Organic Chemistry Lab March 27, 2022
Sodium Borohydride Reduction of Benzil Lab
I.
Purpose The purpose of experiment 6 is to reduce Benzil with sodium borohydride and create Hydrobenzoin.
II.
Introduction Sodium Borohydride is a mild and selective reducing agent. In ethanol, it reduces aldehydes and ketones rapidly at 25C. This mechanism involves the transfer of a hydride from the borohydride ion to the carbonyl carbon and protonation of the resulting alkoxide by the solvent. The addition of the first equivalent of hydride forms an alpha-hydroxy ketone. The second equivalent of hydride forms the diol.
III. Procedure 1. Add 200 mg of Benzyl and add it to a 10ml Erlenmeyer flask. 2. 2. Add 2 ml of 95% ethanol. Swirl to mix. 3. 3. Add 75 mg of sodium borohydride (MW 37.85) to the solution of benzyl 4. 4. Let stand for 10 minutes with occasional swirling. (Yellow color of benzyl will disappear as it is reduced.) 5. 5. Add 2 ml of water SLOWLY as foaming may occur. Heat the solution to boiling. 6. The solution should be clear and colorless. If necessary, filter off any insoluble impurities using a Pasteur pipette. 7. 6. Transfer the hot clear solution to a clean Erlenmeyer using a filter pipette. 8. 7. Add an additional 2 ml of hot water. 9. 8. Allow the solution to crystalize undisturbed to form shiny thin plates. Cool in an ice bath and then vacuum filter the crystals using a pre weighed filter paper and Buchner funnel. 10. 9. Wash the crystals on the filter paper with a minimum amount of cold water. 11. 10. Scoop out the funnel, fold the filter paper in half and press dry. 12. 11. Reweigh the filter paper and the crystal and determine the actual yield of the actual yield of the product.
IV.
Results and Discussion *filter paper: 0.12g
*filter paper + product : 0.26g *product: 0.26g - 0.12g= 0.14g *moles of benzil = 0.05 g/210.22g=2.38 x 10^-4 *moles of NaBH4 = 0.08 g/37.83g= 2.14 x 10^-3 * theoretical yield of benzoin = 2.38 x 10^-4 x 2.14 x 10^-3 = 5.09 x 10^-2 --> 0.059g >0.59g
*percent yield= 0.14g/0.59g x 100% = 23.7%
By subtracting the values of the filter paper and filter paper + sample, the amount of sample is obtained which is 0.14g. Due to significant figures, 0.059g was turned into 0.59g. Plugging in the values in the percent yield formula which is actual/theoretical x 100, it resulted in 23.7%. The results showed a low yield percentage meaning there was a loss of product. During the boiling of the product, it turned cloudy to clear. Following the clear solution, the formation of solid lustrous thin plates occurred.
V. Conclusion It is concluded that the percent yield is 23.7%. Majority of the product was gathered but due to a low actual yield, it is shown that some of the product was lost. The loss of product could have been due to the suctioning process ,or pipettin