ACJC H2 Chem 2013 Prelim P2 QP

2 © ACJC 2013 9647/02/Prelim/13 [Turn over 1 The Grignard reagent, R1-MgBr, can be made by reaction between alkyl bromide and magnesium. In a Grignard reaction, the Grignard reagent may be added to a carbonyl compound for the formation of an alcohol. This reaction is useful for the formation of carbon–carbon bonds. R1Br + Mg → R1-MgBr For Examiner’s use (a) The above reaction between the Grignard reagent and ketone can give a yield of about 80 %. You are required to prepare 4 g of 2-methylpropan-2-ol. (i) Identify the alkyl bromide and ketone required to synthesise 2-methylpropan-2-ol. …………………………………………………………………………………… [1] (ii) Assuming that the alkyl bromide can be completely converted into the Grignard reagent, determine the mass of each reactant in (a)(i) needed to prepare the desired amount of the pure organic product. [2] 257

3 © ACJC 2013 9647/02/Prelim/13 [Turn over (iii) Similar to the lithium aluminium hydride, the Grignard reagent is rapidly destroyed by water and air. Give a suitable solvent and the condition required for the solvent in the synthesis of the Grignard reagent. ……………………………………………………………………………………. [1] For Examiner’s use (b) The Grignard reagent was first prepared by adding an appropriate mass of magnesium to an appropriate mass of alkyl bromide dissolved in an appropriate volume of solvent. The mixture was then heated under reflux for about 30 minutes. After cooling the reaction mixture which now contains the prepared Grignard reagent, a fixed amount of ketone was t hen slowly added dropwise to it with intermittent swirling and warming. (i) You are provided with the following apparatus: two-necked round bottom flask, condenser, heating mantle and dropping funnel. Draw a suitable set-up using the glassware provided for conducting the above experiment. [2] 258

4 © ACJC 2013 9647/02/Prelim/13 [Turn over (ii) Write a step-by-step procedure on how you would prepare 4 g of the pure 2-methylpropan-2-ol using provided magnesium as well as the suggested alkyl bromide and ketone from your answers in (a)(i) bearing in mind that the yield in such a procedure is about 80 %. Your plan should:  show how the quantities of reactants calculated in a(ii) are used;  show that the mass of magnesium used is in excess for complete conversion of the alkyl bromide to the Grignard reagent;  use an appropriate volume of the solvent from your answer in a(iii);  ensure that the reactions are conducted under the appropriate condition. ……………………………………………………………………………….…… ……………………………………………………………………………………. ………………………………………………………………………………….… ………………………………………………………………………………….… ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ………………………………………………………………………………….… ………………………………………………………………………………….… ………………………………………………………………………………….… ………………………………………………………………………………….… ……………………………………………………………………………………. ……………………………………………………………………………………. …………………………………………………………………………….……… ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. For Examiner’s use 259

5 © ACJC 2013 9647/02/Prelim/13 [Turn over ……………………………………………………………………………………. ……………………………………………………………………………........... ………………………………………………………………………………….… ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ………………………………………………………………………………….... ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. [5] For Examiner’s use 260

6 © ACJC 2013 9647/02/Prelim/13 [Turn over (c) Excess unreacted magnesium in the reaction mixture could be destroyed by addition of sulfuric acid. Give a safety precaution, with reason, which you would take to perform the above step. .............................................................................................................................. ……………………………………………………………………………………….….. [1] For Examiner’s use [Total: 12 marks] 2 The graph for the second ionisation energies of Period 3 elements is given below: 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 (a) (i) State the electronic configuration of element D and deduce the group which element D belongs to. …………………………………………………………………………….……… ……………………………………………………………………………………. [2] D E F G H J B C 261

7 © ACJC 2013 9647/02/Prelim/13 [Turn over (ii) The figure below shows the melting points of six consecutive elements in the periodic table. Explain in terms of structure and bonding the variation in melting points of the elements. ………………………………………………………………………………….… ………………………………………………………………………………….… ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………............................................................................ …………………………………………………………………………………… …………………………………………………………………………………… ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. …………………………………………………………………………………... ………………………………………………………………………………….… …………………………………………………………………………………….