Mirror-image molecules

Figure 9.1 Tetrahedral carbon atoms and their mirror images. Molecules of the type CH3X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not. A CHXYZ CH3X X l X hK vh H ...

To appreciate the difference between the two cis isomers, look carefully at the complex and its mirror image (or better, build models). No matter how we twist and turn the molecules, we cannot superimpose the mirror image molecule on the original molecule. It is like trying to superimpose your right hand on your left hand. The two molecules are, in fact, two distinct compounds. Optical isomers can also occur whenever four different groups form a tetrahedral complex, but not if they form a square-planar complex. 

For example, (7) is trans-2-methylcyclohexanol in addition the chirality of C-l and C-2 should be designated by the R/S) notation, and the correct systematic name is therefore (lR,2R)-2-methylcyclohexanol (7a), since the mirror image molecule, (lS,2S)-2-methylcyclohexanol (7b) is also the trans isomer. The cis isomer is also chiral, and diastereoisomeric with the trans isomer the two enantiomers would be (lS,2R)-2-methylcyclohexanol (24a) and ( R,2S)-2-methylcyclohexanol (24b). 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

Since the enantiomeric resolution of tartaric acid in 1848 by Pasteur, a longstanding issue is whether mirror-image molecules are energetically identical with respect to the origin of biomolecular homochirality [100]. In 1860 Pasteur first conjectured that the homochirality may come from certain intrinsically handed force existing in the Universe [1]. In 1898, Kipping and Pope reported experimental results in relation to NaC103 with l- and d-... 

Since the historical PV weak force origin /3-decay experiment of 60Co [ 106], theoreticians presumed that the tiny parity violating WNC at molecular and subatomic levels may also allow a distinction between mirror image molecules at the macroscopic level as well. This is because PV-WNC at the molecular level may be a candidate for the homochiral scenario under terrestrial and extraterrestrial conditions [1,2,104,109-118]. The WNC, however, did not induce any observable PV effects between enantiomers in their ground states because of the minuscule PV energy difference (PVED) of 10 19 eV and/or negligibly small 10 - % ee in racemates. Theoreticians also proposed several possible amplification mechanisms at reproducible detection levels within laboratory time scales and at terrestrial locations [113,117,118]. 

Fig. 3 Orange and lemon odor in mirror image molecules R-(+)- and S-(-)-limonene, respectively...

The mirror image of trans- 1,2-dichlorocyclopentane is different from (nonsuper-imposable with) the original molecule. These are two different compounds, and we should expect to discover two mirror-image isomers of trans- 1,2-dichlorocyclopentane. Make models of these isomers to convince yourself that they are different no matter how you twist and turn them. Nonsuperimposable mirror-image molecules are called enantiomers. A chiral compound always has an enantiomer (a nonsuperimposable mirror image). An achiral compound always has a mirror image that is the same as the original molecule. Let s review the definitions of these words. 

Mirror-image molecules have nearly identical physical properties. Compare the following properties of (K)-2-bromobutane and (S)-2-bromobutane. 

A molecule whose geometrical structure is not identical to its mirror image possesses chirality. For example, enantiomers are mirror-image structures of a chiral molecule. Two mirror-image molecules are identified as l- and D-enantiomers. Amino acids and deoxyribose in DNA are chiral molecules. Asy mmetry in biochemistry requires the constant catalytic production of the preferred enantiomer in the reactions between enantiomers, a process known as racemization. In systems with appropriate chiral autocatalysis, instability may appear. Due to random fluctuations, the instability occurs accompanying the bifurcation of asymmetric states in which one enantiomer dominates. These states of broken symmetry can be observed in the following simple model reaction scheme with chiral autocatalysis (Kondepudi and Prigogine, 1999)... 

Schmidt, Karen F. Mirror-Image Molecules. Science News 143 (May 29,1993) 348-350. 

Mirror-image molecules that are not superimposable are called enam tiomers (Greek enantio, opposite ). Enantiomers are related to each oth as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (-H, -OH, -CH , -COOK) bonded to the central carbon atom. The enantiomers are called ( + )-Iactac acid and (—)-lactic acid. 

As discussed in Section 17.3, the analogy can be made between these mirror-image molecules and your left and right hands. Your hands are, indeed, mirror images of one another you cannot draw a plane of symmetry through your hand, nor can you superimpose your left and right hands on one another. 

We know from the standard model of elementary particle physics [116] that there is a tiny weak interaction contribution to every Coulomb interaction. For ordinary matter, where particle interconversion can be ignored, weak interactions due to exchange of neutral Z vector bosons are involved. Unlike the Coulomb interaction, the (neutral and charged variants of) weak interactions do not conserve parity. This leads, in consequence, to a very small energy difference between mirror-image molecules (enantiomers), which in turn might prove to be of importance for the development of a homochiral biochemistry on our planet [117]. 

R. A. Hegstrom, D. W. Rein, P. G. H. Sandars, Calculation of the parity nonconserving energy difference between mirror-image molecules, J. Chem. Phys. 73 (1980) 2329-2341. 

The two nonsuperimposable mirror image molecules are called an enantiomeric pair and each is the enantiomer of the other. The separated enantiomers have identical properties with respect to achiral environments. They have the same solubility, physical, and spectroscopic properties and the same chemical reactivity toward achiral reagents. However, they have different properties in chiral environments. The enantiomers react at different rates toward chiral reagents and respond differently to chiral catalysts. Usually enantiomers cause differing physiological responses, since biological receptors are chiral. For example, the odor of the R- (spearmint oil) and S- (caraway seed oil) enantiomers of carvone are quite different. 

Nonsuperimposable mirror-image molecules are called enantiomers (from the Greek enantion, which means opposite ). The two stereoisomers of 2-bromobutane are enantiomers. A molecule that has a nonsuperimposable mirror image, like an object that has a nonsuperimposable mirror image, is chiral. Each of the enantiomers is chiral. A molecule that has a superimposable mirror image, like an object that has a superimposable mirror image, is achiral. To see that the achiral moleule is superimposable on its mirror image (i.e., they are identical molecules), mentally rotate the achiral molecule clockwise. Notice that chirality is a property of the entire molecule. 

A mixture of equal amounts of two enantiomers—such as (/ )-(—)-lactic acid and (6 j-(+)-lactic acid—is called a racemic mixture or a racemate. Racemic mixtures do not rotate the plane of polarized light. They are optically inactive because for every molecule in a racemic mixture that rotates the plane of polarization in one direction, there is a mirror-image molecule that rotates the plane in the opposite direction. As a result, the light emerges from a racemic mixture with its plane of polarization unchanged. The symbol ( ) is used to specify a racemic mixture. Thus, ( )-2-bromobutane indicates a mixture of (-l-)-2-bromobutane and an equal amount of (-)-2-bromobutane. 

A chiral molecule has a nonsuperimposable mirror image. An achiral molecule has a superimposable mirror image. The feature that is most often die cause of chirality is an asymmetric carbon. An asymmetric carbon is a carbon bonded to four different atoms or groups. An asymmetric carbon is also known as a chirality center. Nitrogen and phosphorus atoms can also be chirality centers. Nonsuperimposable mirror-image molecules are called enantiomers. Diastereomers are stereoisomers that are not enantiomers. Enantiomers have identical physical and chemical properties diastereomers have different physical and chemical properties. An achiral reagent reacts identically with both enantiomers a chiral reagent reacts differently with each enantiomer. A mixture of equal amounts of two enantiomers is called a racemic mixture. 

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