HCI Prelim ANS

H3 Chemistry Preliminary Examination 2009 Mark Scheme Setter & Marker: Ms Grace Chua 1 a) Narcotic analgesics work by depressing the CNS , hence affecting the capacity of the brain to appreciate pain [1] Non–narcotic analgesics work on the pain receptors themselves, preventing them from responding normally to pain stimuli. [1] b) Natural products, current drugs etc [2] ci) Mode of action (1) Aspirin Mode of action (2) Ibuprofen Mode of action (3) Indomethacin [1] Mechanism (1) takes place with the formation of a covalent bond with serine. Aspirin can form an ester linkage with the hydroxy residue. [1] Mechanism (2) should take place with substances that mimic arachidonic acid. Ibuprofen has the most similar structure to arachidonic acid. [1] Mechanism (3) should involve electrostatic interactions between the basic group on arginine and hydrogen bonding/ dispersion forces with the tyrosine group for it to be reversible. Indomethacin fits the description. [1] cii) N NS O O H2N CF3 1 m for each reasonable interaction [2] di) Precursor: N HO O CH3 Cl CO2H [1] Reagents and conditions: (1) NaOH(aq) (2) CH3I, heat [1] dii) SN2. [1] If the mechanism were SN1, the carbocation formed is very unstable/ less steric hindrance Hydrogen bonding with tyrosine/ serine Electrostatic forces of interaction with arginine Hydrophobic interactions with t yrosine C2 Chemistry H3 Prelim 2009_answers 9812/01/HCI 1

C2 Chemistry H3 Prelim 2009_answers 9812/01/HCI 2 r primary halide therefore S 2 favoured. [1] , if not no marks awarded. fo N Note: Answer must be accompanied with reason e) OHO OH H N CH3 R S S R S Z [1/2] for each correct assignment. [3] (f) H CH2SHR CH3Ph H H CH2SHR PhH3C H H RHSH2C PhH3C H H RHSH2C CH3Ph H Enantiomers Enantiomers Diastereomers Diastereomers Diastereomers Compound A Diastereomers [3] [Total: 20m] Setter & Marker: Ms Jessie Koh 2 ai) Rate of hydrolysis A<penicillin-G<B [1] A should hydrolyse slower due to the electron-withdrawing C l group, B should hydrolyse faster due to the electron-donating CH3CH2 group. [1] The electron-withdrawing C l group withdraws electrons from the C=O attached to the R group, thus makes the O of this C=O a poorer nucleophile which will have lower tendency to attack the carbonyl carbon of -lactam ring to cause ring-opening. [1]

ii) 1 mark for each equatio n: NHCH3 C Cl H H + NaOH N N H2N NH2 S-Na+ + N N H 2N NH2 SH N N H 2N NH2 S-Na+ N N H 2N NH2 NHCH3 S C H H 2H2O2 N N