DHS Prelim P2_Suggested_Solution_updated

This question paper consists of 18 printed pages and 0 blank page. © DHS 2018 [Turn over Suggested solutions DUNMAN HIGH SCHOOL Preliminary Examination 2018 Year 6 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 13 September 2018 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Write in dark blue or black pen. 3 You may use an HB pencil for any diagrams or graphs. 4 Do not use staples, paper clips, glue or correction fluid. 5 Answer all questions in the spaces provided on the Question Paper. The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings in calculations. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use Question No. Marks 1 13 2 12 3 25 4 10 5 15 Total 75

2 © DHS 2018 9729/02 Answer all questions in the spaces provided. 1 Isoprene, E, is an organic compound that could be used to synthesise limonene, which is commonly used in fragrances. (a) E can be synthesised from 3–methylbut–1–ene, A, in a 4–step process as follows. C A E I II III IV HCl (g) B D excess concentrated H2SO4, heat (i) B is a major product of step I. Draw the structures of compounds B, C and D. [3] B : Cl C : D : OH OH (ii) Suggest the reagents and conditions for steps II and III. [2] step II : alcoholic NaOH heat under reflux step III : cold alkaline KMnO4 (iii) Predict, with reasoning, whether the mixture of products formed in step I is optically active. [2] The reaction mixture in B is not optically active. During electrophilic addition, a trigonal planar carbocation is formed. C l– has a 50% chance each of attacking the carbocation from the top or bottom of the plane . Hence, resulting in a racemic mixture formed. CH3 H + :Cl- CH3 H Cl H CH3 H ClH 50% 50% Trigonal Planar carbocation

3 © DHS 2018 9729/02 [Turn Over (b) The following reaction shows an alternative route to form intermediates for the synthesis of isoprene, E. Cl Cl F GA + + other products (i) State the reagents and conditions used to form F and G from A. [1] Limited Cl2, uv (ii) Predict the ratio in which F and G will be formed. [1] F : G 6 : 1 (iii) Z was one of the other products formed in the reaction in (b). It was suggested that the radical, •G, is involved in the formation of Z. The mechanism of •G converting to Z is thought to involve 2 steps. 1. Delocalisation of unpaired electron forming • Z. 2. • Z reacts with Cl2 to form Z and a radical. The structures of Z, •G and •Z are as follows. G Z Z Cl Use the information above to draw out the mechanism for the conversion of •G