom is directly bonded to the benzene ring. Deduce the structural formula of benzenesulfonic acid and give the equation for the reaction of benzene with sulfuric(VI) acid to produce this acid. 8 Knowledge check A student was asked to give two reasons how he knew that iron(III) bromide was acting as a catalyst in the bromination of benzene. What do you think he gave as his correct answers? Extra Help In electrophilic aromatic substitution, the dotted line in the formula of the positively charged intermediate should not extend all the way around the ring. It should not reach the carbon atom at which substitution is occurring. St u dy point When giving a complete equation for the halogenation of benzene, you should also show the other product of the reaction (usually HBr or HCl). anhydrous AlCl 3 Cl Cl Cl Cl –H 2 O DDT C 6 H 4 Cl C 6 H 4 Cl