Right- and Left-Handed Molecules

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

There is still another kind of isomerism that stems from the existence of right- and left-handed molecules. It is sometimes referred to as optical isomerism because the molecules that make up a pair of these isomers usually differ only in the way they rotate plane polarized light. 

A molecule is said to be chiral if it is not superimposable on its mirror image, like your right and left hands. Molecules that will exhibit chirality are required to have a stereogenic unit, a grouping within the molecule that is a focus of stereoisomerism at least one of these must be present in any enantiomer. For a molecule to be chiral, three conditions must be fulfilled. There must be no plane of symmetry in the molecule, no center of symmetry, and no low-energy path for the interconversion of the mirror images. At this point, we need a... 

This illustrates an important distinction in chemical enumeration that between the enumeration of "structural" isomers, in which only the connections between the atoms are considered, and that of stereoisomers, in which the situation of a molecule in space is important, so that as above we can have right- and left-hand forms of a molecule. This distinction will occur, for example, when a carbon atom is bonded to four distinct substituents (it can occur in many other ways). Such a carbon atom is said to be asymmetrical. 

Figure 9.10 (a) and (b) Example of a circular-difference effect from an isotropic Langmuir-Blodgett film composed of enantiomerically pure chiral molecules. Experimental data points are fitted to Eq. (42) (solid curve), (a) The -polarized second-harmonic signal, fit coefficients fs = 0.87 + 0.39z and hs = —0.38. (b) The / -polarized second-harmonic signal, fit coefficients fp = 0.63 — 0.20i, gp = 0.12 + 0.003z, and hp = 1.24. The right- and left-hand circular polarization are indicated by open and filled circles. 

It is to be noted that/(r) is normalized to unity. Due to discontinuity problem in the number of electrons [13] in atoms and molecules, the right- and left-hand side derivatives at a fixed number of electrons introduces the concepts of EF for nucleophilic and electrophilic attack, respectively. Introducing the finite difference approximation and the concept of atom condensed Fukui function (CFF) [14], the working equations are... 

As noted above, molecules that are not superimposable on their mirror images are said to be chiral. Nature makes important distinctions between right-handed and left-handed molecules. 

SCHEME 1. Access to optically active compounds Structure relationship between the chiral source and chiral products (flags refer to right- or left-handed molecules). 

Asymmetry Right-Handed and Left-Handed Molecules... 

In chemistry, chiral is a term used to describe asymmetric molecules Ibat arc tnirror-images of each other, i,e ihey are related to each other optically as right and left hands. Such molecules are also called enantiomers and arc characterized by optical activity, An excellent summary of chirality in chemistry is given by Prelog. Science, 193, 17-24 1197(h). 

However, it is possible to introduce two new factors into the problem of multilayer structure, either of which can influence structure in an interesting way. It is possible to replace the ordinary hydrocarbon chain by a perfluorinated chain and it is possible to dip over a subphase containing a trivalent cation. The perfluorinated carbon chain has a helical structure which produces a chain which appears to be approximately cylindrical. The handedness of a chain is determined randomly so that an equal number of right handed and left handed molecules exist in a given batch. 

Helices are chiral objects often encountered in nature, for example helical shells. The absolute configuration of these objects is designated P (plus) and M (minus) for right- and left-handed helices, respectively. A number of chemical structures resort to helicity, the most famous example being the class of molecules known as helicenes. Thus, the compound shown in diagram XXXVIII is (/>)-( + )-hexahelicene [33]. 

This kind of chiral crystallization of an achiral molecule always gives both right- and left-handed crystals. However, under ordinary crystallization conditions, enantiomorphous control is not possible. We have reported that a cocrystal is formed by the crystallization of a solution of a 1 1 mixture of 3-indolepropionic acid and phenanthridine in acetonitrile [28]. When the acetonitrile solution was divided into six parts and spontaneously evaporated in six containers, P-crystals appeared in four containers and M-crystals in two. If such chiral crystallization is carried out using a much larger number of flasks, the ratio of the formation of both enantiomeric crystals will approach 1 1. 

Right-handed and Left-handed Molecules. Every amino acid except glycine can exist in two isomeric forms. These two forms, called L (levo) and n (dextro) forms, are identical with one another except for the arrangement in space of the four groups attached to the a-carbon atom. The two molecules are mirror images of one another- one can be called the left-handed molecule, and the other the right-handed... 

This is a very puzzling fact. Nobody knows why it is that we are built of L-amino acid molecules, rather than of D-amino acid molecules. All the proteins that have been investigated, obtained from animals and from plants, from higher organisms and from very simple organisms—bacteria, molds, even viruses—are found to have been made of L-amino acids. Now right-handed molecules and left-handed molecules have exactly the same properties, so far as their interaction with ordinary substances is concerned—they differ in their properties only when they interact with other tight-handed or left-handed molecules. The... 

Chirality is concerned with objects and their mirror images. A chiral object is one that cannot be superimposed on its mirror image in such a way that the two appear identical. Right and left hands provide simple examples of chiral objects. Alan Bassindale wrote A left hand and a right hand are, in appearance, nonsuperimposable. A right hand viewed in a mirror appears to be a left hand. The analogy with hands is often carried through to molecules. Chiral molecules are often said to have a handedness. ... 

FIGURE 14.1. The importance of lack of mirror symmetry in defining a chiral object, (a) Right and left hands, and (b) two chiral molecules [B(ADEF)] are shown. One is the mirror image of the other, but they cannot be superimposed on each other. E and F, which are out of the plane of the page, cannot be superimposed on the mirror image of B(ADEF) by a rotation about A-B. F will lie on E, and E will lie on F after such a rotation. 

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