Racemic mixture


Pfeiffer effect The change in rotation of a solution of an optically active substance on the addition of a racemic mixture of an asymmetric compound.  [c.302]

Now consider such a symmetrical system, that of a racemic mixture of the enantiomers plus the inert third component. A pair of mirror-image conjugate phases will not physically separate or even become turbid, since they have exactly the same density and the same refractive index. Unless we find evidence to the contrary, we might conclude that this is a binary mixture with aT,x phase diagram like one of those on the right-hand side of figure A2.5.30. In particular any syimnetrical tliree-phase region will have to shrink symmetrically, so it may disappear at a tricritical point, as shown in two of the four pseudobinary diagrams. The dashed lines in these diagrams are two-phase critical points, and will show the properties of a second-order transition. Indeed, a feature of these diagrams is that with increasing temperature, a first-order transition ends at a tricritical point that is followed by a second-order transition line. (This is even more striking if the phase diagram is shown in field space as a /i, J or p, T diagram.)  [c.659]

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40].  [c.418]

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic  [c.77]

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  [c.288]

Rotation of the plane of polarized light m the clockwise sense is taken as positive (+) and rotation m the counterclockwise sense is taken as a negative (—) rotation Older terms for positive and negative rotations were dextrorotatory and levorotatory from the Latin prefixes dextro ( to the right ) and levo ( to the left ) respectively At one time the symbols d and / were used to distinguish between enantiomeric forms of a substance Thus the dextrorotatory enantiomer of 2 butanol was called d 2 butanol and the levorotatory form / 2 butanol a racemic mixture of the two was referred to as dl 2 butanol Current custom favors using algebraic signs instead as m (+) 2 butanol (—) 2 butanol and ( ) 2 butanol respectively  [c.288]

Because of the high degree of chiral recogni tion inherent in most biological processes (Section 7 8) It IS unlikely that both enantiomers of a chiral drug will exhibit the same level or even the same kind of effect At one extreme one enantiomer has the desired effect and the other exhibits no biologi cal activity at all In this case which is relatively rare the racemic form is simply a drug that is 50% pure and contains 50% inert ingredients Real cases are more complicated For example the S enantiomer IS responsible for the pain relieving properties of ibuprofen normally sold as a racemic mixture The 50% of racemic ibuprofen that is the R enantiomer is not completely wasted however because enzyme catalyzed reactions in our body convert much of it to active (S) ibuprofen  [c.296]

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  [c.297]

Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane Figures 7 13 and 7 14 depict the stereochemical relationships associated with anti addition of bromine to (E) and (Z) 2 butene respectively The trans alkene (E) 2 butene yields only meso 2 3 dibromobutane but the cis alkene (Z) 2 butene gives a racemic mixture of 2R 3R) and 2S 3S) 2 3 dibromobutane
The separation of a racemic mixture into its enantiomeric components is termed resolution The first resolution that of tartaric acid was carried out by Louis Pasteur m 1848 Tartaric acid IS a byproduct of wine making and is almost always found as its dextrorotatory 2R 3R stereoisomer shown here m a perspective drawing and m a Fischer projection  [c.310]

Pasteur s technique of separating enantiomers not only is laborious but requires that the crystals of the enantiomers be distinguishable This happens very rarely Conse quently alternative and more general approaches for resolving enantiomers have been developed Most are based on a strategy of temporarily converting the enantiomers of a racemic mixture to diastereomeric derivatives separating these diastereomers then regenerating the enantiomeric starting materials  [c.310]

FIGURE 7 15 The general procedure for resolving a chiral substance into its enantiomers Reac tion with a single enantiomer of a chiral resolving agent P(+) converts the racemic mixture of enantiomers C(+) and C(-) to a mixture of diastereomers C(+) P(+) and C(-) P(+) The mixture of diastereomers IS separated—by fractional crystallization for example A chemical reaction is then carried out to convert diastereomer C(+) P(+) to C(+) and the resolving agent P(+) Like wise diastereomer C(-) P(+) is converted to C(-) and P(+) C(+) has been separated from C(-) and the resolving agent P(+) can be recovered for further use  [c.311]

