ACJC H2 Chem 2013 Prelim P2 Soln

1 © 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 (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] Bromomethane/ Methylbromide and propanone (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] Mass of product based on 100% yield = 80 100 x 4 = 5 g nproduct = 1) 16 12 15 (3 5     = 0.0676 mol [Assuming both are limiting reagents] Mass of methyl bromide = 0.0676 x 94.9 = 6.41 g Mass of propanone = 0.0676 x 58.0 = 3.92 g (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] Anhydrous/Dry diethyl ether (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. 272

2 © ACJC 2013 9647/02/Prelim/13 [Turn over (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] The drawing should include: + A two-necked round bottom flask with a condenser and dropping funnel; The condenser should be connected to a water supply with correct connection. + A heating mantle should be placed under the round bottomed flask. Heating mantle Water in Water out 273

3 © 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 a