Achiral compounds, definition

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

The third approach is the main topic of this volume. According to the definition given above it involves enantiomerically pure starting materials which at some point must be provided by resolution or ex-chiral-pool synthesis. It is more or less equivalent to the term asymmetric synthesis defined by Marckwald in 19047 as follows Asymmetric syntheses are those reactions which produce optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . In today s language, this would mean that asymmetric syntheses are those reactions, or sequences of reactions, which produce chiral nonracemic substances from achiral compounds with the intermediate use of chiral nonracemic materials, but excluding a separation operation. 

An interesting phenomenon whereby achiral compounds occupy chiral cavities has been reported. Steroidal host compounds give rise to the attachment of definite chiral conformations of achiral compounds within cavities, making it possible to observe solid-state circular dichroism spectra. Gdaniec and Polonski reported this type of property for the inclusion compounds of DCA and CA with various aromatic ketones [40a] and benzil [40c], Furthermore, it is possible for the selected conformers to maintain their chiral state temporarily in solution. That is, soon after the inclusion compounds are dissolved, the chirality may be retained for some time. /V-Nitrosopiperidines were found to display this type of dynamic chiral recognition in DCA and CA inclusion compounds [40b], In this case, one can observe the decay of the circular dichroism signal after dissolution of these inclusion compounds in methanol. 

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. 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

One source defines a meso compound as an achiral compound with stereocenters. Why is this a poor definition ... 

The original definition of meso is an achiral compound that has chiral diastereomers. Our working definition of meso is an achiral compound that has chirality centers (usually asymmetric carbon atoms). The working definition is much easier to apply, because we don t have to envision all possible chiral diastereomers of the compound. Still, the working definition is not quite as complete as the original definition. 

Asymmetric synthesis is a term first used in 1894 by E. Fischer and defined4 in 1904 by W. Markwald as a reaction which produces optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . A modem definition was proposed 5) by Morrison and Mosher 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 stereosiomeric products (enantiomeric or diastereomeric) are formed in unequal amounts. This is to say, an asymmetric synthesis is a process which converts a prochiral6) unit into a chiral unit so that unequal amounts of stereoisomeric products result . When a prochiral molecule... 

C is correct, Meso compounds are (by definition) achiral although they do contain chiral carbons. They are not optically active (do not rotate plane-polarized light). 

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... 

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. 

You need to ask which is the first chiral intermediate Can it be conveniently resolved Will the hirality survive subsequent steps The first intermediate is chiral but it enolizes very readily and the rnol is achiral so that s no good. The second compound is definitely chiral but it has three chiral entres and these are evidently not controlled. We should have to separate diastereoisomers before resolution and that would be a waste of time since all the diastereoisomers give the next mtermediate. 

Strictly, ICD encompasses all kinds of circular dichroism effects except for those of inherently chiral chromophores. This is so because most systems (molecules) may be more or less arbitrarily divided into one achiral chromophore in which the ICD is observed and a remaining component that includes the chirality. Thus, the CD exhibited by a chiral molecule may be considered ICD if the chromophoric entity itself is achiral. With this broader definition of ICD, molecules such as carbonyl compounds (in which the carbonyl chromophore is achiral) display an intramolecular ICD in the first allowed n —> 7t transition around 300 nm due to perturbation from a surrounding chiral (e.g. steroid) skeleton. Distinction between this type of molecule and one which is an inherently chiral chromophore could be difficult to make as the boundaries of the chromophore are often diffuse. For example, when a diene is in immediate proximity of a carbonyl, a composite chromophore is obtained, which, if skewed, is inherently chiral. Therefore, it is practical to restrict the definition of ICD to cases in which the achiral part is... 

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