Asymmetric molecule, enantiomorphic forms

Asymmetric carbon atom, description, 57 Asymmetric molecule, enantiomorphic forms, 57... 

Consider a methane molecule CH, and suppose that some or all of its hydrogen atoms are replaced by some other monovalent atom. If the atoms attached to the carbon are all different, that is, the carbon atom is asymmetric, the resulting molecule is chiral and exists in two so-called enantiomorphic forms mirror images of each other. (For further information on chirality see the interesting expository paper [PreV76]). 

An ideal approach to achieving chiral induction in a constrained medium such as zeolite would be to make use of a chiral medium. No zeolite that can accommodate organic molecules, currently exists in a stable chiral form. Though zeolite beta and titanosilicate ETS-10 have unstable chiral polymorphs, no pure enantiomorphous forms have been isolated. Although many other zeolites can, theoretically, exist in chiral forms (e.g., ZSM-5 and ZSM-11) none has been isolated in such a state. In the absence of readily available chiral zeolites, one is left with the choice of creating an asymmetric environment within zeolites by the adsorption of chiral organic molecules. 

Let us now differentiate between structures which are asymmetric and dissymmetric. The word asymmetric conveys the idea that the molecule is completely devoid of the elements of symmetry. Dissymmetric on the other hand means not completely devoid of elements of symmetry but possessing so few elements of symmetry that on the whole it will posses two structures which will be the mirror images of each other. Therefore to avoid confusion the term asymmetric is used to cover examples which rotate the plane polarized light. The two forms of an optically active compound are called enantiometers or enantiomorphs or optical antipodes. They are also said to have enantiomeric relationship to each other. 

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

The formation of an isotactic polymer requires that insertion always occur at the same prochiral face of the propylene molecule. Theoretically, both a chiral catalytic site (enantiomorphic site control) and the newly formed asymmetric center of the last monomeric unit in the growing polymer chain (chain end control) may... 

Chiral crystals, like any other asymmetric object, exist in two enantiomorphous equienergetic forms, but careful crystallization of the material can induce the entire ensemble of molecules to aggregate into one crystal, of one-handedness, presumably starting from a single nucleus (Figure 2). However, it is not uncommon to find both enantiomorphs present in a given batch of crystals from the same recrystallization. 

The coadsorption of chiral molecules into racemic layers is an efficient way to induce further asymmetrization towards single handedness. While in heterogeneous chiral catalysis the stationary ratio of modifier and reactant at the surface is assumed to be one, a small amount of chiral dopant can be sufficient for induction of homochirality on the entire surface SU on Cu( 110), for example, forms two enantiomorphous domains in its bisuccinate phase [27]. 

Indeed, the group of radicals R, R, R"", A when considered as material points differing among themselves form a structure which is enantiomorphous with its reflected image, and the residue, M, cannot re-establish the symmetry. In general then it may be stated that if a body is derived from the original type M A4 by the substitution of three different atoms or radicals for A, its molecules will be asymmetric, and it will have rotatory power. 

P2i2i2i Z = 8 D, 1.56 R = 0.058 for 1,884 intensities. The crystal structures of two monoclinic forms of this compound had previously been reported. - In the orthorhombic form here examined, there are two independent molecules in the asymmetrical unit. The glycosyl dispositions of these are syn (—134.2°, —132.2°). The D-ribosyl groups are 2 (162.8°, 42.3° 161.6°, 41.8°). The exocyclic, C-4 -C-5 bond torsion-angles are gauche (55.7°, 65.1 °). The syn-g combination results in intramolecular hydrogen-bonds (278.7, 288.8 pm) between the 0-5 atom and the N-3 atom. (Note The coordinates reported were for the wrong enantiomorph. To obtain the correct enantiomorph, all coordinates must be multiplied by — 1.)... 

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

Achiral molecules can also crystallize into enantiomorphic 3-D crystals such that, when the conformational deracemization is faster than the processes of crystallization, unequal amounts of the two enantiomorphous crystals will be formed. Such a crystallization event will, therefore, lead to a neat process of mirror-symmetry breaking. A topochemical asymmetric transformation in appropriate motifs of such single crystals should result in the formation of polymers of a single handedness. The pioneering study of such class of absolute asymmetric transformations, by Penzien and Schmidt [35], was the gas-sohd asymmetric bromination of the nonchiral p, pf -dimethyl-chalcone, which crystaUizes in an enantiomorphous crystal. Following this study, the details of over 20 such photoinitiated and thermally initiated reactions have been reported [36-42]. 

Chiral molecules, only left or only right, form chiral phases, left and right chiral molecules in equal amount form achiral (enantiomorphic) phases [6]. Consider a chiral molecule of a popular compound DOBAMBC (D(or L)-p-decyloxybenzyli-dene-p -amino-2methylbutyl cinnamate). It has an asymmetric carbon in its tail and form a chiral SmC phase in the range of 95-117°C, Fig. 3.5a. A molecule with a chiral tail looks like an ice-hockey stick and forms a helical liquid crystal phase. Feft and right forms of a chiral tail result in the left and right handedness of a molecule Fig. 3.6. On the other hand, chirality of cholesterol esters is exclusively due to a curvature of the molecular skeleton Fig. 3.5b. 

Pasteur ascribed the existence of this so-called optical isomerism to the formation of enantiomorphous molecules, that is to say, of molecules capable of existing in right- and left-handed forms, and related to each other as an object to its non-coincident image. His researches in this field of chemistry, carried out mostly between 1848 and 1858, led to the conclusion that organic molecules, like tangible objects, fall into two categories, designated by the terms symmetric and asymmetric (or non-symmetric). 

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