Resolving agents, racemate chiral

We have seen that the spectra of enantiomers, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples, and racemic ones. The wording is carefully chosen, particularly normal conditions , because it is often possible to distinguish enantiomers, by running their spectra in abnormal conditions - in the presence of a chiral resolving agent. Perhaps the best known of these is (-)2,2,2,trifluoro-l-(9-anthryl) ethanol, abbreviated understandably to TFAE. (W.H. Pirkle and D.J. Hoover, Top. Stereochem., 1982,13, 263). Structure 7.4 shows its structure. 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

A Chiral Amines as Resolving Agents. Resolution of Racemic Acids... 

A is correct. Strychnine is the only chiral molecule and thus the only possibility. The passage states that the only chemical difference between enantiomers is their reactions with chiral compounds. Strychnine is often employed as a resolving agent for racemic acids. 

The method relies on the p and n salts having different solubilities, and they must not form solid solutions or double salts. The more insoluble salt is filtered and the purified acid recovered by adding mineral acid. This method of chiral resolution is well established, and lists of resolving agents for many classes of racemic compounds are available [22], Inclusion chemistry may be employed for the same purpose by preparing host-guest compounds with a chiral host ... 

In Section III, it was shown that the introduction of an optically active resolving agent (S ) into a pair of enantiomers (R)/(S) gives two diaste-reoisomers (R.S1) and (S,S ) with different metal configurations. Instead of an optically active resolving agent (5 ), a racemic mixture of a chiral ligand (R )I(S ) can also be used. Then, not two, but four, isomers are formed two diastereoisomeric pairs of enantiomers (R,R ) and (5,5 ) as well as (R,S ) and (S,R ). An example is presented in Scheme 23,... 

An overwhelming majority of classic resolutions still involve the formation of diastereomeric salts of the racemate with a chiral acid or base (Table 6.1). These chiral-resolving agents are relatively inexpensive and readily available in large quantities (Table 6.2). They also tend to form salts with good crystalline properties.8... 

Chiral acids or bases are tested for salt formation with the target racemate in a variety of solvents. The selected diastereomeric salt must crystallize well, and there must be an appreciable difference in solubility between the two diastereoisomers in an appropriate solvent. This type of selection process is still a matter of trial and error, although knowledge about the target racemate and the available chiral-resolving agents and experience with the art of crystallization does provide significant help.9-11... 

The resolution of racemic compounds through the formation of reversible diastereomer complexes is certainly an example of the generation of chirality upon association of an achiral solute (the racemate to be resolved) and a chiral solute (the resolving agent). Such interactions are normally considered solely from the separations point of view [11], and only rarely is CD used to follow the association mechanism. It is evident, however, that the spectroscopic method would be of great value to characterize the associated species. 

III. Chiral Discrimination of Racemates by Conventional Resolving Agents... 

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