2021 NJC H3 Chemistry P1 Answer sheet

[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. OH NO2 NO2 4a (i) Draw the structure of the organocatalyst most suitable for the synthesizing cyclohexanol 4a. [1] N H Ar OTMS Ar Ar = 3,5-(CF3)2-C6H3 (ii) Based on the mechanism in figure 1, write a balanced equation for the reaction to synthesize cyclohexanol 4a. [1] R1 O R2 NO2 NO2 + OH NO2 R2 NO2 R1 N H Ar OR Ar Ar = 3,5-(CF3)2-C6H3 DABCO R1 = –CH2CH3 R2 = –C6H5 (iii) Suggest the condition(s) best for the formation of highly substituted optically active cyclohexanol 4a. Explain your answer. [1] According to table 1, the use of CH 2Cl2 under room temperature produces the highest yield of stereospecific 4a. (iv) Suggest three advantages of asymmetric domino reactions using organocatalysts in organic synthesis. [3] [Any valid answer] • Domino reactions reduces wastage due to the lack of need for excess solvent used to purify intermediate compounds. • Through the use of organocatalysts, no toxic metals are used. Less issue with disposal of toxic metals which may harm environment. • Highly stereospecific compound formed with fewer purification steps, hence increasing yield obtained.

NJC/H3 Chemistry Preliminary Examination/01/2021 [Turn over 3 (v) Draw the 2 conformers of cyclohexanol 4a. Suggest which structure is the major conformer. Explain your answer. [2] OH NO2 Ph NO2 EtOH NO2 Ph O2N Et MajorHigh Steric Hindrance (1,3-diaxial interaction) (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] R1 H N NO2 H O2N H Ar Ar OR R1 H N NO2 HO2N H Ar Ar OR bottom face Steric hindrance positioning NO2 group away from phenyl group Steric hindrance causing nucleophile to attack from bottom face d e d e (ii) Draw a suggested mechanism for step 3. [2] N R H2O N R O H H N R O H H N H R O H +R O H++ [Total: 13]

NJC/H3 Chemistry Preliminary Examination/01/2021 4 2 (a) (i) Using suitable diagram, calculate the bond order of Li2+. [2] Bond Order = (2 – 0) /2 = 1 (ii) Suggest whether Li2+ exhibits paramagnetism. [1] Yes, Li2+ contains unpaired electron and also will exhibit paramagnetism. (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 2 3.1 Polyacetylene n 1.5

NJC/H3 Chemistry Preliminary Examination/01/2021 [Turn over 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] LUMO: Conduction band HOMO: Valence band (iii) Polyacetylene is known to be an organic semiconductor. Explain how polyacetylene is able to function as an organic semiconductor. [2] Extensive π conjugated system => smaller energy gap between HOMO LUMO (small band gap) Electron has sufficient energy to cross over from HOMO to LUMO This allows electrons to delocalize and act as charge carrier.

NJC/H3 Chemistry Preliminary Examination/01/2021 6 (c) It is understood that compound, such as compound A, bearing charged leaving groups like –NR3+ and –SR2+ promotes elimination. N(CH3)3 + A (i) Elimination of compound A can yield two alkenes B and C. Two different mechanisms were employed to produce the two alkenes respectively. N(CH3)3 + A +AgO H2O B C Draw the two mechanisms that lead to alkenes B and C being formed. [3] H N(CH3)3H H + H2O + N(CH3)3 + H3O+ E2 Mechanism N(CH3)3 + + N(CH3)3 E1 Mechanism H2O + H3O+ H +slow

NJC/H3 Chemistry Preliminary Examination/01/2021 [Turn over 7 (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] N(CH3)3 + B Br NH2 NH3, heat in sealed tube Heat, excess CH3I Br H2N (H3C)3N + Br2(l) AgO H2O [Total: 18]

NJC/H3 Chemistry Preliminary Examination/01/2021 8 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. S HR1 R R O H R2 + O R1R2 + SR2 O R1R2 + D E (i) Draw the mechanism for the epoxidation. [2] S HR1 R R O H R2 + O R1R2 S H R1 R R O − HR2 (ii) Suggest, with suitable diagram, why epoxides D and E are the only 2 stereoisomers being formed. [2] 4 possible products from the nucleophilic addition but only 2 has conformation suitable for SN2 mechanism. R2 O − H H SHR2 R1 S HR1 R R O H R2 + Top and bottom face R2 O − H R1 SHR2 H H O − R2 R1 SHR2 H H O − R2 H SHR2 R1

NJC/H3 Chemistry Preliminary Examination/01/2021 [Turn over 9 O R1R2 O R1R2 DE S H R1 R R O − HR2 R2 O − H R1 SHR2 H H O − R2 H SHR2 R1 (b) Epoxide F can be further reacted under different reagent and condition to give compound G and H. O OH OCH3 G + H3CO OH HF 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] OH (S) OCH3 O(H) C(O,H,H)C(C,C,H) C(C,H,H) 1 23 4 (ii) Explain the yield of compound G and H obtained. [2] Without acid, likely reaction pr