TJC H2 Chem 2013 Prelim P2 Soln

1 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use TJC Paper 2 Answers 1 Planning (P) (i) Which of the following two organic solvents would be a good choice to dissolve crude γ-butyrolactone to perform a liquid-liquid extraction? Explain your answer.  Diethyl ether γ-butyrolactone is polar with the presence of electronegative O atom and is able to form permanent dipole-permanent dipole interactions (pd-pd) with polar diethyl ether (ii) One common mistake when performing a liqui d-liquid extraction is to mix-up the layers and hence the organic product would be lost in the discarded layer. Using the following data in the table, Solvent Density (g cm -3) Benzene 0.877 Diethyl ether 0.713 Water 1.000 Identify if the organic solvent identified in (i) would be the upper or lower layer? Explain your answer.  Upper layer. Diethyl ether is less dense (  = 0.713 g cm –3) than water (  = 1.000 g cm –3). Therefore, diethyl ether will be the upper layer and water will be the bottom layer. (iii) The experimental procedures of the purification of crude γ-butyrolactone via liquid- liquid extraction is described below: 1. Dissolve the crude γ-butyrolactone in 30 cm3 of the organic solvent. 2. Cool the flask in an ice bath and then add 20 cm 3 of 6 mol dm –3 hydrochloric acid dropwise, while stirring, to quench the reaction. 3. Place the reaction mixture in a separating funnel and separate the aqueous layer from the organic layer. Reject the aqueous layer. 4. Add about half its volume of chemical reagent A. Shake the reaction mixture in the separating funnel cautiously, releasing the pressure at intervals. 5. Transfer the organic layer into a conical flask and add some chemical reagent B. Filter off chemical reagent B and evaporate the solvent at room temperature. State the identities of chemical reagents A and B , explaining the role each plays in 1352

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