DHS Prelim P3_Suggested_Solutions

This question paper consists of 13 printed pages and 1 blank page. © DHS 2018 [Turn over Suggested solutions DUNMAN HIGH SCHOOL Preliminary Examination 2018 Year 6 H2 CHEMISTRY 9729/03 Paper 3 Free Response 18 September 2018 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Writing Paper Cover Sheet Graph Paper INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page and on the Cover Sheet provided. 2 Write your answers on the separate writing papers provided. 3 Write in dark blue or black pen. 4 You may use an HB pencil for any diagrams or graphs. 5 Start each question on a fresh sheet of paper. *[Marks will be deducted if you fail to do so.] 6 At the end of the examination, fasten all your work securely together with the Cover Sheet on top. 7 Do not use staples, paper clips, glue or correction fluid. Section A 8 Answer all questions Section B 9 Answer one question. 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.

2 © DHS 2018 9729/03 Section A Answer all questions. 1 The Williamson ether synthesis is an organic reaction, forming an ether from a halogenoalkane and alcohol in the presence of sodium. This reaction was developed by Alexander Williamson in 1850 and still remains the simplest and most popular method of preparing ethers till today. The following equation shows the formation of dimethyl ether, a common aerosol propellant. CH3Cl CH3OH CH3OCH3 HCl+ Na CH3OH + (a) (i) State the purpose of sodium used in the Williamson ether synthesis. [1] Sodium reacts with CH3OH to form a stronger nucleophile, CH3O– (ii) Hence, name the mechanism of the reaction. [1] SN2 / bimolecular nucleophilic substitution (iii) Suggest suitable reagent(s) to synthesise each of the following ethers. • CH 3CH2OCH2CH3 • O [2] • CH 3CH2OH (in the presence of Na) and CH3CH2Cl • Cl OH (in the presence of Na) (b) Dimethyl ether is known as a symmetrical ether whereas tert-butyl ethyl ether, CH3CH2OC(CH3)3 is an example of an unsymmetrical ether. To prepare tert-butyl ethyl ether via the Williamson ether synthesis, there are two possible combinations of reagents, as shown in the table below. Combination Reagents A CH3CH2Br and (CH3)3COH B (CH3)3CBr and CH3CH2OH Identify the combination of reagents that might favour the mechanism identified in (a)(ii) and justify your choice, with reasoning. [2] Combination A CH3CH2Br is a primary alkyl halide , and hence it will be less sterically hindered for the nucleophile / alkoxide ion to