Reduction Lab 1 Report PDF

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Princy Mathew 01/23/2020 The Borohydride Reduction of a Ketone: Hydrobenzoin from Benzil Aim The objective of this experiment was to synthesize hydrobenzoin from Benzil using Borohydride as a reducing agent which involved the reduction of a Ketone. Theory In Organic chemistry, Organic reductions are usually defined as a net gain of hydrogen or a net loss of oxygen atoms. In this experiment, carbonyl groups on the compound benzil was reduced with sodium borohydride. Reductions occur when hydrogen is incorporated into a functional group, either through high pressure hydrogenation using hydrogen gas and a metal catalyst or through a metal-hydride transfer reaction. The most common reducing agents for carbonyl chemistry are lithium aluminum hydride, and sodium borohydride. Both are excellent reducing agents; however, Lithium aluminum hydride is so strong that it reacts violently with water or any other protic solvent, making sodium borohydride the preferred reducing agent. It is stable to most protic solvents, and not nearly as reactive in water. Mechanistically, sodium borohydride works through a hydride transfer, in which a hydride is transferred from an electropositive boron to an electropositive alpha carbon in a carbonyl. The hydride can attack from either the front or back side of a ketone, creating a new chiral center. As the hydride attacks the carbonyl’ electrons in the carbonyl move onto the oxygen. This in turn causes a lone pair from the oxygen to bond to the electropositive boron. As a final step, this intermediate is hydrated by a protic solvent, creating the newly formed chiral alcohol and an oxyborohydride by product.

Benzil has two chemically equivalent carbonyls, and thus it can produce an array of stereoisomeric products. The first carbonyl on benzil can be attacked by the hydride either on the front, or the back resulting in (+) and (-) benzoin enantiomers. Materials 10 * 100-mm reaction tube 50 mg of benzil 0.5 mL of 95% ethanol Beaker with ice 10 mg of sodium borohydride 0.5 mL of water Heating apparatus 1 mL of Hot water Pasteur pipette Buchner funnel Filter paper Suction apparatus Methods A 10 * 100-mm reaction tube was obtained. 50 mg of benzil was dissolved in 0.5 mL of 95% ethanol in the reaction tube. The solution was cooled in an ice bath to produce a fine suspension. To this suspension, 10 mg of sodium borohydride in excess was added. As the mixture warmed up, the benzil dissolved, and the yellow color disappeared in about two to three minutes. After a total of ten minutes, 0.5 mL of water was added, and the solution was heated to boiling point. This solution was then filtered with hot water to the point of saturation or until it looked cloudy.

This process required about 1 mL of water. The hydrobenzoin was then separated by withdrawing some of the solvent using a Pasteur pipette and the crystals were collected using a Büchner funnel which was connected to the suction apparatus. Results Chemical reactions are as follows: 4 R2C=O + Na+BH4-  (R2CHO)4B-Na+ (R2CHO)4B-Na+ + 2H2O  4 R2CHOH + Na+ BO2Weight of the crystals separated = 0.040g Benzil initial amount = 0.05g MW of Benzil = 210.228g/mol Moles of benzil = 0.05g * mol/210.228g/mol =2.38 * 10^-4 moles Sodium borohydride initial amount = 0.01g MW of sodium borohydride = 37.83 g/mol Moles of sodium borohydride = 0.01g * mol/37.83g/mol =2.64 * 10^ -4 moles Therefore, the limiting reagent for this reaction is Benzil. Theoretical yield of product = (Moles of starting product * Molecular weight of product) (2.38 * 10^-4 moles) * (210.228 g/mol) =.050034g Percent yield = (Actual/Theoretical) * 100 =(0.040g/.050034g) * 100 =80.0% Melting point of Hydrobenzoin Melting point of actual product = 138-139 degree C Melting pint literature value of Hydrobenzoin Hydrobenzoin (R, R or S, S) =148.5-149.5 degree C

Hydrobenzoin (meso) = 137-139 degree C Hydrobenzoin (racemic) = 122-123 degree C Conclusion This lab focused on the reduction of ketones to alcohols. Sodium borohydride was used as a reducing agent because of its mild reactivity compared to other hydrides as well as its selectivity. From the experiment, it was concluded that benzil was the limiting reagent and based on that information, theoretical and percent yield were calculated. The theoretical yield was calculated to be.050034g and the percent yield was calculated to be 80.0%. From the literature values, the product obtained was identified to be a meso compound based on the melting point. Overall, this experiment went well, and the conversion of a diketone dissolved in ethanol to a dialcohol using solid sodium borohydride in a reduction reaction was studied. Works Cited

Wigal, C. T.Signature Labs Series: Chemistry Lab Experiments CHEM 226;Jefferes, J., Ed. ; Cengage Learning: Mason, OH, 2008;p 135-152 Macroscale and Microscale Organic Experiments 6th edition by Kenneth L. Williamson and Katherine M. Masters

Questions S, S 1.

R, R

R, S

2 0.05g benzil * 1 mol benzil/210.22g * 1 mol NaBH4/2 mol benzil * 37.83g/1 mol = .004499g = 4.5 mg of NaBH4....