One approach called enzymatic resolution, involves treating a racemic mixture with an enzyme that catalyzes the reaction of only one of the enantiomers Some of the most commonly used ones are lipases and esterases enzymes that catalyze the hydrol ysis of esters In a typical procedure one enantiomer of the acetate ester of a racemic alcohol undergoes hydrolysis and the other is left unchanged when hydrolyzed m the presence of an esterase from hog liver  [c.312]

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  [c.316]

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  [c.316]

Section 7 14 Resolution is the separation of a racemic mixture into its enantiomers It IS normally carried out by converting the mixture of enantiomers to a mixture of diastereomers separating the diastereomers then regenerating the enantiomers  [c.317]

What alkene gives a racemic mixture of (2R 3S) and (2S 3R) 3 bromo 2 butanol on treat ment with Br2 in aqueous solution" Hint Make a molecular model of one of the enantiomeric 3 bromo 2 butanols arrange it in a conformation in which the Br and OH groups are anti to one another then disconnect them )  [c.325]

Which product is achiral Which one is formed as a racemic mixture  [c.622]

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers  [c.735]

Epoxidation of cis 2 butene gives meso 2 3 epoxybutane trans 2 butene gives a racemic mixture of (2R 3R) and (2S 3S) 2 3 epoxybutane  [c.1212]

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  [c.1233]

R (Section 4 1) Symbol for an alkyl group Racemic mixture (Section 7 4) Mixture containing equal quantities of enantiomers  [c.1292]

Resolution (Section 7 14) Separation of a racemic mixture into Its enantiomers  [c.1292]

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.  [c.47]

Enantiomers are perhaps the substrate type most difficult to distinguish. As is well known, they are stereochemical species that have exactiy the same stmcture except for their mirror image (chirahty) relationship (33) (see also Chiral separations). This causes a problem. On the other hand, chiral (enantiomer) recognition in complexation is one of the most important means by which receptor sites of biological systems such as in genes or enzymes act and regulate (2). From the principle point of view, recognition of a substrate enantiomer from racemic mixture (50 50 % mixture of enantiomers) requires an enantiomeric optically resolved receptor stmcture in order to make possible two diastereomeric receptor—substrate complexes allowing differentiation (Fig. 22a) (136).  [c.186]

Lactic acid is also the simplest hydroxy acid that is optically active. L (+)-Lactic acid [79-33-4] (1) occurs naturally ia blood and ia many fermentation products (7). The chemically produced lactic acid is a racemic mixture and some fermentations also produce the racemic mixture or an enantiomeric excess of D (—)-lactic acid [10326-41-7] (2) (8).  [c.511]

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. 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.
A recent estimate places the number of prescrip tion and over the counter drugs marketed throughout the world at about 2000 Approxi mately one third of these are either naturally occur ring substances themselves or are prepared by chemical modification of natural products Most of the drugs derived from natural sources are chiral and are almost always obtained as a single enantiomer rather than as a racemic mixture Not so with the over 500 chiral substances represented among the more than 1300 drugs that are the products of synthetic or game chemistry Until recently such substances were with few exceptions prepared sold and adminis tered as racemic mixtures even though the desired therapeutic activity resided in only one of the enan tiomers Spurred by a number of factors ranging from safety and efficacy to synthetic methodology and eco nomics this practice is undergoing rapid change as more and more chiral synthetic drugs become avail able in enantiomerically pure form  [c.296]

A much more serious drawback to using chiral drugs as racemic mixtures is illustrated by thalidomide briefly employed as a sedative and antinausea drug in Europe during the period 1959-1962 The desired properties are those of (/ ) thalidomide (S) Thalido mide however has a very different spectrum of bio logical activity and was shown to be responsible for over 2000 cases of serious birth defects in children born to women who took it while pregnant  [c.296]

