RI H2 CHEM P3 QP Prelim

© Raffles Institution 2015 9647/03/S/15 [Turn Over RAFFLES INSTITUTION 2015 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response September 2015 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet Cover Page 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 on the cover page. Write in dark blue or black pen on both sides of the paper. 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 fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. 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 examina tion, fasten all your work securely together, with the cover page on top. ______________________________________________________________________________ This document consists of 13 printed pages.

2 © Raffles Institution 2015 9647/03/S/15 Answer any four questions. Begin each question on a fresh sheet of paper. 1 Calcium is the fifth most abundant element found in the Earth’s crust. It is commonly found in the form of salts and oxides as limestone, marble and chalk. (a) Finely powdered calcium metal can cause spontaneous explosions in air. Explain briefly why this is so, writing chemical equations where appropriate. [2] (b) Calcium metal is produced mainly by the electrolysis of molten calcium chloride at 800 °C. (i) Determine the mass (in tonnes ) of calcium metal that can be obtained when 1 x 10 5 A of current is passed through an electrolytic cell for 3 hours. [1 tonne = 1000 kg] [2] (ii) With the use of the Data Booklet, explain why the electrolysis of calcium chloride cannot be conducted in aqueous medium. [1] (c) When 2.0 g of calcium ethanoate, Ca(CH3CO2)2, was heated to 450 °C, only propanone and a white solid, M, were produced. (i) Write a balanced equation for the thermal decomposition of calcium ethanoate and state the identity of the white solid M. [1] (ii) On further heating, partial decomposition of solid M occurs to give off carbon dioxide and a white residue. The white residue contains 70% by mass of solid M. Calculate the mass of carbon dioxide evolved from the partial decomposition. [Mr of Ca(CH3CO2)2 = 158.1; CO2 = 44.0] [3] (iii) Predict whether barium ethanoate, Ba(CH 3CO2)2, decomposes at a higher or lower temperature than Ca(CH3CO2)2. Explain your answer. [1] Propanone is widely used as a solvent. (d) (i) Propanone reacts with hydrogen cyanide in the presence of trace amount s of potassium cyanide to form a cyanohydrin. Describe the mechanism, showing all partial and full charges and using cu rly arrows to show the movement of electron pairs. [2] (ii) Suggest a two -step synthesis starting from the cyanohydrin in (d)(i) to form compound P as shown below. [2]

3 © Raffles Institution 2015 9647/03/S/15 [Turn Over (e) In aqueous solutions , p ropanone reacts readily with compound Q in the prese nce of hydroxide ions to form compound R, as shown in the equation below. Four experiments were carried out to study the kinetics of the reaction. Experiment Initial concentrations / mol dm3 Initial rate of formation of R /mol dm3 s1 [Q] [Propanone] [OH] 1 0.0100 0.200 0.100 6.60 x 105 2 0.0080 0.200 0.100 5.28 x 105 3 0.0100 0.100 0.100 3.30 x 105 4 0.0080 0.100 0.150 3.96 x 105 (i) Use the data to deduce the order of reaction with respect to each of the three reagents, showing clearly how you arrive d at your answer. Hence, write a rate equation for the reaction. [3] (ii) It was proposed that the reaction between propanone and Q proceeds via the following mechanism. Based on your rate equation in (e)(i), suggest whether step A or step B is the rate determining step. Explain your choice. [2] (iii) State the role of hydroxide ions in the reaction. [1] [Total: 20]

4 © Raffles Institution 2015 9647/03/S/15 2 Methionine and tyrosine are two essential amino acids responsible for the structural conformation of prion proteins, which are linked to neurodegenerative diseases such as Creutzfeldt-Jakob disease (CJD), the human form of mad cow disease , and Alzheimer ’s disease. (a) (i) A common feature among these neurodegenerative diseases is the conversion of a protein , predominantly composed of -helical structures , into highly ordered fibrous forms with structures rich in -pleated sheets. With the aid of a suitable diagr am, describe the structure of a -pleated sheet protein. [2] The structures of methionine and tyrosine are shown below: (ii) Both methionine and tyrosine dissolve readily in water. Explain why. [2] (iii) Tyrosine residues in proteins are easily modified to 3 -nitrotyrosine residues by enzymes. Explain why the pKa of the 3-nitrotyrosine residue (pKa = 7.2) is lower than that of the tyrosine residue (pKa = 10.1). [1] (iv) At physiological pH of 7.34 –7.45, 3-nitrotyrosines residues were shown to induce conformation change in the tertiary structures of the prion proteins. By considering the p Ka values given above , suggest a possible reason why this happens. [2]

5 © Raffles Institution 2015 9647/03/S/15 [Turn Over (v) The oxidation of methionine to methionine sulfoxide inhibits the conformation change of -helix to -pleated sheet in a normal prion protein.

methionine residue methionine sulfoxide residue oxidation One plausible suggestion is that methionine sulfoxide is able to form R -group interactions with other polar amino acid s, like tyrosi ne, which then stabilise the normal prion proteins. State the type of R-group interaction present between tyrosine residue s and methionine sulfoxide residue s. I llustrate your answer with a suitable diagram. [2] (b) In the 1970s, bio chemists discovered that methionine acts as a precursor to the production of ethene in plants. This process is known as the Yang cycle. During the process, an intermediate, X, C4H7NO2 is formed. X is both soluble in sodium hydroxide and hydrochloric acid. When irradiated with UV light together with chlorine, X forms only one monochlorinated compound (ignoring stereoisomers). (i) Suggest a structure for intermediate, X. [1] (ii) Compound Y is an isomer of X. It can be easily synthesised from a brominated carboxylic acid. Y does not decolourise aqueous bromine. NH2CH2CH2CH(Br)CO2H  Y + HBr State the type of reaction, and hence suggest a structure for compound Y. [2] (iii) 4-oxobutanamide is another structural isomer of X. 4-oxobutanamide Describe what would be observed when 2,4 -dinitrophenylhydrazine is added to 4-oxobutanamide. Draw the structure of the organic product formed.