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CIE Jun 2015 v1 Paper 2 Q3
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![Ethanal reacts with hydrogen cyanide, in the presence of a small amount of NaCN, as shown. CH3CHO + HCNCH3CH(OH)CN (a) Use bond energies from the Data Booklet to calculate the enthalpy change for this reaction. Include a sign with your answer. enthalpy change =kJ mol1 [3] (b) The product of this reaction shows stereoisomerism as it contains a chiral centre. This reaction produces an equimolar mixture of two optical isomers. (i) Explain the meanings of the terms stereoisomerism and chiral centre. stereoisomerismchiral centre[2] (ii) Suggest why the two optical isomers are produced in equal amounts by this reaction.[1]UCLES 2015 9701/21/M/J/15<br />
(c) (i) Complete the diagram to show the mechanism of this reaction. Include all necessary charges, partial charges, lone pairs and curly arrows and show the structure of the intermediate. C H3C CN H3C H + OH CH [5] (ii) With reference to your mechanism in (i), explain the role of the NaCN in this reaction.[1] [Total: 12]UCLES 2015 9701/21/M/J/15 [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v1_QP_-_Paper_2_CIE_Chemistry_A-level_3-0.jpg)
![Page 4 Question Cambridge International AS/A LevelMay/June 2015 9701 21 Mark Scheme Syllabus Paper Mark Scheme Mark Total (a) Bond breaking = C=O = 740 CH = 410 = 1150 kJ Bond forming = CC = 350 CO = 360 OH = 460 = 1170 kJ Enthalpy change = 11501170 = 20 kJ mol 1 Stereoisomerism = (molecules with the same molecular formula and) same structural formula but different spatial arrangements of atoms Chiral centre = atom with four different atoms/groups attached (b) (i) (ii) (Planar) carbonyl so (equal chance of nucleophile) attacking either side (c) (i) [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [1] [3] [2] [1] [5] [1] M1 = lone pair AND curly arrow from lone pair to carbonyl C M2 = partial charges on C=O AND curly arrow from bond (=) to O M3 = structure of intermediate including charge M4 = lone pair AND two correct curly arrows (from lone pair to H AND from HC to C) M5 = CN (ii) (CN regenerated so) catalyst [12]Cambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v1_MS_-_Paper_2_CIE_Chemistry_A-level_3.jpg)