Optical activity racemic forms

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Before 1940 optically active compounds could only be obtained in stereo-isomerically pure form by isolation from natural sources, by resolution of racemic mixtures, or by a few laboratory controlled enzymic reactions. Many of the chemical reactions described in this book lead to products which contain chiral centres, axes, or planes, but in which the isolated material is the optically inactive (racemic) form. This is a direct consequence of the fact that the reactants, reagents, or solvents are achiral or are themselves racemic. The following selection of reactions drawn from the text illustrate this statement they may be cross-referenced to the relevant discussion sections, namely (a) Section 5.4.1, p. 519, (b) Section 5.4.3, p.542, (c) Section 5.11.7, p.687, (d) Section 8.1.3, p. 1133, e) Section 5.2.4, p. 504 and (/) Section 5.4.2, p. 531. 

Neither the cis nor the trans epoxide is optically active when formed from the alkene. The cis epoxide is achiral it cannot be optically active. The trans epoxide is capable of optical activity but is formed as a racemic mixture because achiral starting materials are used. 

The reaction of optically active carbinolamines formed by an enzymatically controlled addition of acetaldehyde to amines, illustrated in Fig. 2, may be of theoretical interest, but lacks experimental verification it also would require the presence of acetaldehyde. The more likely pyruvic acid route to optically active TIQs, however, also remains inconclusive. If it indeed proceeds through TIQ-1-carboxylic acids to DIQ intermediates by an oxidative decarboxylation (176,217,218), it requires that it be followed by an asymmetric enzymatic reduction. Although achieved in vitro (35), this reaction has not been realized in vivo. The formation of unequal amounts of the optical isomers of salsolinol and other TIQs in vivo could arise from racemic 1-carboxy-TIQ in an enzymatic decarboxylation, proceeding with (S) and (R) enantiomers at a different rate and thus affording different amounts of (5)- and (/ )-TIQ. With the availability of optically active TIQ-1-carboxylic acids, this possibility can now be tested. 

Optical Resolution. This reagent is commonly used for the optical resolution of various compounds. Racemic spiro[4.4]nonane-1,6-dione was the first compound to be resolved (55% overall yield). The corresponding semioxamazone was obtained in optically active pure form in two or three recrystallizations and hydrolyzed to (—)-(5)-spiro[4.4]nonane-l,6-dione in a refluxing methanol-water mixture in the presence of Iodine (eq 2). 

As a consequence of the general lack of availability of simple chiral phosphoryl compounds, often the result of difficulties in synthesis which may be associated with problems of optical stability of enantiomeric forms [for example, ethyl ethylphosphonochloridate Et(EtO)P(0)Cl, which has been isolated in optically active enantiomeric forms, racemizes during storage], and also partly by the desire to conduct studies in which ligand reorganization (pseudorotation) in the reaction intermediate or transition state might be restricted, many studies on displacements at phosphorus have employed phosphorus esters and... 

A detailed investigation of the stereochemistry of cis- and tran5-[RuCl2(L-L)2] (l L = o-QH< (EMePh)2 E = As, P) has been undertaken. The optically active, racemic, meso, syn, and anti forms of the trans compound were isolated for both ligands each of these subsequently isomerized to the corresponding cis complex by reaction with AlEts and the various diastereoisomers of the latter separated and characterized. Whereas the trans isomers are relatively inert, the cis complexes readily undergo stereospecific halogen substitution by 1 and A minor product obtained... 

C5H7N3O4, Mr 173.13. Light yellow to greenish cryst., mp. 153-155°C (decomp.), [a]p +9.T (2 N HCI). A. is an antibiotic, isolated in optically active L-form, from Streptomyces species, the racemic form can be synthesized. As antagonist of glutamine it has mutagenic, bactericidal, fungicidal, and cytostatic activities. 

Different independent approaches were used to investigate the mechanism of polymerization of a-olefms with heterogeneous catalysts. As a result, it was shown that isotactic polymerization of racemic mixtures of a-olefms are stereoselective. Also, optically active polymers form with optically active catalysts.Stereoelectivity and stereoselectivity are due to the intrinsic asymmetry of the catalytic centers. This conclusion comes in part from knowledge that propylene coordinates... 

The sensitivity of this method was demonstrated by Jorda and Wilkes (1988) who showed, after aging, that blends of racemic polylactide and its optically active L-form were two phase, a convincing demonstration that two stereo-regular forms of a polymer may be immiscible. 

