ACJC H2 CHEM P3 Questions Prelim

2 © ACJC 2015 9647/03/Prelim/15 [Turn over Answer any four questions. 1 (a) Bradykinin is a nonapeptide ( polypeptide with nine residues) which is a pain - causing agent. Complete hydrolysis of this polypeptide yields the following amino acids. Amino acid Pro Ser Arg Gly Phe No. of residues 3 1 2 1 2 Partial acid hydrolysis of bradykinin gave the following di- and tripeptides. Phe-Ser; Pro-Gly-Phe; Pro-Pro; Ser-Pro-Phe; Phe-Arg; Arg-Pro Given that both the terminal residues in the nonapeptide are arginine (Arg), deduce the primary structure of bradykinin. [2] (b) Frederick Sanger used 2,4-dinitrofluorobenzene (2,4-DNFB) in a basic solution to bind to the free amino group of the N -terminal of a polypeptide thereby labelling the N-terminal residue leading to its identification after hydrolysis. The reaction is shown below: A nucleophilic aromatic substitution occurs between 2,4-DNFB and the N-terminal residue in the first step of the process. (i) Suggest why a basic solution is used in reaction step 1, involving 2,4-DNFB and the peptide. (ii) Generally, halobenzenes do not undergo nucleophilic aromatic substitution. However, in the above reaction, 2,4 -DNFB seemingly undergoes nucleophilic aro matic substitution. Explain why this is possible , in this instance. [3]

3 © ACJC 2015 9647/03/Prelim/15 [Turn over 1 (c) Emil Fischer used the following reaction scheme to convert serine into alanine. (i) Draw the structural formula for A and state the type (s) of reactions that have taken place. (ii) Draw the structure of B and use it to explain why the first step is necessary for the overall conversion of serine into alanine. . (iii) Identify C and hence state what type of reaction has taken place in the conversion of C to alanine. (iv) Would you expect the absolute configuration about the chiral centre to have changed in the conversion of serine to alanine in the above scheme? Explain. [8]

4 © ACJC 2015 9647/03/Prelim/15 [Turn over 1 (d) A small polypeptide, oxytocin, when treated with certain reducing agents ( eg. Na/NH3) brings about a single chemical change that can be reversed by air oxidation. Based on the structure of oxytocin shown below, describe and explain what chemical changes were involved. [2] (e) The guanidine group (in brackets) of arginine, shown below, is one of the most strongly basic of all organic groups, with a pK a of 12.5. Explain why this is so, using appropriate illustrations. [2] (f) There are three pKa values associated with arginine: 2.2, 9.0 and 12.5. Derive the pI value of this amino acid, explaining your reasoning. [3] [Total: 20]

5 © ACJC 2015 9647/03/Prelim/15 [Turn over 2 Transition elements have different properties that distinguish them from the main group elements. This question hence discusses a bout cobalt and its properties, such as, able to exist in variable oxidation states and form coloured complexes etc. (a) When air is bubbled through an aqueous solution containing CoC l2, NH 4Cl and NH3, and the resulting solution evaporated, crystals of a salt W can be isolated. W has the following composition by mass: Co, 23.4 %; N, 28.0 %; H, 6.0 %; Cl, 42.6 % On adding an excess of aqueous silver nitrate to an aqueous solution containing 0.01 mol of W, 2.87 g of silver chloride is precipitated. (i) Determine the structural formula of the cation in W. Hence, calculate the oxidation number of the cobalt atom in W. (ii) If a solution of hydrogen peroxide is added to a separate solution containing only CoC l2 and NH 4Cl, what observation(s) would you expect? Explain your answer with reference to the Data Booklet. [5] (b) The traditional method for synthesis of organic compounds is via chemical routes, which usually involves multiple steps. However, over the last century, there is an increase in the use of electrochemical methods for organic synthesis, otherwise known as electrosynthesis. Electrosynthesis of organic compounds is carried out with the application of a constant current. The yield of electrosynthesis is expressed both in terms of the chemical yield and current efficiency, which is the ratio of Coulombs consumed in forming the products to the total number of Coulombs passing through the cell. Occurrence of side reactions decreases the current efficiency. The most successful organic electrosynthesis process is the cathodic reduction of acrylonitrile to form adiponitrile (ADN), which is a key intermedi ate for the production of nylon-6,6 polymers: 4 CH2=CHC≡N (l) + 2 H2O (l) → 2 N≡C(CH2)4C≡N (l) + O2 (g) An example of the operation conditions of this process is as follows: Cathode: Pb Anode: Fe (Ni 9%) Supporting electrolyte: Aqueous solution of (EtBu3N)2HPO4 + K2HPO4 + K2B4O7 Current efficiency: 90.7% Production per year: 300,000 tonnes (i) Write a half-equation, including state symbols, for the cathodic reduction of acrylonitrile to form adiponitrile. (ii) Based on the overall equation, deduce the half -equation, including state symbols, for the anodic reaction. [2]

6 © ACJC 2015 9647/03/Prelim/15 [Turn over 2 (b) (iii) Given that the o redE (acrylonitrile/adiponitrile) is x V, what is the minimum voltage required for the electrosynthesis of adiponitrile? (iv) Based on the operation conditions in the electrosynthesis of adiponitrile, calculate the current required for the production plant to operate 24 hours all year round. [5] (c) A coordination polymer contains metal cation centers lined by organic ligands. In recent years, due to their unique structural architectures, these polymers have increased applications in fields, such as biochemistry, material science etc. One such example is a cobalt(II) coordination polymer, conta ining the adiponitrile (ADN) ligand. (i) Co2+ ion forms a pale yellow complex with adiponitrile. Various samples containing different volumes of 1  10–5 mol dm–3 Co2+ and 3  10–5 mol dm –3 adiponitrile were prepared. The following graph was obtained when the colour intensity of the samples was measured using a colorimeter: Based on the above information, deduce the coordination number of Co 2+ ion. [2] Vol. of Co2+ solution/ cm3 Vol. of adiponitrile solution/ cm3

7 © ACJC 2015 9647/03/Prelim/15 [Turn over 2 (c) (ii) The following table provides information about the wavelengths absorbed when Co2+ ion is coordinated with different ligands: Cobalt(II)-complex ion aqueous solution of Co2+ Co(II)-adiponitrile solution Wavelength absorbed Green Violet By considering the above information and the wavelengths of the visible light spectrum, what deduction can be made about the size of the d -orbital splitting in the two complexes? Explain your answer. (iii) The stability constant, Kstab, is an equilibrium constant for the formation of a complex ion relative to its aqua-complex in a solution: The following table shows the lg K stab values of cobalt(II) -complex ion with the various ligands: Ligands H2O NH3 EDTA4- lg Kstab (of cobalt(II)-complex ion) - 4.9 16.3 (1) Write a balanced equation, with state symbols, representing the ligand exchange reaction between NH 3 and EDTA4- in the cobalt(II)-complex ion. (2) Hence account for the lg K