Question

EXPERIMENT 11 THE MICROSCALE REDUCTION OF BENZIL WITH SODIUM BOROHYDRIDE INTRODUCTION Oxidation-reduction (redox) reactions occur in concert. One substance undergoes oxidation by donating electrons to another substance that is then reduced. In organic chemistry certain rules are followed in determining whether a particular substrate is being oxidized or reduced. Oxidation: The loss of hydrogen and/or gain of oxygen by a substrater Reduction: The gain of hydrogen and/or loss of oxygen by a substrate When an organic substrate is reduced, the reagent being oxidized is called the reducing agent. A common reagent for reducing aldehydes and ketones to alcohols is sodium borohydride. Normally this is carried out in a protic solvent such as water, methanol, or ethanol. An equivalent of hydride (H:) is provided to the substrate as visualized below: H-O-R OH H-B-H н In this experiment you will study the reduction of a diketone, benzil, by NaBH4. Before beginning the experiment you need to look up the structure and melting point of benzil as well as hydrobenzoin, the product. Because hydrobenzoin has two chiral carbons, various stereoisomers exist, and the Fischer projections of all of these compounds should be drawn in the table on the next page. The identity of the correct diastereomer of hydrobenzoin that results can be determined from its melting point and verified by its Re value on a silica gel TLC plate. 99

Answer 1

Benzil OH K NaBH4 NaBH4 HO I Ph benzoin HSOH NaBH4 H OH * HOS ROH Ph HOS ROH NaBH4/ benzoin NaBH4 NaBH4 HO POH 1 HOS SOH L ROH HH Ph DI > meso-hydrobenzoin HOH . OH racemic-hydrobenzoin Ph HS-OH HOSH Ph (R, R)-hydrobenzoin on (S,S)-hydrobenzoin

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