CJC Prelim P3_Answers

9729/03/CJC JC2 Preliminary Examination 2018 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response Friday 24 August 2018 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and 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. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 8 printed pages. Catholic Junior College JC2 Preliminary Examinations Higher 2 WORKED SOLUTIONS

2 9729/03/CJC JC2 Preliminary Examination 2018 Section A Answer all the questions in this section. 1 Propene is a colourless gas that is produced in large quantities by the petrochemical industry. It is used in the production of synthetic rubber and as a propellant in aerosols. (a) When propene reacts with HBr, 2-bromopropane is produced as the major product. CH2=CHCH3 + HBr → CH3CHBrCH3 (i) Describe the mechanism of the above reaction, using curly arrows to show the movement of electrons. [4] Electrophilic addition Correct partial charges, lone pair of electrons, slow/fast steps Correct curly arrows Correct secondary carbocation intermediate (ii) When propene reacts with BrC l, 1-bromo-2-chloropropane is produced as the major product. CH2=CHCH3 + BrCl → CH2BrCHClCH3 With reference to your answer in (i), explain why the two reactions give major products with bromine in different positions. [2] Br is more electronegative than H but less electronegative than C l, hence Br acquires a partial positive charge in BrCl .Thus, Br adds first to form the more stable carbocation where the positive charge is on the second carbon (iii) The product of the reaction in (ii) has a chiral carbon, but it is optically inactive. Explain why this is so. [2] There is equal chance of C l‒ attacking the trigonal planar carbon in the carbocation intermediate from either side of the plane. Thus, both enantiomers are formed in equal amounts/a racemic mixture is formed.

3 9729/03/CJC JC2 Preliminary Examination 2018 [Turn over (b) 2-bromopropane reacts with hot aqueous sodium hydroxide, whereas 2-bromopropene does not. Suggest reasons for this difference in reactivity. [2] In 2-bromopropene, p orbitals of Br overlap with π bond (one of the lone pairs of electrons on Br is delocalised into the C=C π bond), hence there is a partial double bond character for the C-Br bond. This makes the C