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.

thioacetal with H-atoms in equivalent environments