The Mosher Method of Determining Enantiomeric Ratios: A Microscale NMR Experiment for Organic Chemistry

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 9 Issue 6 (2004) pp 359-363

The Mosher Method of Determining Enantiomeric Ratios: A Microscale NMR Experiment for Organic Chemistry

Steve Lee

School of Science and Mathematics, Roosevelt University, Chicago and Schaumburg, IL, splee@roosevelt.edu
Received August 4, 2004. Accepted September 16, 2004.

Published online: 16 November 2004

Abstract. Mosher’s established technique of measuring the enantiomeric ratio of a chiral compound has been adapted to be suitable for an undergraduate organic chemistry experiment. In this technique a chiral alcohol or amine reacts with the Mosher acyl chloride, a-methoxy-a-(trifluoromethyl) phenylacetyl chloride (MTPA-Cl), to yield the corresponding diastereomeric ester or amide. The ester or amide products are analyzed by ¹H and/or ¹⁹F NMR spectroscopy, and signals for diastereotopic groups on the products are integrated to determine the ratio of the diastereomeric products and consequently the ratio of enantiomeric substrates. This experiment has the benefits of helping students (1) apply advanced topics in stereochemistry, such as stereoheterotopic ligands and kinetic resolution; (2) learn a quantitative application of NMR spectroscopy, including ¹⁹F NMR; and (3) be introduced to microscale techniques. Furthermore, even though this is a microscale NMR technique, it can be conducted on a low-field (60- or 90-MHz) NMR spectrometer with satisfactory results.

Key Words: Laboratories and Demonstrations; organic chemistry; synthesis

(*) Corresponding author. (E-mail: splee@roosevelt.edu)

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Supporting Materials:

A student handout, assignment, and experimental notes for the instructor are available in a Zip file (193 KB).