The critical role of stereochemistry in life and industrial/pharmaceutical chemistry mandates that stereochemistry is well represented in undergraduate lab curricula. This has largely been achieved via experiments in the enantiomeric resolution of asymmetric acids and bases followed by polarimetric analysis, NMR analysis using chiral shift reagents, and NMR, GC, and HPLC analysis of diastereomeric derivatives of the chiral molecules.
To achieve the stereochemical lab requirement without sacrificing existing lab techniques, we have modified two common laboratory experiments to include chiral GC analysis herein described.
The extraction of carvone from spearmint leaves and caraway seed via steam distillation or other extraction methods is a lab widely used to demonstrate stereochemistry. However, the analysis of the products from the extraction is often limited to thin-layer chromatograms that confirm the presence of carvone and shows that the enantiomers have similar polarity. By including a GC method that uses beta DEX 225 column in this lab, students were able to compare retention times of the spearmint and caraway extracts with those in a racemic mixture, then predict whether they are R or S by comparing with an injection of a pure R-carvone sample.
The reduction of aldehydes and ketones is another common experiment done in most institutions. Sodium borohydride reduction of acetophenone produced a racemic mixture which was observed as two retention times on the chromatogram. Reduction using the two enantiomers of commercially available CBS catalysts provided relatively enantiopure alcohols. Students were able to predict which enantiomers resulted from each enantiomer of the CBS catalyst by comparing it with a commercially available enantiopure alcohol. They also used the chromatographic data to calculate the enantiomeric excess from their reactions.

Leigha Dillard

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