Optically inactive mixtures

The reaction of (—)-(65) ([a]3g5 — 0.95) with dimethylmercury, which does not occur in the dark, yields, in daylight, the corresponding optically inactive mixture of meso and racemic dl hexaorganoditins (55)... 

An optically inactive mixture of meso and racemic dl hexaorganoditins (59) is also obtained when (+)-methylneophyl-i-propyltrityltin, (+)-( ) ([oc]p° + 1.8) reacts with lithium aluminum hydride 44) ... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

The conversion of an optically active compound into an optically inactive mixture (dl mixture) is known as racemisation. The (+) and (-) forms of most compounds are capable of racemisation under the influence of heat, light or chemical reagents. Which agent can bring about racemisation, depends on the nature of the compound. For example (+) tartaric acid when heated strongly with water to 175° is transformed into a mixture of racemic and mesotartaric acids. 

The alcohol is chiral, but the carbocation is not. Thus, irrespective of which enantiomer of 2-phenyl-2-butanol is used, the same carbocation is formed. The carbocation reacts with ethanol to give an optically inactive mixture containing equal quantities of enantiomers (racemic). 

The two glyceraldehyde isomers of 4-13 are identical in all physical properties except that they rotate the plane of polarized light in opposite directions and form enantiomorphous crystals. When more than one asymmetric center is present in a low-molecular-weight species, however, stereoisomers are formed which are not mirror images of each other and which may differ in many physical properties. An example of a compound with two asymmetric carbons (a diastereomer) is tartaric acid, 4-16, which can exist in two optically active forms (d and L, mp 170 C), an optically inactive form (meso, mp 140 C), and as an optically inactive mixture (dl racemic, mp 206°C). 

Atropine is the optically inactive mixture of dextro- and laevo-hyoscyamine. Laevo-hyoscyamine alone occurs in nature. The hest source of hyoscyamine is a variety of henbane indigenous in Egypt, Soudan and India, known as Hyoscyamus muticus, in the various parts of which it has been shown to be present in the following proportions leaves 1.4% stems 0.6 % seeds 0.87-1.34%. Atropine is also manufactured from the root of Scopolia carniolica, in which hyoscyamine is present to the extent of 0.43-0.51% and from A ropa belladonna, the leaves of which contain, on the average, 0.4%, and the roots 0.5%, of hyoscyamine. Many other solanaceous plants of the Datura species contain these alkaloids, in varying, and smaller, amounts, often associated with hyoscine or scopolamine. 

The naturally occurring optically active molecules, (S)-alanine, and the laboratory synthesised optically inactive mixture (R, S)-... 

The 3-hydroxy-p end group is the most abundant chiral end group in carotenoids and is often called the zeaxanthin end group. Zeaxanthin (55) possesses two chiral centres at C(3) and C(3 ), making possible three optical isomers, namely the (3R,3 R)-isomer (most abundant in Nature) and the (3S,3 S)-isomer as well as the (3R,3 S)-isomer which constitutes a meso-form. It is this optically inactive mixture of isomers which is usually obtained by synthesis of the so-called racemate [50]. 

Acid production via microorganisms is important to the flavor of many fermented foods. The acid of greatest importance to the flavor of fermented dairy products is lactic acid [88]. Lactic acid is an optically active acid existing as the D, L, or optically inactive mixture, depending upon the microorganism involved in its synthesis. There appears to be no difference in flavor between the optical isomers. Lactic acid is characterized as being odorless and having a sour mUk taste. 

In the presence of base, (R)-a-methylbutyrophenone is deprotonated to form an achiral enolate, which can then be protonated from either face to form an optically inactive mixture of two enantiomers. 

Figure A2.5.30. Left-hand side Eight hypothetical phase diagrams (A through H) for ternary mixtures of d-and /-enantiomers with an optically inactive third component. Note the syimnetry about a line corresponding to a racemic mixture. Right-hand side Four T, x diagrams ((a) tlirough (d)) for pseudobinary mixtures of a racemic mixture of enantiomers with an optically inactive third component. Reproduced from [37] 1984 Phase Transitions and Critical Phenomena ed C Domb and J Lebowitz, vol 9, eh 2, Knobler C M and Scott R L Multicritical points in fluid mixtures. Experimental studies pp 213-14, (Copyright 1984) by pennission of the publisher Academic Press.

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

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... 

Structures A and A are nonsuperimposable mirror images of each other Thus although as 1 2 dichloro cyclohexane is chiral it is optically inactive when chair-chair interconversion occurs Such interconver Sion IS rapid at room temperature and converts opti cally active A to a racemic mixture of A and A Because A and A are enantiomers interconvertible by a conformational change they are sometimes re ferred to as conformational enantiomers... 

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

Section 7 4 Optical activity, or the degree to which a substance rotates the plane of polarized light is a physical property used to characterize chiral sub stances Enantiomers have equal and opposite optical rotations To be optically active a substance must be chiral and one enantiomer must be present m excess of the other A racemic mixture is optically inactive and contains equal quantities of enantiomers... 

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... 

Optically Inactive Chiral Compounds. Although chirality is a necessary prerequisite for optical activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers, no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic and is designated as ( ) and not as dl. Racemic mixtures usually have melting points higher than the melting point of either pure enantiomer. 

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive. 

Microorganisms and their enzymes have been used to functionalize nonactivated carbon atoms, to introduce centers of chirahty into optically inactive substrates, and to carry out optical resolutions of racemic mixtures (1,2,37—42). Their utifity results from the abiUty of the microbes to elaborate both constitutive and inducible enzymes that possess broad substrate specificities and also remarkable regio- and stereospecificities. 

Composition. Shellac is primarily a mixture of aUphatic polyhydroxy acids in the form of lactones and esters. It has an acid number of ca 70, a saponification number of ca 230, a hydroxyl number of ca 260, and an iodine number of ca 15. Its average molecular weight is ca 1000. Shellac is a complex mixture, but some of its constituents have been identified. Aleuritic acid, an optically inactive 9,10,16-trihydroxypalmitic acid, has been isolated by saponification. Related carboxyflc acids such as 16-hydroxy- and 9,10-dihydroxypalmitic acids, also have been identified after saponification. These acids may not be primary products of hydrolysis, but may have been produced by the treatment. Studies show that shellac contains carboxyflc acids with long methylene chains, unsaturated esters, probably an aliphatic aldehyde, a saturated aliphatic ester, a primary alcohol, and isolated or unconjugated double bonds. 

Papaverine was condensed with formaldehyde to methylenepapaverine (XXII), which on successive catalytic and electrolytic hydrogenation yielded two dZ-methyltetrahydropapaverines (XXIII), which on successive demethylation, condensation with formaldehyde and re-methylation yielded a mixture of bases, from which the two optically inactive corydalines (XXIV), jwesocorydaline, m.p. 163-4° (nac.), and dZ-corydaline, m.p. 132-3°, identical with the products obtained by the hydrogenation of dehydrocorydaline were isolated. For the conversion of corycavine to corydaline, see p. 304. 

---