Achiral molecules, definition

The equilibrium ensemble of the configurations of meso-tartaric acid 10 a—c has umin = 0. From the definition of umm follows immediately that for all achiral molecules min = 0 holds. This applies also to those cases where asymmetric C-atoms are present but cancel each other because of overall molecular symmetry. 

Definition An ensemble of molecules is chemically achiral under given observation conditions if and only if it is composed of racemic pairs of enantiomers and achiral molecules. 

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, a molecule is achiral if it is left invariant by some improper operation (reflection or rotary reflection) of the point group of the skeleton. Writing the permutation s corresponding to a given improper operation in cyclic form,... 

In retrospect, it seems unfortunate that in 1971 Morrison and Mosher8 generalized the definition, while keeping the term, an asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereoisomeric products arc produced in unequal amounts ( Footnote The substrate molecule must have either enantiotopic or diastereotopic groups or faces) . Obviously the phrase "an achiral unit in an ensemble of substrate molecules is too inexact and requires a great deal of additional explanation, which was partially given by the footnote (note that molecule, i.e., singular, was used ). Currently, the Morrison-Mosher term appears to be equivalent to stereoselective reaction. Unfortunately, this term was only defined in the modem sense by Izumi in 1971, i.e., in the same year the Morrison-Mosher definition was published. 

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... 

In these definitions, object refers to any rigid array, such as a set of points, or to a geometrical figure, or to a model of a molecule ideally realized (of which more below) properly congruent can be replaced by superposable and homomorphs, which may be either chiral or achiral, are congruent counterparts in Kant s terminology. 

The immediate question is are selected nuclei in a molecule chemical shift equivalent, or are they not If they are, they are placed in the same set. The answer can be framed as succinctly as the question Nuclei are chemical shift equivalent if they are interchangeable through any symmetry operation or by a rapid process. This broad definition assumes an achiral environment (solvent or reagent) in the NMR experiment the common solvents are achiral (Section 3.17). 

Note that according to the foregoing definition, chirality occurs only in molecules that do not have a rotation/reflection axis. However, if the molecule has only ( ) an axis of rotation, it is chiral. For example, both trans-1,2-dibromocyclohexane (D in Figure 3.3) and the dibromosuccinic acid E have a two-fold axis of rotation (C2) as the only symmetry element. In spite of that, these compounds are chiral because the presence of an axis of rotation, in contrast to the presence of a rotation/reflection axis, is not a criterion for achirality. 

The mirror image of trans- 1,2-dichlorocyclopentane is different from (nonsuper-imposable with) the original molecule. These are two different compounds, and we should expect to discover two mirror-image isomers of trans- 1,2-dichlorocyclopentane. Make models of these isomers to convince yourself that they are different no matter how you twist and turn them. Nonsuperimposable mirror-image molecules are called enantiomers. A chiral compound always has an enantiomer (a nonsuperimposable mirror image). An achiral compound always has a mirror image that is the same as the original molecule. Let s review the definitions of these words. 

Ring inversion (when strictly defined) of achiral conformations is nothing more than a pseudorotation. Ring inversion in the cyclohexane chair, for example, leaves the molecule apparently rotated by 60° along the C3 axis. Nevertheless, in order to conform with common usage, we will exclude ring inversion from the definition of ring pseudorotation. 

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... 

Another case that falls within the scope of the definition is the generation of an excess of one configuration at a new chiral center in the reaction between a chiral molecule and an achiral reagent. The control of the stereochemistry of addition of Grignard reagents to carbonyl centers adjacent to a chiral carbon is exemplary. In... 

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