HCI_H3_CHEM_P1_KEY

C2 Chemistry H3 Prelim 2011 Answers 9812/01/HCI 1 Answers for 2011 H3 Chemistry Prelims 1 (a) (i)  Disruptions of the synthesis of folic acid  Disruptions of protein synthesis  Plasma membrane disruption  Disruption of nucleic acid transcription Any 2 of the above. (ii) N S R O OH O HN S R O OH O OR RO N SR O OH O O R H H HN SR O OH O O R O H H HN SR O OH O OH O H H + ROH NH S R O OH O O R HN S R O OH O OHRO H (iii) Penicilloic acid does not retain the rigid conformation as that of penicillin and thus will not be able to effectively interact with the D-Ala-D-Ala termini in glycopeptides transpeptidase. Accept if students talk about the fact that the strained -lactam ring is essential in the SAR. (iv) It is an irreversible inhibitor as it covalent ly modifies the enzymes and this inhibition cannot be reversed. (b) O OR OH H2NOCO H3CO CH3 H3C O OR OHH3C CH3 OCH3 OCONH2 The more stable conformation is indicated in the box above. It is more stable as it has minimal 1,3-diaxial interaction (two as compared to four). [Accept if student mention anomeric effect] (c) (i) Pivampicillin is more lipophilic (non-polar) and thus can be better absorbed through the gut wall. (ii) The ester group further away from the penicillin nucleus, is less sterically hindered as compared to the other ester group and thus more susceptible to attack by nucleophiles. Hence it undergoes hydrolysis more easily.

C2 Chemistry H3 Prelim 2011 Answers 9812/01/HCI 2 (iii) esterase O H O (d) (i) Around the molecular ion peak, isoniazid-A and isoniazid-B will give the following pattern respectively: isoniazid-A isoniazid-B MM + 2 M + 4 MM + 2 M + 4 3 : 4 : 1 1 : 2 : 1 (ii) (I) NaCN; nucleophilic substitution (II) H2NNH2 2 (a) (i) Irreversible inhibition of the cyclo-oxygenase (COX) enzyme that it needed to turn arachidonic acid into prostaglandins. As prostaglandins are hormones responsible for the transmission of pain information to the brain, inhibiting the enzyme will reduce the pain felt, hence the analgesic effect. (ii) Since aspirin is less polar than benzoic acid, aspirin has greater affinity for the non-polar stationary phase and it eluted later. Thus peak 3 is due to aspirin. (iii) Peak area of component 1 = 930.02.63.02 1  Peak area of component 2 = 625.05.25.02 1  Peak area of component 3 = 800.06.10.12 1  Ratio of component 1 : 2 : 3 = 0.930 : 0.625 : 0.800

C2 Chemistry H3 Prelim 2011 Answers 9812/01/HCI 3 (b) Number of carbon = (100/1.1)(15.4/100) = 14 Molecular formula = C14H22N2O  Deduction 1.13 (t 6H) 2 CH 3 groups next to CH2 2.29 (q 4H) 2 CH 2 groups next to CH3 2.23 (s 6H) 2 CH 3 groups with no neighbouring proton 3.22 (s 2H) CH2 with no neighbouring proton; deshielded  next to electronegative atom