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 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. [5] Mass of magnesium required = 0.0676 x 24.3 = 1.64 g Step 1: Weigh, using a weighing balance, 6.41 g of methyl bromide, 3.92 g of propanone and 2 g of magnesium (excess). Step 2: Add both 6.41 g of methyl bromide and 2 g of magnesium reagents to the 50 ml of anhydrous diethyl ether in the two-necked round bottom flask. Step 3: Heat the reaction mixture and allow the mixture to reflux gently for 30 minutes. Cool the reaction mixture to room temperature. Step 4: Add the 3.92 g of propanone dropwise to the Grignard reagent at room temperature from the dropping funnel . Warm and swirl the mixture intermittently. (Students were not expected to give details on the follow-up reaction of adding dilute acid to form the 2-methyl-propan-2-ol, the extraction with diethyl ether and purification by recrystallization after extraction.) (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] Add the dilute acid drop wise. The mixture will foam out of the flask if the acid is added too quickly. [Total: 12 marks] 274

4 © ACJC 2013 9647/02/Prelim/13 [Turn over 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. 1s22s22p63s23p1 D belongs to group III. [2] (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. [4] D E F G H J B C Al Si P S Cl Ar Temperature/ oC 275

5 © ACJC 2013 9647/02/Prelim/13 [Turn over - Aluminium has giant metallic structure, Large amount of energy is required to overcome the strong electrostatic attraction between the positive metal cations and free electron cloud in the giant metallic lattice. - Silicon has giant covalent structure, Large amount of energy is required to break the strong covalent bonds between the Si atoms in the giant molecular structure. - P4,S8 and Cl2 has simple covalent structures, Lesser energy is required to break the weaker Van der Waals’ (VDW) forces between the discrete molecules (for P 4, S8 and Cl 2) or atoms (for Ar) in the simple molecular (or atomic) structure. - Strength of VDW forces increases with size of the molecule or atom, therefore melting points of S 8 > P 4 >Cl 2. Larger molecules have more electrons in the electron cloud and this electron cloud is more easily distorted. (b) Draw the dot and cross diagram for the compound SiCl4. [1] (c) Silicon tetrachloride is a liquid at room temperature, phosphorous (V) chloride, however exist as an ionic solid due to the presence of the ions [PCl4]+ and [PCl6]-. (i) Draw the structures of the cation, [PCl 4]+ and anion, [PCl 6]-. State the shapes of the two ions. P Cl Cl ClCl P Cl Cl Cl Cl Cl Cl Tetrahedral octahedral [3] (ii) Silicon tetrachloride reacts readily with water but carbon tetrachloride does not hydrolyse readily. Explain. Si has low-lying, vacant 3d-orbitals available for dative bonding with water molecules. [1] (iii) Suggest another physical property that is different between silicon tetrachloride and phosphorous (V) chloride. PCl 5 is able to conduct electricity when dissolved in polar solvents where it does not react but not SiCl4. [1] [Total: 12 marks] Cl xx x x x x Cl xx x xxx Cl x xxx x x Cl xx x x xx Si x x x x 276

6 © ACJC 2013 9647/02/Prelim/13 [Turn over 3 (a) (i) Compound M, shown below, reacts with hydrogen bromide. Describe the mechanism for the reaction. CC H H H C O OH Electrophilic Addition C C H H H C O OH + H Br C C H H H C O OH H + + Br- C C H H H C O OH H + + Br- slow fas t C C H H H C O OH H Br Or