Physical Properties of Enantiomers

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Chapter: Organic Chemistry : Stereochemical and Conformational Isomerism

If both enantiomers are present in a solid sample, the melting point and the solubility of the solid mixture are often found to be different than those of the pure enantiomers.


PHYSICAL PROPERTIES OF ENANTIOMERS

If both enantiomers are present in a solid sample, the melting point and the solubility of the solid mixture are often found to be different than those of the pure enantiomers. This is due to the fact that the solid-state interaction of two R enantiomers or two S enantiomers is often different than the solid-state interaction of an R and an S enantiomer. (In fact, these interactions are diastereomeric.) The result is that three different scenarios are possible when a racemic mixture is crystallized from solution:

1. If an enantiomer has a greater affinity for molecules of like configuration, then two sets of crystals will be formed, one set composed only of the (+) form and the other composed only of the () form. This racemic mixture is called a conglomerate because it is a mixture of two different types of crystals. Moreover it behaves as a typical mixture —the melting point is lower than the pure enantiomeric components and the solubility is higher (Figure 6.2). This is a relatively rare situation.


2. If an enantiomer has a greater affinity for molecules of opposite configu-ration, then crystals are produced which contain equal numbers of the (+and () forms. 

The solid compound which has properties different than either pure enantiomer and exists only in the solid state is called a race-mate or a racemic compound or a racemic mixture. A racemate is often higher melting and less soluble than a pure enantiomer and behaves as a mixture in the presence of either pure enantiomer (Figure 6.3). This is the most common situation and allows an unequal mixture of enantiomers to be purified. Upon crystallization, the racemate will precipitate first, leaving behind the enantiomer in excess.

3. If one enantiomer has similar affinity for molecules of either configuration, then the enantiomers are randomly distributed in the crystal and the solid is a “racemic solid solution” or mixed crystal. Such solids are identical with either enantiomer (Figure 6.4).



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