Bonds handedness

Figure 1.3 The "handedness" of amino acids Looking down the H-Ca bond from the hydrogen atom, the L-form has CO, R, and N substituents from Cq going In a clockwise direction. There is a mnemonic to remember this for the L-form fhe groups read CORN in clockwise direcfion.

Chiral (Section 9.2) Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, one right-handed and one left-handed. The most common cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents. 

Fig. 2 Differences between wurtzite (WZ) and zincblende (ZB) forms, (a, b) Handedness of the fourth interatomic bond along the right (R) for WZ and left (L) for ZB. (c, d) Eclipsed (WZ) and staggered (ZB) conformation of atoms. Reprinted with permission from [23], Copyright 1992 by the American Physical Society...

Nerol can also be used as a substrate. The stereochemical outcome is shown in Scheme 1, which indicates that the BINAP-Ru species differentiates the C(2) enantiofaces. The C(6)-C(7) double bonds are left intact. Thus, both R and S enantiomers are accessible by either variation of ailylic olefin geometry or choice of handedness of the catalysts. 

At first thought, it might be considered that the steric restrictions posed by the large fluorine atoms would cause conformational rigidity due to restricted bond rotations. Hsu, however, showed that the conformations of the TFE chains in the crystalline regions in Nafion are in fact dynamic in that they can undergo helix reversals that is, the handedness of the helix is easily reversed.These helix reversals are also seen in PTFE.2 o This disorder phenomenon causes considerable conformational entropy and is... 

In contrast, the constitutional isomers of butane have different physical properties. Thus, n-butaneboils at-0.5°C but Mobutane boils at-12 C. Although the composition of the two butanes is the same, their three-dimensional geometries are quite different as a result of different patterns of chemical bonding. Note, for example, that one of the carbon atoms of wobutane makes bonds to three other carbon atoms. No carbon atom in n-butane does. Thus, the chemistry of the two butanes differs in many ways, but not as a consequence of handedness. They are simply examples of different ways to put together the same set of structural elements. Think about a Lego set. 

Enantiomers are characterized as nonsuperimposable mirror images. Enantiomers are said to be chiral (note that some diastereomers may be chiral as well). In the context of the same bonding pattern or connectivity, which atoms are bonded to which, enantiomers have handedness and are related to each other as the right hand is related to the left hand. In the specific example we saw earlier, the carbon atom is linked to four different atoms. Such molecules have non-superimposable mirror images. Stereoisomerism occurs in some molecules that do not have such a carbon atom but these cases are more exotic than we need to worry about here. Stereoisomers frequently have different, and sometimes strikingly different, biological properties, exemplified by the thalidomide case. 

This hydrogen-bond-driven large-scale structural organization has parallels in abiotic systems as well. For example, 1 1 mixtures of ra -l,2-diaminocyclohexane and C2 symmetric 1,2-diols self-assemble into well-defined supramolecular structures which have been characterized by X-ray diffraction analysis. The structures, some of which are stable to sublimation, are helical, the handedness of the helices being determined by the handedness of the 1,2-diamine [222]. 

Diastereoisomers are relatively easy to distinguish because the various bonds in the cis and trans isomers point in different directions. Enantiomers, however, are stereoisomers that differ in a more subtle way Enantiomers are molecules or ions that are nonidentical mirror images of each other they differ because of their handedness. 

Figure 20 The crystal structure of pure diol 14 projected in the be plane, and showing an edge-on view of two corrugated hydrogen-bonded layers. Each layer in the compound is built from enantiomerically pure molecules, but adjacent layers have opposite handedness. Fluorine atoms are designated by vertical hatching.

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