ACJC Prelim P3 QP

ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/03 Higher 2 Paper 3 Free Response 24 August 2018 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet Cover Page READ THESE INSTRUCTIONS FIRST Write your index number and name, form class and tutorial class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Start each question on a new sheet of writing paper. A Data Booklet is provided. The use of an approved calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely behind a cover sheet. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 15 printed pages and 1 blank page. 9729/03/Prelim/18 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2018 Department of Chemistry [Turn over

2 © ACJC 2018 9729/03/Prelim/2018 [Turn over BLANK PAGE

3 © ACJC 2018 9729/03/Prelim/2018 [Turn over Section A Answer all questions in this section. 1 (a) (i) Due to reactivity of A lCl3 and PCl5 in water, the following reactions are carried out under anhydrous conditions. Write equations showing the reactions of aluminium chloride and phosphorus pentachloride in water. State the pH of resulting solutions. [3] (ii) Chlorobenzene and 2-chloro-2-methylbutane are separately boiled with aqueous sodium hydroxide and acidified with nitric acid before aqueous silver nitrate is added. State and explain the difference in observations. Write equations for the reactions that occur. [3]

4 © ACJC 2018 9729/03/Prelim/2018 [Turn over (b) To study the kinetics of the reaction between 2-chloro-2-methylbutane and sodium hydroxide in alcoholic medium , two experiments with different initial concentrations of 2-chloro-2-methylbutane were carried out at constant temperature. A [NaOH]–time graph was plotted using the results obtained from the experiments. (i) Deduce the orders of reaction with respect to 2-chloro-2-methylbutane and NaOH. [2] (ii) Hence state the rate equation of this reaction. [1] (iii) The proposed mechanism of 2-chloro-2-methylbutane with alcoholic sodium hydroxide is given below. Using your answer in (b)(ii) and the above steps, suggest the complete mechanism by • drawing curly arrows, • showing lone pair(s) of electrons and • identifying the slow step. [3] [NaOH] / mol dm–3 Reaction 1: 0.25 mol dm–3 of 2-chloro-2-methylbutane Reaction 2: 0.50 mol dm–3 of 2-chloro-2-methylbutane 0 0.002 0.004 0.006 0 5 10 15 20 25 0.008 0.010 time / minutes

5 © ACJC 2018 9729/03/Prelim/2018 [Turn over (c) Chlorobenzene has the following resonance structures which represent the alternative distribution of electrons on the molecule. Using these structures, suggest why the electrophilic substitution of chlorobenzene is 2,4-directing. [2] (d) When butane is reacted with chlorine gas under strong UV rays, a mixture of compounds is formed. (i) Assuming that primary and secondary hydrogen atoms have similar reactivity, draw the structures of alkyl radicals formed during the reaction and state their likely ratio of formation. [2] (ii) During the termination stage, the alkyl radicals in (d)(i) form three structures with the molecular formula C 8H18. Suggest their structures and the ratio of these structures. [4] (e) Chlorofluoroalkanes, CFCs, were once used as refrigerant fluids and aerosol propellants. In many applications, they have now been replaced by alkanes. This is because CFCs contribute to the destruction of the ozone layer. (i) Suggest one reason why CFCs were originally used for this purpose. [1] (ii) Suggest one potential hazard of using alkanes instead of CFCs. [1] [Total: 22]

6 © ACJC 2018 9729/03/Prelim/2018 [Turn over 2 Recreational drugs like methamphetamine and methcathinone can be synthesised easily from pseudoephedrine, a common nasal decongestant found in cough medicine. OH H N O H N X pseudophedrine methcathinone HI, followed by neutralisation (a) (i) The structures of methcathinone and methamphetamine, with pKb values at 298 K, are shown in the table below. compound p Kb methcathinone 5.98 methamphetamine 3.79 Explain why methcathinone has a higher pKb value than methamphetamine. [2] (ii) Suggest the structure of intermediate X. [1] O H N H N

7 © ACJC 2018 9729/03/Prelim/2018 [Turn over (b) Commercially, methcathinone is sold in its salt form as solid methcathinone hydrochloride. Methcathinone hydrochloride undergoes the following reaction to form a possible analogue, Y, with hallucinogenic properties. Name the type of reaction and describe its mechanism, showing curly arrows, charges and any relevant lone pairs. [3] (c) Hydrogen iodide, H I, is listed as a controlled substance as it is often used illegally to produce popular recreational drugs like methamphetamine. The commercial preparation of hydrogen iodide, H I, involves the reaction of iodine I 2, with hydrazine, N2H4, which has ammonia-like properties. N2H4(l) + 2I2(aq) N 2(g) + 4HI(g) (i) During the preparation, gaseous hydrogen iodide produced must be removed immediately. Suggest a reason why this is done. [1] (ii) Hydrogen halides can be unstable to heat. Write an equation for the reaction undergone on heating hydrogen iodide, HI. [1] (iii) Using your knowledge of the chemistry of Group 17, deduce with reason how the thermal stability of