Separating enantiomers is called resolution

There are other types of chiral molecule but they all share the same feature—there is no plane of symmetry. 

Early in this chapter, we said that most of the molecules in nature are chiral, arid that Nature usually produces these molecules as single enantiomers. We ve talked about the amino acids, the sugars, ephedrine, pseudoephedrine, and tartaric acid—all compounds that can be isolated from natural sources as single enantiomers. On the other hand, in the lab, if we make chiral compounds from achiral starting materials, we are doomed to get racemic mixtures. So how do chemists ever isolate compounds as single enantiomers, other than by extracting them from natural sources We ll consider this question in much more detail in Chapter 45, but here we will look at the simplest way using nature s enantiomerically pure compounds to help us separate the components of a racemic mixture into its two enantiomers. This process is called resolution. 

Imagine the reaction between a chiral, but racemic alcohol and a chiral, but racemic carboxylic acid, to give an ester in an ordinary acid-catalysed esterification (Chapter 12). 

The product contains two chiral centres, so we expect to get two diastereoisomers, each a racemic mixture of two enantiomers. Diastereoisomers have different physical properties, so they should be easy to separate, for example by chromatography. 

Rpmember that it) means the compounds are racemic vie re Showing only relative, net absolute,. ,  

We could then reverse the esterification step, and hydrolyse either of these diastereoisomers, to regenerate racemic alcohol and racemic acid. 

If we repeat this reaction, this time using an enantiomerically pure sample of the acid, available from (Jl)-mandelic acid, the almond extract you met on p. 310, we will again get two diastereoisomeric products, but this time each one will be enantiomerically pure. Note that the stereochemistry shown here is absolute stereochemistry. 

If we now hydrolyse each diastereoisomer separately, we have done something rather remarkable we have managed to separate two enantiomers of the starting alcohol. 

A separation of two enantiomers is called a resolution. Resolutions can be carried out only if we make use of a component that is already enantiomerically pure it is very useful that Nature provides us with such compounds resolutions nearly always make use of compounds derived from nature. 

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The separation of a racemic compound into its enantiomers is called resolution. Various methodologies have been used for the resolution of the enantiomers on both analytical and preparative scales. The different techniques may be categorized into two classes the classical approach, using enzymatic degradation of one of the enantiomers, and preferential crystallization. Modem technologies include spectroscopic, electrophoretic, and chromatographic methods. 

If we need one pure enantiomer of butan-2-ol, we must find a way of separating it from the other enantiomer. The separation of enantiomers is called resolution, and it is a different process from the usual physical separations. A chiral probe is necessary for the resolution of enantiomers such a chiral compound or apparatus is called a resolving agent. 

The separation of a racemic mixture into its component enantiomers is called resolution. Thus, a racemic mixture is resolved into its component enantiomers. 

Since all living cells and organisms involve reactions of enantiomerically pure materials such as carbohydrates, proteins, and DNA, most naturally occurring chiral compounds exist in enantiomerically pure form. Chemical reactions, however, often produce racemic mixtures. This is always the case if only racemic and/or achiral reactants, reagents, catalysts, and solvents are used. The products of chemical reactions can be enantiomerically enriched or enantiopure only if chiral starting materials, reagents, catalysts or solvents are used. (See Section 2.5 for a discussion of enantiose-lective reactions.) Racemic mixtures can be separated into the two enantiomeric forms. The process of separating a racemic mixture into its enantiomers is called resolution, and it can be accomplished in several different ways. 

Note that the physiological effects of enantiomers are sometimes profoundly different. Thus the (-)-nicotine that occurs naturally in tobacco is much more toxic than the (+)-nicotine that is made in the laboratory. Specific effects such as these are attributed to dissymetric reaction sites in biological systems. Since enantiomers are so similar, and since in chemical reactions the two forms are produced in equal amounts, special techniques are required to separate the two. This separation process is called resolution. Some resolution methods are described in Section 4.10. Often, a single optical isomer will rearrange to give a racemic mixture the process is called racemization. 

We have just seen (eq. 5.5) that, when reaction between two achiral reagents leads to a chiral product, it always gives a racemic (50 50) mixture of enantiomers. Suppose we want to obtain each enantiomer pure and free of the other. The process of separating a racemic mixture into its enantiomers is called resolution. Since enantiomers have identical achiral properties, how can we resolve a racemic mixture into its components The answer is to convert them to diastereomers, separate the diastereomers, and then reconvert the now-separated diastereomers back to enantiomers. 

The separation of a racemic mixture into its enantiomers is called resolution. 

The separation of enantiomers is called resolution. It is achieved by the reaction of the racemate with the pure enantiomer of a chiral compound to yield separable diastereomers. Chemical removal of the chiral reagent frees both enantiomers of the original racemate. Another way of separating enantiomers is by chiral chromatography on an optically active support. 

Unlike other stereoisomers, enantiomers have identical physical properties and, consequently, are difficult to separate. A process by which enantiomers can be separated is called resolution. 

A number of compounds exist as racemic mixtures ( ), i.e. a mixture of equal amounts of two enantiomers, (—) and (+). Often, one enantiomer shows medicinal properties. Therefore, it is important to purify the racemic mixture so that active enantiomer can be obtained. The separation of a mixture of enantiomers is called the resolution of a racemic mixture. 

An equimolar mixture of two enantiomers is called a racemate. The separation of two enantiomers that constitute a racemate is called optical resolution or resolution. Their crystalline forms best characterize types of racemates. A racemic mixture is a crystal where two enantiomers are present in equal amounts. A conglomerate is a case where each enantiomer has its own crystalline form. Sometimes their crystals have so-called hemihedral faces, which differentiate left and right crystals. For over a hundred years, crystallization processes have been used for the separation and purification of isomers and optical resolution, both in the laboratory and on an industrial scale. 

Separation of two enantiomers by a chemical reaction that selectively occurs for only one of the enantiomers is called kinetic resolution. 

Separation of one or both enantiomers from a racemic mixture is called resolution, and it is the converse of racemization. Since resolution usually involves molecules or systems with more than one stereogenic centre, it is dealt with in Chapter 3. 

Pasteur was only 26 years old at the time and was unknown in scientific circles. He was concerned about the accuracy of his observations because a few years earlier, the well-known German organic chemist Eilhardt Mitscherlich had reported that crystals of the same salt were all identical. Pasteur immediately reported his findings to Jean-Baptiste Biot and repeated the experiment with Biot present. Biot was convinced that Pasteur had successfully separated the enantiomers of sodium ammonium tartrate. Pasteur s experiment also created a new chemical term. Tartaric acid is obtained from grapes, so it was also called racemic acid (racemus is Latin for a bunch of grapes ). When Pasteur found that tartaric acid was actually a mixture of enantiomers, he called it a racemic mixture. Separation of enantiomers is called the resolution of a racemic mixture. 

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