TJC H2 Chem 2013 Prelim P2 QP

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2 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use 1 Planning (P) Scientists in the 1970s have discovered that the reduction of cyclic anhydrides using sodium borohydride (NaBH 4) is more convenient and versatile method than lithium aluminium hydride (LiAlH4). γ-butyrolactone, a pro-drug to GHB, can be synthesized from its starting cyclic anhydride using NaBH4, as shown in the diagram below. (a) A research student wishes to synthesize γ-butyrolactone to determine the enthalpy change of combustion of γ -butyrolactone. Professor Hamilton pointed out to him that the product synthesized has to be purified before further use. This is because in the preparation of crude γ-butyrolactone, sodium borohydride was added in excess to ensure maximum yield in synthesizing γ-butyrolactone. After the reaction is complete, the excess sodium borohydride is dec omposed by acidifying the reaction mixture to pH 6 or below (slowly and while stirring) using excess 6 mol dm -3 aqueous hydrochloric acid. Hydrogen gas is evolved during this process as the excess sodium borohydride decomposes. The purification of crude γ -butyrolactone involves perform ing a liquid-liquid extraction initially using a separating funnel seen in Figure 1 below, before final purification of the product by recrystallization or distillation. Figure 1: Liquid-liquid extraction using a separating funnel