ACJC H2 CHEM P3 Answers 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. Arg-Pro Pro-Pro Pro-Gly-Phe Phe-Ser Ser-Pro-Phe Phe-Arg Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg [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:

3 © ACJC 2015 9647/03/Prelim/15 [Turn over 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. N-terminal is usually protonated at physiological/normal pH. Hence need to use a base to deprotonate the amino group so that it can act as a nucleophile. [1] (b) (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. Fluorine is very electronegative and hence produces a partial positive charge on the carbon atom to which is bonded, drawing the nucleophile. The two strongly electron withdrawing nitro groups help to disperse the negative charge in the reactive intermediate thereby stabilising it. [2] (c) Emil Fischer used the following reaction scheme to convert serine into alanine.

4 © ACJC 2015 9647/03/Prelim/15 [Turn over (i) Draw the structural formula for A and state the type (s) of reactions that have taken place. Acid-base reaction (no “neutralisation”) and esterification/condensation/nucleophilic (acyl) substitution [3] (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. Esterification of the carboxyl group is necessary to prevent it from being converted to the acyl chloride which will render it susceptible to unnecessary nucleophilic substitution reactions. Or To protect the carboxyl group from being converted into an acyl chloride thereby leading to other undesirable reactions. [2] (iii) Identify C and hence state what type of reaction has taken place in the conversion of C to alanine. Reduction [2] (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. No, None of the reactions/conversions involved the chiral carbon and therefore did not affect the stereochemistry of the chiral carbon. [1]

5 © ACJC 2015 9647/03/Prelim/15 [Turn over (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. Reduction cleaved the disulfide linkage and hence affected the primary structure of oxytocin. Air oxidation allowed the disulfide linkage to be re-formed. [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. Protonated guanidine group is very stable due to resonance delocalisation of the positive charge over the three nitrogen atoms. [2]

6 © ACJC 2015 9647/03/Prelim/15 [Turn over (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. pI = (9.0 +12.5) / 2 = 10.75 [3] [Total: 20] 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. [5] (i) Determine the structural formula of the cation in W. Hence, calculate the oxidation number of the cobalt atom in W. Co N H Cl % mass 23.4 28.0 6.0 42.6 % mole 0.397 2.00 6.00 1.2 Mole ratio 1 5 15 3 n(AgCl) = 2.87 / 143.5 ≈ 0.02 mol 1 mol of complex contains 2 mol of free chloride ions. Hence, the formula of the cation in W is [Co(NH3)5Cl]2+ Oxidation number of cobalt = +3

7 © ACJC 2015 9647/03/Prelim/15 [Turn over (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. A greenish-yellow, pungent gas, which turns blue litmus paper red, then bleaches moist litmus paper will also be evolved. o redE (H2O2/H2O) is more positive than that of o redE (Cl2/Cl-), hence Cl - ions will be oxidised to Cl2 gas. (OR o cellE = (+1.77) – (+1.36) = +0.41 V > 0, hence reaction is feasible.) (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 applicatio n 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 [7] (i) Write a half-equation, including state symbols, for the cathodic reduction of acrylonitrile