Specific rotation enantiomers, chiral

This chapter will introduce the concepts of asymmetry and chirality as they apply to stereoisomers. Enantiomers are nonsuperimposable mirror images that are different compounds, identifiable only by differences in the physical property known as specific rotation. Enantiomers arise when a molecule has one or more atoms (including carbon) with different substituents (from different substituents for carbon). Such atoms are known as stereogenic (chiral) atoms (most of the examples in this book will deal with a stereogenic carbon atom). With more than one stereogenic center, another type of stereoisomer results known as a diastere-omer. All of these are types are stereoisomers, and a nomenclature system is in place to correlate the structure with what is known as absolute configuration. 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In the... 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

A carbon atom with four different groups attached is chiral. A chiral carbon rotates plane-polarized light, light whose waves are all in the same plane, and has an enantiomer (non-superimposable mirror image). Rotation, which may be either to the right (dextrorotatory) or to the left (levorotatory), leads to one optical isomer being d and the other being 1. Specific rotation (represented... 

Later, Pasteur 15) had arrived at the general stereochemical criterion for a chiral or dissymmetric molecular structure. Thus, the specific rotations of the two sets of sodium ammonium tartrate crystals in solution, isolated from the racemic mixture by hand-picking, were equal in magnitude and opposite in sign, from which Pasteur inferred that enantiomorphism of the dextro- and laevorotatory crystals is reproduced in the microscopic stereochemistry of the (+)- and (—)-tartaric acid molecules. The term dissymmetry or chirality is used when there is no superimposability between the two enantiomers, as seen in Sect. 2.1. 

Sensors for the detection of enantiomers are of great interest, as so far the on-line monitoring of production processes and medical diagnostics using standard chemical analytical methods is not possible. Quite often only one enantiomer of a chiral compound is actually a bioactive therapeutic. Therefore a proper analysis of the final product is essential. Currently, this involves separation techniques like liquid chromatography, GC and capillary electrophoresis, and determination of enantiomeric purity with circular dichro-ism and specific rotation. These are all off-line procedures and therefore no real-time analysis can be performed. Sensing devices for the distinction of different enantiomers would be a much cheaper, faster and easier-to-use alternative for this task, amenable to automation. 

In order to determine the Absolute Configuration (AC) of an enantiomer of a chiral molecule of defined specific rotation, its VCD spectrum is measured and compared with the predicted DFT/GIAO VCD spectra of the two enantiomers. Assuming that the... 

The essence of asymmetric synthesis is the creation of asymmetric centers under the influence of other asymmetric centers in such a way that the resulting enantiomers or diastereoisomers are formed in unequal proportions. Most reactions in asymmetric synthesis that have been described involve the conversion of trigonal carbon atoms into asymmetric, quadrivalent carbon atoms, and this article will be principally concerned with such reactions, although, in many instances, the principles involved may also be applied to asymmetric reactions in which, for example, chiral phosphorus or sulfur atoms are formed. In all reactions in which are formed mixtures of enantiomers having one enantiomer in preponderance, it is possible to describe the stereoselectivity of the reaction in terms of optical yield (optical purity, or enantiomeric yield). The precise significance of these terms has been described in detail elsewhere,1 but, practically, where at a selected wavelength, [a] is the specific rotation of the reaction product and [A] is the specific rotation of a pure enantiomer, the optical yield = [a]/[A]. Thus, the value of the optical yield is a measure of the excess of one enantiomer over the other. 

Reactions in which bonds to chiral centers are not broken can be used to get one more highly important kind of information the specific rotations of optically pure compounds. For example, the 2-methyl-1-butanol obtained from fusel oil (which happens to have specific rotation -5.756°) is optically pure—like most chiral compounds from biological sources—that is, it consists entirely of the one enantiomer, and contains none of its mirror image. When this material is treated with hydrogen chloride, the l-chloro-2-methylbutanc obtained is found to have specific rotation of 4-1.64°. Since no bond to the chiral center is broken, eveyy... 

Enantiomers and the Tetrahedral Carbon 307 The Reason for Handedness in Molecules Chirality Optical Activity 312 Specific Rotation 313... 

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