Problem in Diastereoselectivity

Topics: Laboratory glassware, Sodium, Distillation Pages: 3 (955 words) Published: November 25, 2012
The purpose of this experiment is to determine the stereochemical outcome of a reaction. A second chiral center is being formed in this experiment and two diastereomeric compounds are created. Table of Reagents:

Safety Precautions or Hazards:
Do not place pipettes in paper trash. Must be thrown away in the broken glass container •Take caution when adding HCl.
CO2 evolution may be vigorous so be sure to vent funnel. •Never directly heat ethanol over hot plate, use water bath. •Conc. H2SO4 is highly corrosive

Equations for all Reactions:


Experimental Procedure:
Part A:
1.Add a magnetic stir bar, 2g of benzoin, and 20 ml of absolute ethanol in a 125ml Erlenmeyer flask. 2.While stirring, (do not turn on the heater) carefully add 0.4 g (10.6 mmol) of sodium borohydride portion-wise to the mixture over 5 minutes. After the addition is complete, stir the mixture for another 15 minutes at room temperature. A white precipitate will form. 3.Cool the flask in an ice-water bath and decompose the excess sodium borohydride by first adding 30 mL of water followed by the careful and dropwise addition of 2 mL of 3M HCl. The HCl addition should be done quite slowly, no more than about 3 drops per minute (maybe 10-15 minutes total time). The mixture may foam uncontrollably if the acid is added too quickly. If the foam reaches the 100 mL mark on the flask, stop adding HCl until the foaming subsides. 4.When the HCl addition is complete, add another 10 mL of water and stir the mixture for 15 minutes. 5.Collect the white precipitate by suction filtration. Wash the product diol with water on the suction funnel (about 150 mL) and allow the product to air dry on a filter paper. Safely store the product in your drawer until next week. 6.The filtrate should be placed in the aqueous acidic waste container. It contains water, B(OH)3, HCl, and ethanol. 7.Then, record the mp when the...
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