Right-handedness

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 ... 

Don t confuse conformation with configuration. Students confuse these terms all of the time. Conformations are the different positions that a molecule can twist into, but configuration is a matter of right-handedness or left-handedness (R or S). 

Fig. 21 Spiral structiu-e formed by seven beam interference central beam is right-handed circularly polarized, and the side beams linearly [32,33]. Simulated by interference of plane waves according to Eq. 2 with side beams comprising an 80° angle with the optical axis (the -field of the central beam is Eo = o/V2(l, i,0), where + corresponds to the right-handedness i = V )...

Chirality due to a helical shape.52 Several compounds have been prepared that are chiral because they have a shape that is actually helical and can therefore be left- or right-handed in orientation. The entire molecule is usually less than one full turn of the helix, but this does not alter the possibility of left- and right-handedness. An example is hexahelicene,53 in which one side of the molecule must lie above the other because of... 

Molecules of inheient structural asymmetry aie anisotropic they are optically active and exhibit optical rotation in solution. The typical optically active center is a carbon atom with four different substituents. In addition, any structural dissymmetry that results in a spatial left- and right-handedness will cause optical activity. Compounds of these types of come in a right-hand l R) and left-hand (L) form. When equal amounts of these two forms are mixed (racemic mixtures) there is no optical rotation because the activity of the two forms exactly cancel. Internal compensation of optically active centers m complex molecules is also found. Left- and right-handed optical isomers were first studied by Pasteur well over 100 years ago. and extensive surveys are found in most organic chemical texts. 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

So far we have considered only carbon compounds in which single bonds link the central carbon atom to four different atoms or radicals. Van t Hoff s tetrahedrons also work for double bonds between carbon atoms, when two tetrahedrons have an edge in common (see figs. IX and X, p. 145), and for triple bonds, when two tetrahedrons have a triangular face in common (see fig. XI, p. 14 ). Molecules containing such bonds can still be optically active if they possess the necessary overall asymmetry in the form of left- or right-handedness. 

The dominance of right-handedness over left-handedness occurs in all races and cultures. Despite this fact, even identical twins can exhibit differences in hand preference. Pictured are Matthew (right-handed) and Zachary (left-handed), identical twin sons of the author. 

The molecular basis for the left- and right-handedness of distinct crystals of the same chemical substance and the associated differences in optical rotation was developed from the hypothesis of Paterno (1869) and Kekule that the geometry about a carbon atom bound to four ligands is tetrahedral. Based on the concept of tetrahedral geometry, Van t Hoff and LeBel concluded that when four different groups or atoms are bound to a carbon atom, two distinct tetrahedral molecular forms are possible, and these bear a non-superimposable mirror-image relationship to one another (Fig. 3). This hypothesis provided the link between three-dimensional molecular structure and optical activity, and as such represents the foundation of stereoisomerism and stereochemistry. 

It is possible to imagine a universe where space itself has an attribute of left-handedness or right-handedness, or where space does not but materials do. But if we set these possibilities aside and use ordinary ideas about symmetry, it follows that at any point where stresses exist inside a continuum, there are three orthogonal planes across which the tangential stress is zero these planes suffer only normal stresses. The planes themselves are principal planes, their normals are the three principal directions at the point and the normal-stress magnitudes are the principal stress magnitudes. (See Figure 6.1.)... 

A and A prefixes enantiomers of octahedral complexes containing three equivalent didentate ligands (tris-chelate complexes) are among those which are distinguished using A (delta) and A (lambda) prefixes. The octahedron is viewed down a three-fold axis, and the chelates then define either a right- or left-handed helix. The enantiomer with right-handedness is labelled A, and that with left-handedness is A. 

Alpatov, V. V. (1957) Left- and Right- handedness in Structure of Organisms of the Plants and Animals. Bulletin of MOIP, depart, biol., vol. (62) 5 [in Russ]. 

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