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  [c.297]

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  [c.305]

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane  [c.309]

Recall from Section 7 13 that a stereospecific reaction is one in which each stereoiso mer of a particular starting material yields a different stereoisomeric form of the reaction product In the ex amples shown the product from Diels-Alder cycloaddi tion of 1 3 butadiene to as cinnamic acid is a stereo isomer of the product from trans cinnamic acid Each product although chiral is formed as a racemic mixture  [c.410]

Unless a resolution step is included the a ammo acids prepared by the synthetic methods just described are racemic Optically active ammo acids when desired may be obtained by resolving a racemic mixture or by enantioselective synthesis A synthesis IS described as enantioselective if it produces one enantiomer of a chiral compound m an amount greater than its mirror image Recall from Section 7 9 that optically inactive reactants cannot give optically active products Enantioselective syntheses of ammo acids therefore require an enantiomerically enriched chiral reagent or catalyst at some point m the process If the chiral reagent or catalyst is a single enantiomer and if the reaction sequence is completely enantioselective an optically pure ammo acid is obtained Chemists have succeeded m preparing a ammo acids by techniques that are more than 95% enantioselective Although this is an impressive feat we must not lose sight of the fact that the enzyme catalyzed reactions that produce ammo acids m living systems do so with 100% enantioselectivity  [c.1122]

Enantiomeric excess (Section 7 4) Difference between the per centage of the major enantiomer present in a mixture and the percentage of its mirror image An optically pure material has an enantiomenc excess of 100% A racemic mixture has an enantiomeric excess of zero  [c.1282]

A particular point of interest included in these hehcal complexes concerns the chirality. The heUcates obtained from the achiral strands are a racemic mixture of left- and right-handed double heUces (Fig. 34) (202). This special mode of recognition where homochiral supramolecular entities, as a consequence of homochiral self-recognition, result from racemic components is known as optical self-resolution (203). It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often quoted as one of the possible sources of optical resolution in the biological world. On the other hand, the more commonly found process of heterochiral self-recognition gives rise to a racemic supramolecular assembly of enantio pairs (204).  [c.194]

Levonorgestrel andNorgestrel. Both of these compounds aie used in oral contraceptives. Noigestiel (65) is a racemic mixture the biologically active isomer is known as levonorgestrel or (—)-norgestrel (65). Norgestrel can be recrystaUized from ethyl acetate or methanol (95), and levonorgestrel can be recrystaUized from chloroform—methanol (95). Both compounds are soluble in chloroform, slightly soluble in ethanol, and practically insoluble in water (96). Extensive physical, spectral, and analytical properties have been compiled (97).  [c.215]

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6).  [c.512]

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24).  [c.512]

Tartaric acid [526-83-0] (2,3-dihydroxybutanedioic acid, 2,3-dihydroxysuccinic acid), C H O, is a dihydroxy dicarboxyhc acid with two chiral centers. It exists as the dextro- and levorotatory acid the meso form (which is inactive owing to internal compensation), and the racemic mixture (which is commonly known as racemic acid). The commercial product in the United States is the natural, dextrorotatory form, (R-R, R )-tartaric acid (L(+)-tartaric acid) [87-69-4]. This enantiomer occurs in grapes as its acid potassium salt (cream of tartar). In the fermentation of wine (qv), this salt forms deposits in the vats free crystallized tartaric acid was first obtained from such fermentation residues by Scheele in 1769.  [c.524]


See pages that mention the term Racemic mixture : [c.424]    [c.1286]    [c.352]    [c.437]    [c.309]    [c.311]    [c.311]    [c.79]    [c.270]   
Carey organic chemistry (0) -- [ c.288 , c.297 , c.316 ]

Modern spectroscopy (2004) -- [ c.78 ]

Organic chemistry (0) -- [ c.288 , c.297 , c.316 ]