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William Ryan Cook Allison Underwood Kimberly Ray October 15, 2004 Experiment #5 (25 A) REDUCTION OF BENZIL TO HYDROBENZOIN USING NaBH4 Reaction: O OH NaBH4 O OH Benzil Hydrobenzoin Mechanism: O O BH 3 OH BH 3 H 2O O + H H O O OH OH OH H BH 3 H 2O + H H 3B O OH O Safety: 1. Always wear safety goggles. Procedure (microscale): Dissolve 50mg of benzil in 0.5mL of ethanol in a 5mL reaction vial, and cool the vial in an ice bath. Remove the vial from the ice bath and add 10mg of sodium borohydride, swirling the vial to mix the contents. Swirl the vial periodically while the reaction continues for ten minutes. Note any observations: gas evolution, heat evolution, color changes, etc. After ten minutes, carefully add 0.5mL of water to the vial. (Caution: Foaming may occur). Carefully heat the solution to its boiling point on a hot plate, remove the vial from the heat source, and add hot water dropwise until the solution just remains cloudy; about 1 mL will be required. Set the vial aside to cool; the product should crystallize as shiny flakes as the solution cools. When the mixture has reached room temperature, cool the vial in an ice bath to complete crystallization. Collect the diol by suction filtration on a Hirsch funnel, wash it with a small amount of cold water, and allow it to dry in air. Weigh the dry product, calculate the percent yield, and determine the melting point. Data and Observations: Table of Reactants and Product: Compound M.W. Wt( Wt(g) Moles Moles Physical g/mol g) really in actual contents to be used theory used Benzil 210.22 0.05 0.06 2.37E-4 2.85E-4 mp=94-95oC Sodium borohydride 37.83 0.01 0.02 2.64E-4 5.2 E-4 d=400g/mL Hydrobenzoin 214.27 0.19 mp=148.5- (S,S or R,R) 149.5oC Hydrobenzoin 214.27 mp=137-139oC (meso) Hydrobenzoin 214.27 mp=122-123 oC (racemic) When 0.06g of benzil was added to 5mL of ethanol, the benzil did not completely dissolve in the ethanol solution. The sodium borohydride was added and the mixture was swirled periodically for ten minutes, and the undissolved benzil reacted with the sodium borohydride and the solution turned clear. After the solution turned clear, 0.5mL of water was added dropwise to the solution. No foaming was observed. After heating solution to it’s boiling point and adding hot water dropwise, it took quite a while for the solution to turn cloudy. The solution had to be cooled to room temperature before the product would crystallize to shiny flakes. The product was funneled and allowed to dry over the week before the weight and melting point was taken. Results and Discussions: Percent Yield The limiting reagent for this reaction is Benzil. 0.06g (2.85E-4moles) of Benzil were used. The molecular weight of the product is 214.27g/mol, where 1 mole of Benzil gives 1 mole of Hydrobenzoin. Theoretical yield of product: Theoretical yield of product in grams = (Moles of starting product) x (Molecular weight of product) 2.85E-4moles x 214.27 g/mole = 0.080g of product Hydrobenzoin Percent yield: Percent yield = (Experimental yield/ Theoretical yield) x 100 Percent yield = ( 0.03g / 0.061g ) x 100 = 49.2% Melting Points of Hydrobenzoin Melting point of actual product = 138-139oC Melting point literature value of Hydrobenzion(R,R or S, S) = 148.5-149.5 oC Hydrobenzion (meso) = 137-139 oC Hydrobenzoin (racemic) = 122-123 oC Conclusion: The product of this reaction is Hydrobenzoin (meso) Questions: 1. Reduction of just one of the carbonyl groups of benzil gives a compoumd called benzoin. Draw its structure and give the IUPAC names of benzil, benzoin, and hydrobenzoin. O OH O OH OH O benzil benzoin hydrobenzoin 1,2-diphenylethonediole 2-hydroxy-1,2-diphenylethanone 1,2-diphenylethone-1,2-diol 2. Calculate the weight of sodium borohydride needed to reduce 1.00g of benzil to hydrobenzoin. (1.00g of Benzil) / (210.22g/mol of Benzil) = 4.76E-3 moles of Benzil Benzil is the limiting reagent and 1mole of Benzil gives 1mole of Hydrobenzoin, therefore it takes 1mole of NaBH4 to react with 1mole of Benzil. 1mole of Benzil = 1mole of NaBH4 (4.76E-3moles of NaBH4) x (37.83 g/mol of NaBH4)= 0.18g of NaBH4 It takes 0.18g of NaBH4 to reduce 1.00g of benzil to hydrobenzoin.
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