18   8 Esters have many uses due to their characteristic aromas and often have common names. For example, isoamyl acetate is referred to as banana oil and amyl acetate has a scent similar to apples. O O O O isoamyl acetate amyl acetate (a) What is the number of peaks in a 13C NMR spectrum of isoamyl acetate and of amyl acetate? (1) isoamyl acetate amyl acetate A 5 6 B 6 6 C 6 7 D 7 7 (b) State the molecular formula of amyl acetate. (1) .................................................................................................................................................................................................................................................................................... (c) Deduce the structural formula of the carboxylic acid that could be used to form both isoamyl acetate and amyl acetate. (1) (d) Deduce the name of the alcohol that forms isoamyl acetate. (1) .................................................................................................................................................................................................................................................................................... (e) Give the systematic name for amyl acetate. (1) .................................................................................................................................................................................................................................................................................... Turn over 19   (f) The carboxylic acid used to make isoamyl acetate and amyl acetate can also be used to make six further ester isomers. The structures of two of these esters, A and B, are shown. (i) Complete the skeletal formulae of three of the remaining esters. Names are not required. (3) O O isoamyl acetate O O amyl acetate O O O O ester A O O O O ester B O O (ii) Write an equation to show the formation of ester A from an acyl chloride and an alcohol. (2) 20   (g) Esters can be hydrolysed by heating under reflux with aqueous acid or alkali. Compare and contrast these two methods of hydrolysis for amyl acetate. 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(Total for Question 8 = 14 marks) Turn over 21   9 At high temperatures, ethanal decomposes to form methane and carbon monoxide. The reaction is second order with respect to ethanal and second order overall. CH3CHO → CH4 + CO (a) Write the rate equation for this reaction. (1) (b) Deduce the units of the rate constant given that the units of rate are mol dm–3 s–1. (1) (c) The table shows the concentration of ethanal in a sample at different times. Time / s Concentration of ethanal / mol dm–3 0 0.72 420 0.36 1260 0.18 Calculate average values for the rate of reaction between 0 and 420 seconds and between 420 and 1260 seconds. Give your answers to an appropriate number of significant figures. (2) 0 s – 420 s ..................................................................................................... 420 s – 1260 s ..................................................................................................... 22   (d) Explain why the data given and your answers in (c) show that the reaction is neither zero order nor first order. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (e) The rate constant for the reaction was determined at five temperatures. The results are given in the table. Temperature (T) / K 1/ Temperature (1/T) / K–1 Rate constant (k) / units in (b) ln k 700 1.43 × 10–3 0.011 –4.51 730 1.37 × 10–3 0.035 –3.35 760 1.32 × 10–3 0.105 –2.25 790 0.343 810 1.23 × 10–3 0.787 –0.24 Determine the activation energy, Ea , in kJ mol–1, by completing the data in the table and plotting a graph of ln k against 1/T. You should include the value of the gradient of the line and its units. The Arrhenius equation can be expressed as  ln k = – E R a  ×  1 T + constant (7) 23   Total for Question 9 = 13 marks) TOTAL FOR PAPER = 90 MARKS 24  