We saw in the previous sections that as long as an enol or enolate can be formed, hydrogens in the a position can be exchanged for deuterium through treatment with deuterated acid or base (Fig. 19.28). Treatment of a simple optically active aldehyde or ketone with acid or base results in loss of optical activity, racemization, as long as... 

Subcomponent substitution of a racemic mixture of isomers of 1 1 cage complex of 826 with encapsulated triflate anion in the presence of optically active primary amine 462 by Scheme 5.84 allowed to obtain an optically active AAAA form of this complex [84], The use of a racemic mixture of / ,S-isomers of the amine subcomponent afforded / ,S-imine capsules 828 but not its homochiral analogs. Therefore, chiral selfsorting is not observed in this system. 

Let us now discuss the aeetolysis of the enJo-isomer. This compound displays a 57% loss of optical activity that cannot be explained by flie reaction pafliway previously considered for the exo-epimer. In fact, the loss of optical activity in solvolysis reactions is more in agreement with the involvement of a classical carbo-cation intermediate. If we consider the transition state for flic departure of the OBs group (16 in Scheme 40.8) flic forming orbital at the C8 position is now poorly lined-up for participation of flic C3-C4 bond and a localized carbocation like 17 should be formed (at least at first instance). This intermediate could go directly to product 3 by an exo attack of the solvent (path a), or rearrange to form nonclassical cation 13 (path b). The rearrangement of 17 to 13 is very facile, as nonclassical cation 13 is 13.3 kcal/mol more stable than classical cation 17. Finally, the third alternative for cation 17 is to undergo isomerization to form its enantiomer, cation 18. The isomerization of 17 to 18 would account for flic loss of optical activity (racemization) of en fo-brosylate 2 during the solvolysis (Scheme 40.8). 

Borneol and isoboineol are respectively the endo and exo forms of the alcohol. Borneol can be prepared by reduction of camphor inactive borneol is also obtained by the acid hydration of pinene or camphene. Borneol has a smell like camphor. The m.p. of the optically active forms is 208-5 C but the racemic form has m.p. 210-5 C. Oxidized to camphor, dehydrated to camphene. 

The melting-points of the dextto and laevo forms of any optically active compound may, as in this case, be virtually identical with that of the racemic fomi in many compounds however there is a marked difference in melting-point, and often in solubility, between the (-)-) and ( -) forms on one hand and the ( ) form on the other. 

Acetophenone similarly gives an oxime, CHjCCgHjlCtNOH, of m.p. 59° owing to its lower m.p. and its greater solubility in most liquids, it is not as suitable as the phenylhydrazone for characterising the ketone. Its chief use is for the preparation of 1-phenyl-ethylamine, CHjCCgHslCHNHj, which can be readily obtained by the reduction of the oxime or by the Leuckart reaction (p. 223), and which can then be resolved by d-tartaric acid and /-malic acid into optically active forms. The optically active amine is frequently used in turn for the resolution of racemic acids. 

In this as m other reactions m which achiral reactants yield chiral products the product IS formed as a racemic mixture and is optically inactive Remember for a substance to be optically active not only must it be chiral but one enantiomer must be present m excess of the other... 

It IS a general principle that optically active products cannot be formed when opti cally inactive substrates react with optically inactive reagents This principle holds irre spective of whether the addition is syn or anti concerted or stepwise No matter how many steps are involved m a reaction if the reactants are achiral formation of one enan tiomer is just as likely as the other and a racemic mixture results... 

Section 7 9 A chemical reaction can convert an achiral substance to a chiral one If the product contains a single chirality center it is formed as a racemic mixture Optically active products can be formed from optically inactive... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

If the a carbon atom of an aldehyde or a ketone is a chnality center its stereo chemical integrity is lost on enolization Enolization of optically active sec butyl phenyl ketone leads to its racemization by way of the achiral enol form... 

The product is chiral but is formed as a racemic mixture because it anses from an achiral intermediate (the enol) it is therefore not optically active... 

The optical activity of malic acid changes with dilution (8). The naturally occurring, levorotatory acid shows a most peculiar behavior in this respect a 34% solution at 20°C is optically inactive. Dilution results in increasing levo rotation, whereas more concentrated solutions show dextro rotation. The effects of dilution are explained by the postulation that an additional form, the epoxide (3), occurs in solution and that the direction of rotation of the normal (open-chain) and epoxide forms is reversed (8). Synthetic (racemic) R,.9-ma1ic acid can be resolved into the two enantiomers by crystallisation of its cinchonine salts. 

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