RI 2021 Prelim P2 QP

© Raffles Institution 2021 9729/02/S/21 [Turn Over RAFFLES INSTITUTION 2021 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 14 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided at the top of this page. 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. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. Do not write anything in it. 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 together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 25 2 / 10 3 / 12 4 / 28 Total / 75 This document consists of 23 printed pages and 1 blank page.

2 © Raffles Institution 2021 9729/02/S/21 Answer all the questions in the spaces provided. 1 Esters are sweet smelling compounds with widespread industrial applications. (a) Simple esters are flammable liquids. The standard enthalpy change of combustion for some simple esters can be experimentally determined using the set-up shown in Fig. 1.1. Fig. 1.1 Heat loss for such experiments are significant but can be taken into account by using the data collected from an initial experiment. To determine the standard enthalpy change of combustion for ethyl ethanoate (Mr = 88.0), an initial experiment, Experiment 1, was carried out using methyl ethanoate (Mr = 74.0). Experiment 1 Methyl ethanoate was combusted in a spirit burner which heated 300 g of water in a copper can as shown in Fig. 1.1 . It was found that 0.980 g of methyl ethanoate was required to raise the temperature of the water in the copper can by 10.0 C. Table 1.1 provides some relevant data for the experiment. Table 1.1 mass / g specific heat capacity / J g−1 K−1 water 300 4.18 copper can 250 0.384 thermometer copper can 300 g water spirit burner ester

© Raffles Institution 2021 9729/02/S/21 [Turn Over 3 (i) Calculate the total heat energy gained by the water and the copper c an in Experiment 1 . You may assume that the water and copper can are in thermal equilibrium with each other. [2] (ii) The theoretical standard enthalpy change of combustion of methyl ethanoate is −1592 kJ mol−1. Calculate the theoretical heat energy released by the mass of methyl ethanoate combusted in Experiment 1. [1] The same set-up was used to carry out Experiment 2 with ethyl ethanoate. The following results were obtained. Experiment 2 • mass of ethyl ethanoate combusted = 0.948 g • increase in temperature of 300 g of water = 11.5 C (iii) Calculate the standard enthalpy change of combustion for ethyl ethanoate in kJ mol −1, taking into account the heat los s determined in Experiment 1. You may assume that the percentage heat loss in both Experiments 1 and 2 is the same. [3]

4 © Raffles Institution 2021 9729/02/S/21 (b) The structure of another simple ester commonly used as a water -soluble degreaser is shown below. It can be formed by heating ethanol and carboxylic acid P with concentrated sulfuric acid. (i) The yield of the ester is found to be low due to the formation of side products. Two of the side products are A and B. A decolourises aqueous bromine and B is a neutral compound. Draw the structures of A and B. A (C3H4O2) B (C6H8O4) [2] (ii) State and explain how the value of pKa of P compares with that of propanoic acid. …………………………………………………………………………………… ..….......... …………………………………………………………………………………… ..….......... …………………………………………………………………………………… ..….......... …………………………………………………………………………………… ..….......... ……………………………………………………………………………………..…......[2] (iii) An aqueous solution containing 0.10 mol dm−3 of P has a pH of 2.43. Calculate the value of pKa for P. [2]

© Raffles Institution 2021 9729/02/S/21 [Turn Over 5 (c) Linalyl formate is a naturally occurring ester responsible for the characteristic apple smell. It can undergo various reactions under alkaline conditions using potassium hydroxide. (i) Draw the structures of compounds C and D in the boxes above. [2]

6 © Raffles Institution 2021 9729/02/S/21 Potassium hydroxide can also be used to bring about substitution and elimination reactions. (ii) Draw the structures of E, F, G and H in the boxes above. [4] (iii) Potassium hydroxide is said to function as a nucleophile in reaction 1 and a Brønsted base in reaction 2. Using your understanding of the two terms in italics, describe one similarity and one difference between the two terms. …………………………………………………………………………………… ..….......... …………………………………………………………………………………… ..….......... …………………………………………………………………………………… ..….......... …………………………………………………………………………………..………... [2]

© Raffles Institution 2021 9729/02/S/21 [Turn Over 7 (iv) State the conditions required for reactions 1 and 2. reaction 1: ……………………………………….. reaction 2: ……………………………………….. [2] (v) Using an enantiomerically pure sample of E, d escribe the mechanism for reaction 1, showing clearly the stereochemistry of the reactant and products. You may represent the group as R– in your drawing. [3] [Total: 25]

8 © Raffles Institution 2021 9729/02/S/21 2 Boron trihydride, BH 3, is an important reagent in the hydroboration -oxidation reaction, which converts an alkene to an alcohol. The structure of BH3 is given below. (a) (i) State the hybridisation of the boron atom in BH3. ……………………………………………………………………………………..….......[1] (ii) By considering your answer in (a)(i) and the number of valence electrons around boron, explain why BH3 is an electrophile. ……………………………………………………………………………………..…........... ……………………………………………………………………………………..…........... ……………………………………………………………………………………..….......[1] The hydroboration-oxidation reaction is a two-stage hydration reaction. Stage 1: BH3 adds across the alkene. Stage 2: The –BH2 group is replaced by an –OH group. The stages in the hydroboration-oxidation of 3-methylpent-2-ene are shown below.

© Raffles Institution 2021 9729/02/S/21 [Turn Over 9 (b) (i) For 3-methylpent-2-ene, it was proposed that stage 1 proceeds via a two -step electrophilic addition mechanism. Th e first step of the mechanism produces a carbocation and a hydride ion, H–. By considering the electronegativity values in Table 2.1 and the information provided, draw the proposed two-step electrophilic addition mechanism for stage 1. Show all relevant charges and the movement of electron pairs by using curly arrows. Table 2.1 element electronegativity value H 2.2 B 2.0 [2] (ii) Markovnikov’s rule states that for the addition of H –X to an asymmetric alkene, the H atom is added to the doubly bonded carbon with more hydrogen atoms. Fig. 2.1 shows the outcome of the reaction in stage 1. F