RI 2019 Prelim P3 Ans

© Raffles Institution 2019 9729/03/S/19 1 2019 Y6 H2 Chemistry Preliminary Exams Paper 3 Suggested Solutions Section A 1 (a) (i) Examiner’s Comments • Most students were able to correctly draw the structure of the amine. However, many did not balance the equation with HBr. (ii) The p orbital of the Cl atom overlaps with the  electron cloud of the benzene ring. This allows the lone pair of electrons in the p orbital of the chlorine atom to delocalise into the benzene ring and results in partial double bond character in the C–Cl bond. This bond is strengthened and hence not easily broken. Sterically, nucleophilic attack at the electron -deficient carbon atom from the side directly opposite the Cl atom is blocked by the benzene ring . In addition, the pi - electron cloud of the benzene ring will repel the lone pair of electrons of an incoming nucleophile, rendering attack of the nucleophile difficult. Examiner’s Comments • Many students gave incomplete answers, either omitting the C l p-orbital overlapping with the  electron cloud of benzene or omi tting the delocalisation of electrons. • Students should phrase their answers clearly. For example, it is the C -Cl bond that has partial double bond character and not the Cl atom. • Most students did not discuss the steric hindrance effect of the benzene ring. (iii) Add NaOH(aq) to each sample in a test -tube and heat each mixture in a hot water bath. Cool each mixture and acidify each one with dilute HNO3. Then add AgNO3(aq) to each mixture. For compound H, a pale cream precipitate of AgBr will be observed. For bupropion, no cream precipitate is observed. Alternative answer: Br2(aq) – compound H will decolourise orange Br2(aq) Examiner’s Comments • Most students were able to correctly state the distinguishing test and accompanying observations. Do remember to refer to the Data Booklet for the correct colour of the silver halides. • When describing observations, avoid stating “no observable change” and instead, state the opposite of the positive test. • Students should also take note to specify the solvent used. For example, NaOH(aq), Br2(aq), HNO3(aq). (b) (i) To quench the reaction / to stop the reaction at one minute / to neutralise the unreacted amine from the reaction mixture. Examiner’s Comments • Statements such as “slow down” the reaction, no matter how “drastically” or “significantly”, were not accepted. + HBr

© Raffles Institution 2019 9729/03/S/19 2 (ii) The rate of the reaction is dependent on temperature. Having the two solutions at the same temperature before the start of the reaction will minimise any temperature changes during the mixing of solutions. Examiner’s Comments • This question was poorly done. Many answers did not address the question. • For example, “…so that temperature will not affect the rate of the reaction.” Nothing you can do experimentally will stop