Sodium borohydride Reduction of Benzil
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 electrophilic carbons.
Figure 1: reduction mechanism for the reaction
In this experiment, the benzil (diketone) is used as the organic substrate. Sodium borohydride, NaBH4 is used as the reducing agent (metal hydride) and ethanol as the solvent. The H- of the NaBH4 reduces one or both carbonyl groups in benzil and then the acidification processes results in a diol or two secondary alcohols as products. If only one carbonyl group is reduced, the product results in a racemic mixture of benzoin. If both carbonyl groups are reduced, the product results in a meso-hydrobenzion or a racemic mixture of hydrobenzion. The goal of the experiment is for the student to reduce benzil and use melting point, TLC, and IR to determine the stereochemistry of the resulting product.
Figure 2: Benzil reduction mechanism:
Key experimental details, observations and results:
The initial reaction solution including the 1.011g of benzil and 10ml of 95% ethanol appeared as a yellow liquid. After the solution was heated allowing the benzil dissolved, the solution appeared as a yellow liquid with feather like crystals when cooled to room temperature. The feather like crystals dissolved and the solution became a creamy light yellow liquid...
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