Chiral molecules, definition

In a chiral class, which by definition contains chiral molecules with exclusively achiral ligands, all these chiral molecules are -chiral. In an achiral class the chirality is only due to jS -chirality. Therefore, we have for any class... 

By definition, topologically chiral molecules are those whose enantiomers cannot be interconverted by continuous deformation and therefore racemization is totally excluded as long as no bond in their organic backbone is broken. In addition, the combination of this latter topological property with the high thermodynamic stability of copper(I) 2,9-diphenylphenanthroline complexes provides us with potential catalysts for enantioselective processes. 

Although the term prochirality is frequently used, especially by biochemists, it suffers from a limitation which arises from a corresponding limitation in the definition of chirality. Molecules may display purely stereochemical differences without being chiral cis-tram isomers of olefins and certain achiral cis-trans isomers of cyclanes are examples. Thus (Fig. 2) (Z)- and ( )-1,2-dichloroethylene (4, 5) are achiral diastereomers, as are cis- and /rtww-1,3-dibromocyclobutanes (6, 7) being devoid of chirality these compounds have no chiral centers (or other chiral elements). Thus it is inappropriate to associate stereoisomerism with the occurrence of chiral... 

Conformational polymorphism is the existence of different conformers of the same molecule in different polymorphic modifications, as represented in Fig. 5.1(a),(b). Additionally, Corradini has specified fhaf fhe conformers are nearly isoenergefic, and also includes as an example, racemic and optically active crysfals for the case of chiral molecules. While both the general definition of polymorphism and chemists general understanding of conformation are fraught with nuances and special complicating circumstances, we prefer the general definition of conformational polymorphism here, without Corradini s additional qualifications. Specific questionable or unusual cases can be dealt with as encountered. 

The synthesis of an organic molecule generally proceeds in a series of logically connected individual stages. First, obviously, is definition of the target. For the medicinal chemist this includes, in the case of a chiral molecule, a decision on whether to prepare the compound in racemic or enantiomerically homogeneous form. 

However, 9 is not chiral and one can accordingly amplify the definition above to state if a molecule possesses a plane of symmetry in any energetically available conformation, it cannot be chiral. This statement holds whatever the complexity of the molecule. However, it should be noted that chiral molecules can possess axes of symmetry, and such molecules are considered in Chapter 5. 

However, the real potential of enantioselective chromatography for the preparative separation of optical isomers was definitely established in 1973 by Hesse and Hagel who introduced fully acetylated cellulose (triacetylcellulose) as a new efficient chiral CSP [14]. They successfully achieved the preparative separation of the enantiomers of various chiral compounds. For many years, triacetylcellulose was practically the only chiral stationary phase available for preparative separations and it has been used for the chromatographic resolution of a broad variety of chiral molecules [1-3, 15, 16]. 

For heterogeneous enantioselective catalysts there are some more candidates on the borderline of the definition. For example, when a certain a.P-unsaturated ketone was hydrogenated over Pd/C in the presence of 0.5 equiv. ephedrine, up to 36% ee of the product was obtained. It is not clear whether the demanded amount of the chiral molecule indicates a necessity of the interaction with the substrate in solution, or just a weak adsorption constant of the chiral molecule. Thorey, C., Henin, F. and Muzart, J. (1995) Tetrahedron Asymmetry, 7, 975-5. 

For the purposes of this treatise, the definition of asymmetric synthesis is a modification of that proposed by Morrison and Mosher [1] and as such will be applied to stereospecific reactions in which a prochiral unit in either an achiral or a chiral molecule is converted, by utility of other reagents and/or a catalytic antibody, into a chiral unit in such a manner that the stereoisomeric products are produced in an unequal manner. As such, the considerable body of work devoted to antibody-catalysis of stereoselective reactions including chiral resolutions, isomerizations and rearrangements are considered to be beyond the scope of this discussion. For information regarding these specific topics and more general information regarding the catalytic antibody field the following papers... 

As you have been reminded of these definitions, you have also seen several important concepts related to chemical selectivity that will inform any discussion and debate around what is green. We can readily see, for example, that when working with chiral molecules we not only have to worry about reacting with a particular type of bond or functional group, but we also have to do it in such a way that only the bond of interest forms that preserves or creates the desired isomer. As any synthetic organic chemist knows, there remains a considerable amount of chiral chemistry that suffers from a lack of selectivity (stereo-, regio-, and enantioselectivity) and in many cases more than 50% of the starting material ends up as waste. 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

Whyte [2-37] extended the definition of chirality as follows Three-dimensional forms (point arrangements, structures, displacements, and other processes) which possess non-superposable mirror images are called chiral . A chiral process consists of successive states, all of which are chiral. The two main classes of chiral forms are screws and skews. Screws may be conical or cylindrical and are ordered with respect to a line. Examples of the latter are the left-handed and right-handed helices in Figure 2-50. The skews, on the other hand, are ordered around their center. Examples are chiral molecules having point-group symmetry. 

This statement of Pasteur is free of any ambiguity, it is fundamental. Its interpretation simply implies that in order to create dissymmetry it is therefore necessary to separate physically the object in question from its mirror image an object and image which, in the case of a chiral molecule, are by definition nonsuperimposable enantiomers and whose mutual association forms a racemate. Thus the goal of the chemist is not simply to record the geometric property of a molecular structure, but physically to obtain the possible enantiomers that result from this geometry. This clarification is necessary due to the... 

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