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Borohydride Sodium Reduction of Benzil Raymond Oldham, Michael Lester, Audrey Paulzak CH-238 QZ Introduction: There are several methods to reduce aldehydes and ketones to alcohols. Both primary and secondary alcohols can be created by using complex metal hydrides such as lithium aluminum hydride and sodium borohydride, with ketones reducing to secondary alcohols and aldehydes reducing to primary alcohols. Both reagents provide an abundant source of H+ ions that serve as catalysts for the reduction. Between the two reducing agents, lithium aluminum hydride (LiAlH 4) is extremely strong, expensive, and must be used in aprotic solvents. On the other hand, sodium borohydride (NaBH4) is inexpensive and can be used in common solvents such as water and alcohols. Also, all four of the hydrogen atoms can reduce the ketone or aldehyde1. In this experiment, sodium borohydride will be used to reduce diketone benzil in a quick and efficient manner. Table of Reagents: Melting Point (C) 94-95 400 d. -114.3 0 148.5-149.5 137-139 122-123 Molecular Weight (grams/mol) Benzil Sodium Borohydride Ethanol Water Hydrobenzoin (S,S or R,R) Hydrobenzoin (meso) Hydrobenzoin (racemic) 210 37.83 46.07 18.02 214.27 214.27 214.27 Mechanism: O + H H B H H - O + Na O O H BH2 - O - O BH2 H2O O B H2 - O H + HO OH Safety Information: - If any reagents come into contact with your body, wash with plenty of water. - Dispose of excess reagents in labeled containers. Procedure: To begin the experiment, 50.01 mg of benzil and 0.52 mL of ethanol were combined in an Erlenmeyer flask, and swirled gently until the benzil dissolved completely. The vial was then cooled in an ice bath, and 12 mg of sodium borohydride was added to the contents of the flask. The flask was swirled continuously for 10 minutes while the reaction proceeded. It was observed that the solid particles took a couple of minutes to dissolve. After ten minutes, 0.48 mL of water was added drop-wise to the mixture. Foaming occurred at the top of the solution for approximately one minute after the water was added. The mixture was heated a on hot plate until it came to a rolling boil, and then removed from the heat. Approximately 1.78 mL of hot water was added dropwise to the mixture. The amount added was determined by the clarity of the solution. As soon as it turned, and remained, cloudy, the addition of water was terminated. The flask was set aside to cool to room temperature, and allow crystal product to form. Once the solution had reached room temperature, it was placed in an ice bath to further crystallization of the product. The product was then collected through suction filtration. A melting point was determined after the product was completely dry. Results and Discussion: Since benzil is the limiting reactant in our sodium borohydride reduction of a ketone experiment, use the 50.01 milligrams to calculate the expected moles of product for the theoretical yield. Theoretical Yield = 50.01 mg reactant 1 mol reactant 1 mole product 210.22 grams 1 mole reactant 214.27 g product 1 mole product 50.97 mg product Where the "reactant" represents benzil, and "product" represents hydrobenzoin. The percent yield can then be calculated by using the actual weight of product and the theoretical yield. Percent Yield = Experimental weight of product Theoretical Yield 100 = Benzil Hydrobenzoin (meso) Theoretical yield Experimental yield Percent yield Melting Point 50.01 mg 20.02 mg 50.97 mg 20.02 mg 39.28% 136-138 C The product of this experiment was the meso hydrobenzoin, as indicated by the nearly identical melting point to the accepted values given by the lab manual1. REASONS FOR ERROR. DRAWINGS OF DIFFERENT CHIRALITY. 1. R,R AND S,S 2. MESO 3. RACEMIC Questions: 1) Benzil 1,2-diphenylethanedione Benzoin 2-hydroxy-1, 2-diphenylethanone Hydrobenzoin 1, 2-diphenylethane-1, 2-diol O OH Benzoin has the structure 2) 1.00 g Benzil 1 mole Benzil 1 mole NaBH 4 210.22 g Benzil 2 mole Benzil 37.83 g NaBH 4 1 mole NaBH 4 0.0899 g NaBH 4 Reference: 1. "Experiments in Organic Chemistry," R. Hill and J. Barbaro, Contemporary Publishing Company of Raleigh, Inc 3 rd Edition. (2005) ... View Full Document

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