Page 145 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

configuration of the molecule and its absorption spectrum. In many cases, the ORD 125
curve can be used to determine the configuration of a molecule by comparison with
similar molecules of known configuration. Figure 2.2 shows the UV, ORD, and CD SECTION 2.1
spectra of an enantiomerically pure sulfonium ion salt. 3 Configuration
Chiral substances also show differential absorption of circularly polarized light.
This is called circular dichroism (CD) and is quantitatively expressed as the molecular
ellipticity
, where and are the extinction coefficients of left and right circularly
L
R
polarized light:

= 3330 − (2.5)
L
R
Molecular ellipticity is analogous to specific rotation in that two enantiomers have
exactly opposite values at every wavelength. Two enantiomers also show CD spectra
having opposite signs. A compound with several absorption bands may show both
positive and negative bands. Figure 2.3 illustrates the CD curves for both enantiomers
of 2-amino-1-phenyl-1-propanone. 4

Fig. 2.2. UV absorption, ORD, and CD curves of (R)-ethyl methyl p-tolyl sulfonium
tetrafluoroborate. Reproduced from J. Org. Chem., 41, 3099 (1976), by permission of the
American Chemical Society.
3 K. K. Andersen, R. L. Caret, and D. L. Ladd, J. Org. Chem., 41, 3096 (1976).
4
J.-P. Wolf and H. Pfander, Helv. Chim. Acta, 69, 1498 (1986).