2020 HCI Prelim P3 Answers

2020 HCI C2 H2 Chemistry Preliminary Exam / Paper 3 HWA CHONG INSTITUTION 2020 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 3 1 (a) (i) [1] each x 3 1 (a) (ii) [1] Pick one of the products from (a)(i) and draw mirror image s using stereochemical formula with appropriate dash/wedge, showing the tetrahedral shape about the chiral carbon, and dotted line to show the mirror plane. e.g. The product has two isomers that are [1] non-superimposable mirror images, hence they are enantiomers. 1 (a) (iii) [1] The h ost cell recognises a specific conformation/chirality of the spike protein. If the amino acid in the protein is swapped with its enantiomer, it changes the conformation/3 -dimensional arrangement of the protein, and the host cell no longer recognises it and the virus cannot enter 1 (b) (i) [2] Draw one ethanol molecule and indicate correct hydrogen bond with appropriate illustrations. Students may choose any part of the phospholipid that can form hydrogen bond with the ethanol. Hydrogen bond δ+ δ‒ δ+ δ‒

1 1 (b) (ii) [1] Dispersion forces of attraction forms between the hydrocarbon chains of the soap and phospholipid ions [0.5] Water forms hydrogen bonding with the carboxylate group on the soap. [0.5] The above, together with the mechanical action of water forces the phospholipid particles to be pulled apart/ separated from each other/ washed away, hence breaking up the membrane. 1 (b) (iii) [1] Surface charge of the fiber polarises/distorts the electron cloud of the non- polar dust molecules [1] Forms ion – induced dipole attraction, OR electrostatic force of attraction between the charge on the fiber and the induced dipole on the dust molecule 1 (c) Copper metal is made up of a [1]g iant metallic lattice held together by the [1]electrostatic forces of attraction between the Cu2+ metal ions and the sea of delocalised electrons 1 (d) (i) [1] P: 1 (d) (ii) [1] Step 2: LiAlH4/ dry ether OR NaBH4 OR H2/ Ni/ high pressure [1] Step 3: PCl3 OR PCl5 OR SOCl2/ warm OR concentrated HCl/ ZnCl2/ heat [1] Step 4: ethanolic concentrated ammonia, heat in sealed tube [1] R: [1] S: 1 (d) (iii) [1/2] Lone pair of electrons on N a is delocalised into the aromatic ring (s), making the lone pair [1/2] less available for donation/ accept a proton. [1/2] Nb has 3 electron donating alkyl groups attached to it, making its [1/2] lone pair of electrons more available for donation/ accept a proton. [1] Nb more basic than Na. Awarded only if there is some attempt at explaining the difference. 1 (d) (iv) [1] Reagent/Test [1/2] Positive observation [1/2] Negative observation

2020 HCI C2 H2 Chemistry Preliminary Exam / Paper 3 K2Cr2O7(aq)/ H2SO4(aq)/ heat. Orange K2Cr2O7 turns green for hydroxychloroquine but remains orange for chloroquine or KMnO4(aq)/ H2SO4(aq)/ hea