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Unformatted text preview: Sodium borohydride are especially good at hydrogenating carbonyl groups. Benzoin has only one carbonyl group. The carbonyl group is sp 2 hybridized so it is planar. Sodium borohydride can attack either above or below the plane forming two different enantiomers. If the starting benzoin was enantiomer pure (R or S at the hydroxyl group), then reducing it with hydride will form two products. This is because a new stereocenter is formed at the second hydroxyl group. Benzil has two carbonyl groups. So it can form two stereocenters or up to four products, R-R, R- S, S-S, or S-R. The compounds with stereocenters of R-S or S-R are considered meso compounds because they have a plane of symmetry. These meso compounds have a melting point of about 137-139 C. The other compounds R-R and S-S have melting points of about ten degrees higher, 148-150 C. The melting range in the experiment for benzil and benzoin were degrees higher, 148-150 C....
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- Spring '08
- sodium borohydride, Phenyl