Racemic mixtures definition

Two synonyms, two definitions, and an expression of admiration. The word norepinephrine is synonymous with noradrenalin, and the word epinephrine is synonymous with adrenalin. The distinctions are that the first in each case is American and the second British. And the term chiral indicates a potential asymmetry in a molecule that would allow eventual separation into two optical isomers. The term racemic refers to a mixture of these two isomers which has not yet been separated into the individual components. A racemic mixture is called a racemate and, from the point of view of the human animal (which is completely asymmetric), must be considered as a mixture of two structurally identical but... 

When we talk about two chiral diastereoisomers, we have no choice but to draw the structure of one enantiomer of each diastereoisonier, because we need to include the stereochemical information to distinguish them, even if we re talking about a racemic mixture of the two enantiomers. To avoid confusion, it s best to write something definite under the structure, such as (meaning racemic) under a structure if it means this diastereoisomer but not this enantiomer of this diaster eoiso mer . 

In Sec. V we extend the treatment of Sec. IV to the purification of unpolarized racemic mixtures. We show that when the three laser polarization directions are mutually perpendicular (thus forming a system of axes with definite handedness) one can purify ordinary (unpolarized) racemic mixtures. In accordance with the general theorem of Sec. II, a change in the handedness of the three polarization vectors results in a change in the handedness of the enantiomer to which the racemic mixture is being converted. It then follows that, as discussed in Sec. VI, enantioselectivity can also be achieved by a two-pulse process performed on a sample in which the axis of the molecules (rather than their M projection) has been preoriented. 

One of the issues raised in the discussion definition of polymorphs (Section 1.2.1) deals with racemic mixtures vs enantiomerically pure crystals or conglomerates... 

Carbocation Intermediates.—Though the nature of the active centres in polymerizations propagated by carboanions has been well known for some time, and reasonably adequately characterized, the corresponding species in cationic propagations has proved difficult to identify with anything like the same certainty. However, the use of super acid media, e.g. FSOaH/SbFj, has now enabled Olah to obtain a n.m.r. spectrum of the elusive styryl carbocation, following experiments such as that in which a racemic mixture of products was obtained from the reaction of (—)-l-phenylethyl chloride with EtaAl, which provided powerful circumstantial evidence for the existence of the ion. More recently a particular definitive paper on and n.m.r. spectra of styryl species has appeared. 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

Resolution of a complex even to a very small extent could demonstrate tiiat the original complex was a racemic mixture. Thus, complexes like [Co(dimethylglyoximo)2(NH3)Cl] were partly resolved on quartz powder. The spatial configuration of this complex was unknown. From file theoretical point of view it was expected that only the c/5-form of the complex should exhibit optical activity. In agreement with this expectation, resolution of the tris(dimethylglyoximo)cobaltate(lll) complex on quartz exhibited a poor but definite optical activity revealing the configuration of the complex to be cis rather than trans... 

The resolution of racemic mixtures, i.e., the separation of at least one of the enantiomers in pure form, is an important step in the manufacture of chiral products in the pharmaceutical, agrochemical, flavour and perfumery industries and for other specialty chemicals. The necessity to resolve racemic mixtures is particularly strong in the pharmaceutical industry since the two component enantiomers (see section 1.9 for definition of terms) in the equimolar mixture can have very different pharmacological activities. Thalidomide is a frequently quoted example one stereoisomer was the beneficial agent for preventing morning sickness in pregnant women while the other caused serious birth defects. 

The development of Ziegler-Natta-type catalysts (see Section 27.8) has, since the 1980s, included the use of zirconocene derivatives. In the presence of methylaluminoxane [MeAl(p-0)] as a co-catalyst, compounds A, B and C (shown below) are active catalysts for propene pol5mierization. Compounds A and B are chiral because of the relative orientations of the two halves of the organic ligand. A racemic mixture of A facilitates the formation of isotactic polypropene, while use of catalyst C results in syndiotactic polypropene (see Section 27.8 for definitions of... 

On the other hand, many studies on molecular motions occurring in PBLG and also in racemic mixtures of D and L show, as noted later, that above room temperature the side chain undergoes considerable motions and becomes liquid-like. This suggests that the side chain does not adopt any definite conformation. Thus, the two approaches are yielding the conflicting results. 

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