exp 13 lab report

exp 13 lab report - Bayabo Jan-Kristine 1 Jan-Kristine...

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Bayabo, Jan-Kristine 1 Jan-Kristine Bayabo Partner: Rafy Cahill Chemistry 3BL – Section 103 October 19, 2009 Experiment #13: The Sodium Borohydride Reduction of Benzil and Benzoin Discussion: The reduction of benzil can produce benzoin and isomers of hydrobenzoin, a meso compound and two enantiomers (R, R) and (S, S) (see Fig. 1). However, by using an excess amount of sodium borohydride (NaBH 4 ), the possible benzoin product is further reduced to a hydrobenzoin product. NaBH 4 attacks the ketone groups of benzil and benzoin into alcohol groups. Moreover, NaBH 4 is stereospecific and chooses to attack the least sterically hindered position. On the initial attack of benzil, the first ketone group is attacked either front or back forming a racemic benzoin. The second carbonyl, however, is reduced based on hinderance. The borohydride forms a 5-membered ring with the new alcohol group and the second ketonal group. Because of steric, the second borohydride will attack the ketonal group at the opposite side of the ring. Thus, the meso compound product is favored. In our experiments, the reduction of benzil and benzoin produced sparkly, white, fine crystals. The benzil reduction had a percent yield of 37%, starting with .074g of benzil and producing .028g of product. The product obtained from the benzil had a melting point of 118.6°C – 120.0°C, which is lower than the expected meso compound’s melting point (137°C - 139°C) and lower, but slightly closer to the enantiomers’ melting point (122°C - 123°C). The benzoin reduction had a percent yield of 70%, starting with .077g of benzoin and producing .056g of product. The product obtained from the reduction on benzoin had a melting point of 118.6°C – 119.3°C, also
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exp 13 lab report - Bayabo Jan-Kristine 1 Jan-Kristine...

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