16   7 This question is about carbonyl compounds. (a) Ethanal, CH3CHO, and ethanoic acid, CH3COOH, are both soluble in water but ethanoic acid has a much higher boiling temperature than ethanal. Explain these physical properties of ethanal and ethanoic acid in terms of intermolecular forces. Include a labelled diagram to show why ethanal is soluble in water. 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Turn over 17   (b) Propanal reacts with hydrogen cyanide in the presence of potassium cyanide to form 2‑hydroxybutanenitrile. C2H5 C H OH CN (i) Draw the mechanism for this reaction. Include curly arrows and any relevant lone pairs and dipoles. (4) (ii) Explain whether or not the 2‑hydroxybutanenitrile formed will be a racemic mixture. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 18   (c) Carbonyl compounds can be identified by reacting them with 2,4‑dinitrophenylhydrazine (2,4‑DNPH) to form a solid derivative. These derivatives have characteristic melting temperatures. (i) Identify the steps required to prepare a sample of a pure, dry derivative of a carbonyl compound X. (3) Carbonyl compound X Precipitate in reaction mixture Dry solid derivative Impure solid derivative Purified solid derivative 2,4‑DNPH Step 1 Step 2 Step 3 Step 1 .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 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Step 3 .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 19   (ii) The melting temperature ranges of the derivatives of some carbonyl compounds that could be X are shown in the table. Carbonyl compound Melting temperature range of derivative / °C ethanal 165 – 168 propanal 154 – 156 propanone 127 – 129 cyclohexanone 158 – 160 The melting temperature of the derivative of carbonyl compound X is 156–158 °C and X has an absorption at 1717 cm–1 in its infrared spectrum. Deduce the identity of X. Justify your answer. 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(iii) These carbonyl compounds may also be identified using modern methods such as proton NMR spectroscopy. The structure of the pentan‑3‑one derivative formed with 2,4‑DNPH is shown. O2N N CH2 CH3 H H H H N C CH2 CH3 NO2 Label the different proton environments that would give rise to the peaks in the low resolution proton NMR spectrum. (2) (Total for Question 7 = 18 marks)