Enantiomer, enantiomorph

Optical isomers, enantiomorphs or enantiomers, as they are also known, are pairs of molecules... 

If a molecule is nonsuperimposable on its miixor image, the mirror image must be a different molecule, since superimposability is the same as identity. In each case of optical activity of a pure compound there are two and only two isomers, called enantiomers (sometimes enantiomorphs), which differ in structure only in the left-and right-handedness of their orientations (Fig. 4.1). Enantiomers have identical physical and chemical properties except in two important respects ... 

A chiral object and the opposite object formed by inversion form a pair of enan-tiomorphs. If an enantiomorph is a molecular entity, it is called an enantiomer. An equimolar mixture of enantiomers is a racemate. 

Crystals composed of the R and S enantiomers of the same racemic mixture must be related by mirror symmetry in terms of both their internal structure and external shape. Enantiomorphous crystals may be sorted visually only if the crystals develop recognizable hemihedral faces. [Opposite (hid) and (hkl) crystal faces are hemihedral if their surface structures are not related to each other by symmetry other than translation, in which case the crystal structure is polar along a vector joining the two faces. Under such circumstances the hemihedral (hkl) and (hkl) faces may not be morphologically equivalent.] It is well known that Pasteur s discovery of enantiomorphism through die asymmetric shape of die crystals of racemic sodium ammonium tartrate was due in part to a confluence of favorable circumstances. In the cold climate of Paris, Pasteur obtained crystals in the form of conglomerates. These crystals were large and exhibited easily seen hemihedral faces. In contrast, at temperatures above 27°C sodium ammonium tartrate forms a racemic compound. 

A solution or melt of a racemic mixture of enantiomers may crystallize either as a racemic phase or as a mixture of two resolved enantiomorphic phases. The molecules in these two enantiomorphic phases will be exact mirror images of one another. However, a given enantiomer, say R, will have different environments in the racemate and in the resolved crystal and will be conformationally different. Correspondingly, the R molecule in the resolved crystal and the S molecule in the racemate will not be exact mirror images. 

As illustrated for compounds 77 and 78 in Scheme 18, different methods were applied for the syntheses of 77-79 (79 was obtained analogously to 78 according to method a). The racemic products 77a 0.7CH3CN, 78 CH3CN, and 79 were isolated as crystalline solids. In addition, crystals of the racemic compound 77b (an isomer of 77a) were obtained. For the solvent-free compound 78 formation of enantiomorphic crystals was observed. The crystals studied by X-ray diffraction contained (just by accident) the (A)-enantiomer. 

In order to stress the distinction between actual molecules and molecules represented as models, the terms enantiomer/c and diastereower/c are only applied to the former, for the latter the correct terms are enantiomorphic and diastereomorphia5. The Morphic nomenclature is also applied when parts of a molecule, e.g.. isolated groups or fragments, are discussed6. 

Note that these are specified by Ul and Lk, respectively, i.e., begin with a capital letter (see Section 1.1.2.1.). Analogous models with three-dimensionally enantiomorphic groups F,F are known as geometric enantiomers (see Section 1.1.3.3.). Given the modern use of fundamental terms (see Section 1.1.2.1.), this designation appears inappropriate today. 

If a box full of Dreiding models of the enantiomers of alanine had to be sorted into the two types, this simple problem could be solved by comparing models pairwise as to whether they are superposable or enantiomorphic. Lord Kelvin3 introduced terms for this type of relationship and described it thus ... 

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. 

It is called an a-amino acid because the amino group is attached to the a (or number 2) carbon atom. To indicate its three-dimensional structure on a flat piece of paper, the bonds that project out of the plane of the paper and up toward the reader are often drawn as elongated triangles, while bonds that lie behind the plane of the paper are shown as dashed lines. The isomer of alanine having the configuration about the a-carbon atom shown in the following structural formulas is called S-alanine or L-alanine. The isomer which is a mirror image of S-alanine is R-alanine or D-alanine. Pairs of R and S compounds (see Section B for definitions) are known as enantiomorphic forms or enantiomers. 

It should be recalled that whereas the enantiomers in the mixture (or racemate) (1) have identical physical properties (except for their action on the plane of polarised light), the diastereoisomers (2) and (3) have physical properties (e.g. solubility, boiling points, chromatographic behaviour, etc.) which are frequently significantly different. Resolution of the mixture (or racemate) can then be achieved provided that one of the diastereoisomers may be obtained in a pure state, and that regeneration from it of the pure enantiomorphous form is not accompanied by any degree of racemisation. 

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