The fundamentals of structure and stereochemistry have been considered in previous chapters in some detail. There are, however, practical aspects of stereochemistry that have not yet been mentioned, particularly with regard to chiral compounds. How, for instance, can a racemic mixture be separated into its component enantiomers (resolution); what methods can be used to establish the configuration of enantiomers; how can we tell if they are pure; and how do we synthesize one of a pair of enantiomers preferentially (asymmetric synthesis)? In this chapter, some answers to these questions will be described briefly.
Optical activity is an associated phenomenon of chirality and has long been used to monitor the behavior of chiral compounds. Brief mention of this was made earlier (Section 5-1C), but now the origin and measurement of optical rotation will be examined in more detail.
- 19.1: Plane-Polarized Light and the Origin of Optical Rotation
- Electromagnetic radiation involves the propagation of both electric and magnetic forces. At each point in an ordinary light beam, there is a component electric field and a component magnetic field, which are perpendicular to each other and oscillate in all directions perpendicular to the direction in which the beam propagates. In plane-polarized light the component electric field oscillates as in ordinary light, except that the direction of oscillation is within a single plane.
- 19.2: Specific Rotation
- Optical rotation is the usual and most useful means of monitoring enantiomeric purity of chiral molecules. Therefore we need to know what variables influence the magnitude of optical rotation.
- 19.3: Separation or Resolution of Enantiomers
- Because the physical properties of enantiomers are identical, they seldom can be separated by simple physical methods, such as fractional crystallization or distillation. It is only under the influence of another chiral substance that enantiomers behave differently, and almost all methods of resolution of enantiomers are based upon this fact. We include here a discussion of the primary methods of resolution.
- 19.4: Enantiomeric Purity
- Enantiomeric purity (or optical purity) is defined as the fractional excess of one enantiomer over the other. This is expressed in terms of the moles (or weights) of the two enantiomers and is equal to the ratio of the observed optical rotation and to the optical rotation of either pure enantiomer. Thus a racemic mixture has an enantiomeric purity of zero. Any other enantiomeric composition in principle can be determined provided the mixture has a measurable rotation and the rotation of the pure
- 19.5: Absolute And Relative Configuration
- The sign of rotation of plane-polarized light by an enantiomer is not easily related to its configuration. This is true even for substances with very similar structures. Thus, given lactic acid with a specific rotation +3.82°, and methyl lactate with a specific rotation −8.25°, we cannot tell from the rotation alone whether the acid and ester have the same or a different arrangement of groups about the chiral center. Their relative configurations have to be obtained by other means.
- 19.6: The R,S Convention for Designating Stereochemical Configurations
- The R,S or Cahn-Ingold-Prelog convention is a systematic way of denoting configuration that may eventually replace the D,L system, at least for simple compounds. To denote the configuration of a chiral center by the R ,S convention, the groups at the center are assigned an order of precedence according to a specific set of rules based on atomic numbers.
- 19.7: E,Z Notation
- Many compounds cannot be described adequately by the cis-trans system. A system that is easy to use and which is based on the sequence rules already described for the R , S system is call the E,Z notation.
- 19.8: Prochirality
- There is a special term for molecules that are achiral but which can be converted to molecules with chiral centers by a single chemical substitution or addition reaction. They are said to be prochiral.
- 19.9: Optical Rotatory Dispersion and Circular Dichroism
- Nevertheless, much information has been obtained about structure, conformation, and configuration of organic compounds from measurements of optical rotation as a function of wavelength (i.e., optical rotatory dispersion). Like other phenomena involving interactions between electromagnetic radiation and organic molecules, as in infrared, ultraviolet, and NMR spectroscopy, optical rotatory dispersion curves often are quite sensitive to small changes in structure.
- 19.E: More on Stereochemistry (Exercises)
- These are the homework exercises to accompany Chapter 19 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
- 19.10: Asymmetric Synthesis
- If one could prepare 2-hydroxypropanenitrile from ethanal and hydrogen cyanide in the absence of any chiral reagent and produce an excess of one enantiomer over the other, this would constitute an absolute asymmetric synthesis - that is, creation of preferential chirality (optical activity) in a symmetrical environment from symmetrical reagents.
- 19.11: Racemization
- Optically active biphenyl derivatives are racemized if the two aromatic rings at any time pass through a coplanar configuration by rotation about the central bond. This can be brought about more or less easily by heat, unless the 2,2'-ortho substituents are very large. The way in that compounds with asymmetric carbon atoms are racemized is more complicated. One possibility would be for a tetrahedral chiral carbon attached to four groups to become planar and achiral without breaking any bonds.
Contributors and Attributions
- John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."