CJC_H2_CHEM_P3_QP Prelim

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2 9647/03/CJC JC2 Preliminary Exam 2014 Answer any four questions. 1 (a) When an aqueous solution of Co 2+ is reacted with hydrochloric acid and ethane-1,2- diamine (en) under oxidising conditions, a green mixed-ligand cobalt( III) complex, [Co(en)2Cl2]+Cl-, with the following structure is formed: , where L = en (i) State the coordination number of the above complex. (ii) Explain why the above complex is green in colour. (iii) When 0.03 mol of aqueous silver nitrate is added to 0.01 mol of the above aqueous complex, a white precipitate weighing 1.44 g is formed. Identify and calculate the amount of precipitate formed. With reference to the structure of the above complex, explain the above observations fully. (iv) The formula [Co(en) 2Cl2]+Cl- may refer to a structure that is different from what is shown above. Draw this other structure of [Co(en) 2Cl2]+Cl- and state its relationship with the above structure. [9] (b) Cobalt is also found in nitrile hydratase, a bacterial enzyme which catalyses the conversion of nitriles to their corresponding amides: Nitrile hydratase has a quaternary structure with four subunits. Each pair of subunits binds a cobalt atom, which serves as a catalytic centre for the above reaction. (i) Identify the type of reaction performed by nitrile hydratase, and propose a three- step synthesis pathway to perform the same conversion, stating clearly all intermediates, reagents and conditions. (ii) One way to obtain nitriles is to synthesize them from chloroalkanes. State the reagents and conditions required to carry out this synthesis.

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