2021 NJC H3 Chemistry P1 Question Paper

[Turn over NATIONAL JUNIOR COLLEGE SH2 Preliminary Examination Higher 3 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 1 Candidates answer on the Question Paper Additional Materials: Data Booklet Insert 9813/01 Friday 17 September 2021 2 hours 30 minutes READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name 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. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer two questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 Section A 1 /13 2 /18 3 /12 4 /17 Section B (circle the attempted questions) 5 /20 6 /20 7 /20 Percentage /100 This document consists of 28 printed pages.

NJC/H3 Chemistry Preliminary Examination/01/2021 2 Section A Answer all the questions in the spaces provided. 1 The information provided in the insert is taken from a number of published scientific articles. Other published articles may not agree with all of this information. You should read the whole insert before you start to answer any questions and use the information it contains to answer the questions. (a) Cyclohexanol 4a is the major stereoisomer formed in the screening experiments. (i) Draw the structure of the organocatalyst most suitable for the synthesizing cyclohexanol 4a. [1] (ii) Based on the mechanism in figure 1, write a balanced equation for the reaction to synthesize cyclohexanol 4a. [1] (iii) Suggest the condition(s) best for the formation of highly substituted optically active cyclohexanol 4a. Explain your answer. [1] (iv) Suggest three advantages of asymmetric domino reactions using organocatalysts in organic synthesis. [3] (v) Draw the 2 conformers of cyclohexanol 4a. Suggest which structure is the major conformer. Explain your answer. [2] (b) The mechanism for the reaction involves 4 steps. (i) Using suitable diagram, suggest how the stereochemistry at carbon d and e in cyclohexanol 4 was introduced. [3] (ii) Draw a suggested mechanism for step 3. [2] [Total: 13]

NJC/H3 Chemistry Preliminary Examination /01/2021 [Turn over 3 2 (a) (i) Using suitable diagram, calculate the bond order of Li2+. [2] (ii) Suggest whether Li2+ exhibits paramagnetism. [1] (b) Band gap is the energy gap between the valence and conduction band in a conjugated pi system. The following pi systems have varying band gap as stated. Pi system Band Gap (eV) Ethene 6.7 Buta-1,3-diene 5.8 Octa-1,3,5,7-tetraene 3.1 Polyacetylene 1.5 (i) Draw and label the π molecular orbitals for b uta-1,3-diene and octa-1,3,5,7-tetraene on the axis given below . In the same diagram, predict and draw the shape of each molecular orbital. You should indicate the nodes present in each molecular orbital with ( | ). [4] (ii) Suggest which molecular orbitals in the polyene contributes to the conduction and valence band respectively. [1] (iii) Polyacetylene is known to be an organic semiconductor. Explain how polyacetylene is able to function as an organic semiconductor. [2]

NJC/H3 Chemistry Preliminary Examination/01/2021 4 (c) It is understood that compound, such as compound A, bearing charged leaving groups like –NR3+ and –SR2+ promotes elimination. (i) Elimination of compound A can yield two alkenes B and C. Two different mechanisms were employed to produce the two alkenes respectively. Draw the two mechanisms that lead to alkenes B and C being formed. [3] (ii) Draw the energy profile diagram for the two mechanisms on the same axis below. [2] (iii) Using the information in (c), suggest how buta -1,3-diene can be synthesized from B. [3] [Total: 18]

NJC/H3 Chemistry Preliminary Examination /01/2021 [Turn over 5 3 (a) Ylides are dipolar reagents that can be used to synthesize epoxides. This epoxidation process proceeds in 2 steps. The second step is a substitution via SN2 mechanism. (i) Draw the mechanism for the epoxidation. [2] (ii) Suggest, with suitable diagram, why epoxides D and E are the only 2 stereoisomers being formed. [2] (b) Epoxide F can be further reacted under different reagent and condition to give compound G and H. Reaction Reagents and Conditions G H I NaOCH3, CH3OH 86% 5% II H2SO4, CH3OH 7% 83% (i) Using the R and S convention, assign the absolute configuration of compound G. Your answer should include priority assignments as defined by the Cahn-Ingold-Prelog rules. [1] (ii) Explain the yield of compound G and H obtained. [2] (iii) Suggest a possible side product from this reaction. [1] (iv) Suggest what changes to the yield of stereoisomers would you expect if the reagent and conditions were changed to the following: Reaction Reagents and Conditions I NaOCH3, CH3OH, 80oC II H2SO4, (CH3)3COH, 10 oC [2] (c) Thiiranes are analogue of epoxides. Given the C–O–C bond in epoxides have a bond angle of 60o, suggest the bond angle of C–S–C in thiiranes. Explain the answer. [2] [Total: 12]

NJC/H3 Chemistry Preliminary Examination/01/2021 6 4 (a) Organic acids are organic compounds which produces an acidic solution when added into water. (i) Carboxylic acid functional group is one such example with acidic properties. Rank the pH of 1 mol dm–3 solution of the carboxylic acids I, J and K in order of increasing pH. Explain your answer. [2] The Ka value of CH3CH2SH and CH3CH2OH is as follows. Ka / mol dm–3 CH3CH2SH 2.51 x 10–11 CH3CH2OH 1.26 x 10–16 (ii) Suggest a reason for the difference in the Ka values of the two compounds. [1] (iii) With reference to the data booklet, suggest the infra -red absorbance of S –H bond in CH3CH2SH. Explain your answer. [1] Cysteine has the pKa values of 1.92, 8.37 and 10.70. Cysteine (iv) Suggest the structure of the dominant species at  pH 1.52  pH 2.45  pH 10.1  pH 12.2 [2] (v) Isoelectronic point (pI) is the pH when the molecule is electrically neutral. Based on your structures drawn in part (vi), suggest the pI value for cysteine. [1] (b) Hydrogen choride, hydrogen bromide and hydrogen iodide are strong acids with different pKa values. (i) Rank hydrogen choride, hydrogen bromide and hydrogen iodide in order of increasing pKa values. Explain your answer. [3] (ii) Calculate the pH of 0.25 mol dm–3 solution of hydrobromic acid. [1]

NJC/H3 Chemistry Preliminary Examination /01/2021 [Turn over 7 (c) Currently, air on earth is composed of 412 ppm by volume of carbon dioxide. Carbon dioxide gas is soluble in water. Equation 4.1 CO2(g) CO2(aq) KH = 0.800 mol dm–3 atm–1 The solubility of carbon dioxide gas in water can be determined by Henry’s law give n in equation 4.2. Equation 4.2 Cgas, water=KHPgas Where Cgas, water = solubility of gas in water KH = Henry’s Law Pgas = partial pressure of gas Di