14 6 This question is about the preparation and analysis of paracetamol. HO N C CH3 H O paracetamol Paracetamol may be prepared from phenol in three stages. HO HO HO Stage 1 Stage 2 Stage 3 NO2 NO2 HO HO NO2 NH2 HO N C CH3 H O 4-nitrophenol 4-aminophenol HO NH2 paracetamol 2-nitrophenol (a) In Stage 1, phenol is nitrated using dilute nitric acid. The nitration of benzene requires concentrated nitric acid at 55°C with a catalyst of concentrated sulfuric acid. Both these reactions are electrophilic substitution. (i) Explain why phenol can be nitrated using milder conditions than benzene. 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Turn over 15 (ii) A mixture of 2-nitrophenol and 4-nitrophenol is produced in Stage 1. They are separated by steam distillation. The boiling temperature of 2-nitrophenol is 215°C and that of 4-nitrophenol is 279°C. Explain, in terms of intermolecular forces, why 4-nitrophenol has a higher boiling temperature than 2-nitrophenol. You may include a diagram in your answer. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 16 (b) State the type of reagent needed to convert 4-nitrophenol into 4-aminophenol in Stage 2. (1) .................................................................................................................................................................................................................................................................................... (c) The outline procedure for Stage 3 is: • place 1.0 g of 4-aminophenol in a conical flask. Add 9 cm3 of distilled water and stir the mixture • add 1 cm3 of ethanoic anhydride to the flask and shake the mixture until a precipitate of impure paracetamol forms • remove the paracetamol by filtration under reduced pressure • recrystallise the paracetamol using water as the solvent • determine the melting temperature of the pure, dry paracetamol. (i) Draw a labelled diagram of the apparatus used for filtration under reduced pressure. (3) Turn over 17 (ii) Describe the recrystallisation process to obtain a pure, dry sample of paracetamol. 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(iii) The melting temperature of pure paracetamol is 170°C. Describe what happens to the melting temperature if the paracetamol is not pure. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 18 (d) The percentage by mass of paracetamol in three brands of paracetamol tablets are shown in the table. Brand of tablet Percentage by mass of paracetamol P 92.1 Q 93.6 R 99.7 The amount of paracetamol in a tablet can be determined using a titration with cerium(IV) ions. The tablets are crushed and then hydrolysed in acid to form 4-aminophenol. HO N C CH3  +  H2O HO NH2  +  CH3COOH H O 4-aminophenol is oxidised by cerium(IV) ions. HO NH2  +  2Ce4+ O NH   +  2Ce3+  +  2H+ A tablet from one of the three brands of paracetamol was analysed following the outline procedure. • one of the tablets was crushed and 0.500 g of the powder was added to dilute sulfuric acid • the mixture was heated under reflux until the hydrolysis was complete • the solution was made up to 100.0 cm3 in a volumetric flask • 25.0 cm3 portions of the solution were titrated against 0.100 mol dm−3 Ce4+ using ferroin as indicator Result The mean titre was 16.50 cm3. Turn over 19 (i) Give the molecular formula of paracetamol. (1) .................................................................................................................................................................................................................................................................................... (ii) Determine, by calculation, which brand of tablet was analysed. (5) (Total for Question 6 = 21 marks)