William Ryan Cook by qpY3i8

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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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