Macroscopic racemate

Ferroelectric Macroscopic Racemate from Unichiral Mesogen... 

This situation changed dramatically in 1996 with the discovery of strong electro-optic (EO) activity in smectics composed of bent-core, bowshaped, or banana-shaped achiral molecules.4 Since then, the banana-phases exhibited by such compounds have been shown to possess a rich supermolecular stereochemistry, with examples of both macroscopic racemates and conglomerates represented. Indeed, the chiral banana phases formed from achiral or racemic compounds represent the first known bulk fluid conglomerates, identified 150 years after the discovery of their organic crystalline counterparts by Pasteur. A brief introduction to LCs as supermolecular self-assemblies, and in particular SmC ferroelectric and SmCA antiferroelectric LCs, followed by a snapshot of the rapidly evolving banana-phase stereochemistry story, is presented here. 

EO switching. Since the phase is a macroscopic racemate and achiral, the EO behavior of the SmCsPA phase can reasonably be expected to be achiral. But, what of the chiral minority domains ... 

Apparently this switching mode is disfavored since, in fact, the chirality of the layers does not change upon switching to the ferroelectric state rather the layer interface clinicity changes. This occurs when the molecules in alternate layers simply precess about the tilt cone in a manner exactly analogous to antiferroelectric to ferroelectric switching in the chiral SmC phase. As shown in Figure 8.25, the ferroelectric state obtained from the ShiCsPa antiferroelectric phase is a ShiCaPf structure, an achiral macroscopic racemate with anticlinic layer interfaces. 

There are two macroscopic racemates and two conglomerates possible in this model, each conglomerate showing two enantiomers. This situation is quite analogous to the case of a molecule with three tetrahedral stereogenic... 

The monomeric unit is chiral, each individual polymer molecule consists of isochiral monomers, but the macroscopic sample is a racemate. Chains... 

A young Louis Pasteur observed that many salts of tartaric acid formed chiral crystals (which he knew was related to their ability to rotate the plane of polarization of plane-polarized light). He succeeded in solving the mystery of racemic acid when he found that the sodium ammonium salt of racemic acid could be crystallized to produce a crystal conglomerate. After physical separation of the macroscopic enantiomers with a dissecting needle, Pasteur... 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

Definitions It is important to define precisely the stereochemical terms that will be employed in this discussion. The term racemization has often been used loosely by chemists to describe any situation in which a mixture of enantiomers or diastereomers is produced as a result of an amide-bond-forming reaction, without regard to the ratio of stereoisomers formed. For the purposes of this discussion though, the term racemization will be used to describe the situation leading toward the formation of an exact 1 1 mixture of stereoisomers. Racemization, therefore, is a process that occurs to a collection of molecules, and can happen to a single residue or to one residue in a peptide sequence (Scheme 1). This is a macroscopic event, as the result is detected subsequent to the amide bond formation. 

As first realized by Meyer in 1974, when the molecules making up the C phase are non-racemic, the resulting chiral C phase can possess no reflection symmetry. Thus, the maximum possible symmetry of a C phase is C2, and the phase must possess polar order (21). One of the macroscopic manifestations of polar order can be a macroscopic electric dipole moment (the polarization P) associated with orientation of molecular dipoles along the polar axis. While the existence of polar order is not sufficient to assure an observable polarization (just as chirality does not assure optical activity), in fact many FLC materials do possess an observable P. 

We arrive at this conclusion from the lack of more than a slight atropisomeric excess (ca. 0.1% in all but one anomalous experiment) after equilibration of racemic BN in the cholesteric phases at several temperatures (TablelV). The lack of change in the ratio of atropisomers in the cholesteric phases is consistent with our observation that liquid-crystal induced circular dichroism spectra (67) of ISN in cholesteric mixture D are due to a macroscopic property of the solvent the LCICD spectra disappear when mixture D is heated to an isotropic temperature. 

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]. 

On a macroscopic scale, the adjustment of helical hand is an indisputable feature. Indeed, the very fact that different crystal structures (either con-formationally chiral or racemic) can be formed from the same initial melt (whether structured or not by, for example, spinodal decomposition) indicates that this selection of helical hand is operative. Even more demonstrative, the occurrence of mistakes in the adjustment of helical hands can have a profound impact on the growth process. For example, the formation of a sufficient patch of isochiral helices in the antichiral a phase of iPP may induce a growth transition to the chiral /3 phase. The rarity of such growth transi-... 

On the other hand, a-alkoxyorganolithiums are not configurationally stable on a macroscopic timescale when they are secondary and allylic or benzylic. For example, despite the known (see section 5.2.1) stereospecificity of the tin-lithium and lithium-tin exchanges of similar compounds, tin-lithium exchange of 110 with rc-BuLi/TMEDA at -78 °C gives an organolithium 111 which has completely racemised after 10 min stannylation returns racemic stannane 112.55 Similarly, 111 racemises rapidly at -70 °C in pentane/cyclohexane in the... 

No organolithium a to phosphorus has been shown unequivocally to be configurationally stable. The phosphonamide 279 is configurationally unstable on a macroscopic timescale,128 the phosphine oxide 280 gives racemic products on lithiation even in the presence of an internal quench,129 and in a Hoffmann test the phosphine oxide 281 gave the same ratio of diastereoisomers with either racemic or enantiomerically pure 6.129... 

The majority of syntheses of organic compounds is performed under macroscopic symmetrical conditions, thus optical activity is not resulted. The product is the racemate containing two mirror image isomers that can be separated by molecular chiral recognition. 

Consider a sample of racemic 2-butanol at room temperature, prepared by hydrogenation of 2-butanone under achiral conditions. In contrast to cij-1,2-difluorocyclohexane and ethylmethylpropylamine, molecules of 2-butanoI show no evidence of enantiomerization on the leisurely time scale that is associated with optical rotation measurements or with the separation of enantiomers. Because the probability is exactly one half that any macroscopic sample has an odd number of molecules, there is an even... 

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