Experiment 10: Stereochemistry of a Carbonyl Reduction

Benzoin Reduction

Chemical Safety Information:

**Reagents and Solvents**
racemic benzoin	ethanol	sodium borohydride
hydrochloric acid	(R,R)-(+)-1,2-diphenyl-1,2-ethanediol	(S,S)-(-)-1,2-diphenyl-1,2-ethanediol
(R,S)-(-)-1,2-diphenyl-1,2-ethanediol	para-toluenesulfonic acid monohydrate	dichloromethane
2,2-dimethyoxypropane	hexane	Iodine
sodium bicarbonate	sodium chloride	magnesium sulfate
deuterated chloroform

Experimental Spectra:

Sample NMR hydrobenzoin and hydrobenzoin acetal (not available for submission for credit)

Stock IR Benzoin (for reference)

Stock IR 1,2-diphenyl-1,2-ethanediol (available for submission for credit, see laboratory manual for details)

Stock FID ¹H-NMR 1,2-diphenyl-1,2-ethanediol acetal (available for submission for credit, see laboratory manual for details)

Frequently Asked Questions:

Q1) What are enanatiomers, diastereomers, and meso compounds?

A1) In order to understand the stereochemical outcome of this experiment, it is critical to have the basic terminology correct. A short review of the issues is available.

Q2) How do I distinguish the possible products of this reaction by ¹H-NMR?

A2) One of the easiest ways to understand how the acetal products may differ in the ¹H-NMR spectra is to build a model of each and look at the relative positions of each ¹H-atom in each structure. If the molecular structure around the ¹H-atoms is the same, or can be conformationally exchanged, then those ¹H-atoms will have the same chemical shift and will be equivalent in ¹H-NMR spectroscopy.

A handout with an example thioacetal has been prepared showing how these issues work in an actual molecule.