Page 144 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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CHAPTER 2
Stereochemistry,
Conformation,
and Stereoselectivity

Fig. 2.1. Alternative hydrogen-bonding and crystal-packing arrange-
ments for racemic (top) and − (bottom) forms of 2,5-
diazabicyclo[2.2.2]octane-3,6-dione. Reproduced from Tetrahedron
Lett., 35, 8157 (1994), by permission of Elsevier.

Alternatively, e.e. can be expressed in terms of the mole fraction of each enantiomer:

e e = Mole fraction major −Mole fraction minor ×100 (2.3)

The optical purity, an older term, is numerically identical. It represents the observed
rotation, relative to the rotation of the pure enantiomer. Since the two enantiomers
cancel each other out, the observed rotation is the product of % Major −% Minor ×
.If is known, measurement of allows the optical purity and enantiomeric

excess to be determined:
obs ×100
e e = (2.4)

There are several other ways of measuring e.e., including NMR spectroscopy,
chromatography, and capillary electrophoresis (see Topic 2.1).
Measurement of rotation as a function of wavelength is useful in structural
studies aimed at determining the configuration of a chiral molecule. This technique is
2
called optical rotatory dispersion (ORD), and the resulting plot of rotation against
wavelength is called an ORD curve. The shape of the ORD curve is determined by the

2
P. Crabbe, Top. Stereochem. 1, 93 (1967); C. Djerassi, Optical Rotatory Dispersion, McGraw-Hill, New
York, 1960; P. Crabbe, Optical Rotatory Dispersion and Circular Dichroism in Organic Chemistry,
Holden Day, San Francisco, 1965; E. Charney, The Molecular Basis of Optical Activity. Optical Rotatory
Dispersion and Circular Dichroism, Wiley, New York, 1979.