26 8 This question is about some carbonyl compounds with the molecular formula C5H10O. (a) Describe a chemical test, and its result, to distinguish between pentan-2-one, CH3CH2CH2COCH3, and pentan-3-one, CH3CH2COCH2CH3. 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(b) Pentan-2-one reacts with hydrogen cyanide in the presence of cyanide ions to form 2-hydroxy-2-methylpentanenitrile. H    H    H  OH      H H    H    H  CN      H H     C    C     C    C    C     H 2-hydroxy-2-methylpentanenitrile (i) Draw the mechanism for the reaction between pentan-2-one and hydrogen cyanide in the presence of cyanide ions. Include curly arrows and any relevant lone pairs. (4) Turn over 27 (ii) The product of this reaction, 2-hydroxy-2-methylpentanenitrile, has a chiral centre. Explain why a racemic mixture of 2-hydroxy-2-methylpentanenitrile is formed in this reaction. 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(c) An aldehyde with molecular formula C5H10O has a 13C NMR spectrum with three peaks. The high resolution 1H NMR spectrum of this aldehyde has two peaks and neither of them is split. Deduce the displayed formula of this aldehyde. Justify your answer by referring to both NMR spectra. (4) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 28 *(d) Describe the oxidation and reduction reactions of pentanal and pentan-3-one. Include the reagents and the structures of the organic products formed in any reactions that take place. 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(Total for Question 8 = 18 marks)