ASR 2020 J2Prelim H2Chem P3 MS

ASRJC JC2 PRELIM 2020 9729/03/H2 1 Anderson Serangoon Junior College Preliminary Examinations H2 Chemistry (9729) Paper 3 Mark Scheme Section A Answer all questions. 1 Many naturally occurring organic compounds contain either acidic groups or basic groups or both. The strength of the acid or the base depends on the structure of the molecule. (a) The pKa values of ethanoic acid and two amino acids are given in Table 1.1 below. Table 1.1 name structure pKa1 pKa2 pKa3 ethanoic acid CH3COOH 4.7 – – serine (ser) H2N CH CO2H CH2 OH 2.2 9.2 – aspartic acid (asp) H2N CH CO2H CH2 CO2H 1.9 3.7 9.6 (i) Suggest two reasons why the p Ka1 value of serine is so much less than the p Ka1 of ethanoic acid. [2] Electron withdrawing –NH3+ (or –NH2) group is in close proximity to the negative charge of the carboxylate anion of deprotonated serine, hence dispersing the negative charge and stabilizing the anion, and –OH of the R group of serine is able to form intramolecular hydrogen bond with the O atom of –COO–. This stabilizes the carboxylate anion. Hence, carboxylate anion of serine is more stabilised than ethanoate ion (ii) Draw a skeletal structure for the dipeptide, asp–ser. [2] N OH O OH O H2N O HO H

ASRJC JC2 PRELIM 2020 9729/03/H2 2 Electrophoresis is a technique used to separate charged particles placed in an electric field. The system consists of two oppositely –charged electrodes connected by a conducting medium, typically a gel. The separation of the charged particles is based on their electrical charge and Mr. A sample consisting of serine and aspartic acid was analysed by electrophoresis using a gel buffered at pH 5 .7. The small quantity of the mixture was placed at the centre of the gel, at equal distance from the two electrodes. (iii) Using the information given in Table 1 .1, suggest the structures of the major species present in the buffer solution. Label the structures clearly. [2] asp ser (iv) Hence, describe the relative positions of the two species in the gel after the separation by the electrophoresis process. Explain your answer. You may find it helpful to draw a diagram to illustrate your descriptions. [2] Ser has no net charge / electrically neutral / exists as a zwitterion. Hence it will not move and remain at the centre of the strip. Asp has an overall negative charge, and so it will migrate towards the anode. asp Anode (+) Cathode (–)