HCI Prelim P3

This document consists of 11 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 14S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9647/03 17 September 2015 2 hours INSTRUCTIONS TO CANDIDATES Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Begin each question on a new piece of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. Circle the question numbers for the questions that you have attempted on the cover page provided.

2 © Hwa Chong Institution 2015 9647 / 03 / C2 Prelim 2015 Answer any four questions. 1 Margarine was first created in 1869 as a response to a challenge from the French emperor Louis Napoleon III to make a cheap substitute for butter for the army and lower classes. Today, margarine is made by passing hydrogen gas over unsaturated vegetable oils in the presence of a nickel catalyst. A representative reaction may be written as given below: (a) (i) State the type of catalysis that this hydrogenation reaction undergoes. [1] (ii) State the type of isomerism that the product displays and draw diagrams to illustrate the isomerism. [2] (iii) In this type of catalysis, one side (bottom face) of the planar alkene molecule is adsorbed to the surface of the catalyst. Both the hydrogen atoms will be added to the same side of the alkene which is adsorbed to the catalyst and not the other side (top face). The diagram below illustrates how the alkene molecule may be adsorbed to the surface of the catalyst. In the light of this information and your answer to (a)(ii), explain why the product mixture does not rotate plane-polarised light. The use of a diagram may aid in the clarity of your explanation. [3] (b) (i) Using appropriate data from the Data Booklet, calculate the standard enthalpy change of reaction for the reaction given above. [2] (ii) Based on your answer for (b)(i), under what conditions would you expect the hydrogenation reaction to be spontaneous? [2] (iii) Hence explain why a catalyst is still required for the hydrogenation. [2]

3 © Hwa Chong Institution 2015 9647 / 03 / C2 Prelim 2015 (c) A much more effective rhodium-based catalyst for the hydrogenation of alkenes was discovered in 1965. Wilkinson’s catalyst, Rh(PPh 3)3Cl, is added to the alkene dissolved in benzene and then stirred at ambient temperature with hydrogen gas being passed through the solution at atmospheric pressure. The hydrogenation was found to occur within minutes of stirring. The diagram below shows the proposed mode of action for Wilkinson’s catalyst for the hydrogenation of propene. (i) Wilkinson’s catalyst is one of the earliest examples of a transition metal complex behaving as a homogeneous catalyst. Explain what is meant by the word in italics. [1] (ii) The oxidation state of Rh in Wilkinson’s catalyst is +1. The phosphine ligands (PPh 3) are neutral ligands while the hydrogen atoms in compound 2 act as hydride ligands. Hence calculate the oxidation number of Rh in compound 2. [1] (iii) Explain what is meant by the term coordination number and state the coordination number of Rh in Wilkinson’s catalyst and in compound 3. [2] (iv) Based on your answers for (c)(ii) and (iii), state two properties of Rh that allows Rh(PPh3)3Cl, to behave as a homogeneous catalyst. Use the proposed mode of action above to illustrate your answers. [4] [Total: 20]

4 © Hwa Chong Institution 2015 9647 / 03 / C2 Prelim 2015 2 (a) Describe the appearance of chlorine, bromine and iodine at room temperature and explain how the volatility of these elements varies down the group. [3] (b) A sample aqueous solution could contain either the chloride or bromide ion. Describe reactions you would carry out to confirm the identity of the ion, and write equations for reactions that occur, including state symbols. [3] Brominated organic compounds have found uses in drugs and fire retardants. They can be synthesized by reacting bromine with alk enes and arenes. Chemists have studied several alternative methods of bromination to avoid handling bromine directly. An alternative bromine source is pyridinium tribromide, an ionic salt which produces bromine in the reaction mixture via the following equilibrium: (c) (i) Explain one advantage of using pyridinium tr ibromide instead of bromine in organic synthesis reactions. [1] (ii) Draw the structure of the tribromide ion, Br3–, showing its shape. [1] Pyridinium tribromide can be used in the synthesis of mono-brominated anilide. There are two potential routes to this synthesis starting from phenylamine. In the diagram below, the line connecting Br to the benzene ring indicates that Br is attached to one of the carbons in the benzene ring. NH2 O O O HN O pyridinium tribromide in polar solvent HN O Br NH2 O O ONH2pyridinium tribromide in polar solvent HN O BrBr Route 1: Route 2: Step 1 Step 2 Step 1 Step 2 anilide (iii) What types of reaction are Step 1 and Step 2 of Route 1? [2] (iv) Route 1 is the preferred route for the synthesis because it produced mostly mono-brominated anilide while Route 2 produced mostly multi-brominated anilide. Explain this observation as fully as you c an. You may assume pyridinium tribromide in polar solvent has the same reactivity as aqueous bromine. [2]

5 © Hwa Chong Institution 2015 9647 / 03 / C2 Prelim 2015 (v) 4-bromoanilide is obtained from Step 2 of Route 1 at 88% yield. Suggest why it is the major product in this step instead of 2-bromoanilide. [1] (vi) Describe and explain the relative basicities of the three nitrogen-containing compounds in Route 2. [3] (d) Equilibrium systems involving tribromide hav e been studied extensively, one of which is shown below. At 25 oC, the KC value for the above equilibrium is 100. When 16.0 mg of liquid bromine was dissolved in 150 cm 3 of a 0.500 mol dm –3 solution of sodium bromide, the reaction is “virtually complete”. (i) Calculate the initial concentration of Br 2 added and, together with the information given, explain why the reaction is “virtually complete”. [2] (ii) Using your answer in (d)(i), estimate the concentrations of bromide and tribromide in the mixture at equilibrium. [1] (iii) Hence, deduce the concentration of Br 2 in the mixture at equilibrium, without solving quadratic equations. [1] [Total: 20]

6 © Hwa Chong Institution 2015 9647 / 03 / C2 Prelim 2015 3 This question is about reactions of Period 3 elements or their compounds. (a) Sodium iodate, Na IO4 (Mr = 214), is a powerful oxidising agent that can be used to oxidise thioethers into sulfoxides with excellent yield. Thioanisole ( M r = 124.1) is a thioether. In a reaction between thioanisole and 0.500 mol dm ⁻3 solution of sodium iodate, it was found that the reacti