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 purifi