Stereochemistry Part 1 Solutions: #4

4.* (1996 F 5) Vitamin A or retinol is an essential component for the vision receptors in the eyes. Its structure is shown below. Reaction of Vitamin A with H₂ over Pd/C produces a substance whose molecular formula is C₂₀H₄₀O. This substance appeared impure; for example it showed a wide melting range and several bands on a chromatographic column. Explain the nature of this new material and comment on its optical activity.

When Vitamin A is treated with H2 over a palladium/carbon (Pd/C) catalyst, the double bonds are reduced to single bonds. The resulting product is shown below, with the 4 chiral centers labeled:

Since Pd/C is achiral, one would expect a mixture of products with different stereochemistry. Specifically, one would expect 2n=24=16 stereoisomers, or 2n-1=23=8 pairs of enantiomers. Since diastereomers have different properties, you would expect a range of melting points and several bands on a chromatographic column. Now, while each stereoisomer is chiral, it is present in equal amounts as its enantiomer. Thus, you would have a racemic mixture of enantiomers, which does not display optical rotation.