Sodium Borohydride Reduction of Benzil Essay

1105 Words Mar 15th, 2012 5 Pages
Sodium borohydride Reduction of Benzil
Introduction:
The Purpose of this experiment is for the students to learn how to use sodium borohydride to reduce benzil to its secondary alcohol product via reduction reaction. This two-step reaction reduces aldehydes by hydrides to primary alcohols, and ketones to secondary alcohols. In order for the reaction to occur and to better control the stereochemistry and yield of the product, the metal hydride nucleophile of the reducing agents such as LiH, LiAlH4, or NaBH4 must be carefully chosen. Being that LiAlH4 and NaBH4 will not react with isolated carbon-carbon double bonds nor the double bonds from aromatic rings; the chosen compound can be reduce selectively when the nucleophile only react with
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Figure 3: TLC plate

Percent yield calculations:
Moles of excess reagent (Benzil)

1.011g C14H10O2(2mol/210.23g) = 0.009618mole of Benzil

Moles of limiting reagent (NaBH4)

0.212g NaBH4(1mol/37.83g) = 0.005604mole of NaBH4

Theoretical yield of products

0.005604mol NaBh4(214.27g meso/1mol NaBH4)= 1.2008g of meso-hydrobenzoin product

Percent yield of isolated product (pure product)

(0.613g of isolated product/ 1.2008g of theoretical yield) 100= 51.1% product yeild

Table 1: Amount of reagent and product yield compound | weight(g) | Benzil | 1.011 | NaBH4 | 0.212 | Product (meso) | 0.613 |

Table 2: Average melting point compound | Melting point of product(°C) | Literature melting point(°C) | Meso-hydrobenzoin | 135.0-136.3 | 137-139 |

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Table 3: IR data

compound | Peaks (cm^-1) | Functional groups | Structural info | | Hydrobenzoin (meso) | 3375.14 | alcohol | O-H | | Hydrobenzoin (meso) | 2900.47 | carboxyl | C-H | | Hydrobenzoin (meso) | 1860-1940 | ---- | overtones | |

Discussion: The hydrobenzoin (meso) product of the benzil was isolated through the techniques of recrystallization and vacuum filtration. Because there NaBH4 was the limiting reagent in the experiment, 0.005604moles of NaBH4 should yield 1.2008g of hydrobenzoin (meso). The mass of the isolated product was 0.613g, resulting in a 51.1% yield. There are many reasons to account for the loss of 48.9% of
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