プログラム学習・有機化学 Organic chemistry Programmed Study

5.3 Racemates and optical resolution

5.3 Racemates and optical resolution

The two compounds forming a pair of enantiomers have the same physical (e.g., melting point) and chemical (e.g., acidity) properties except optical rotation and reactivity to chiral reagents. A 1 : 1 mixture of enantiomeric pair is named racemateand designated as (±) or dl. Racemates are optically inactive because the effect of one enantiomer is cancelled by the effect of other enantiomer. The chemical property of solution of racemates is identical with that of an enantiomer although sometimes properties of a racemate and an enantiomer may vary in the solid state.

Separation of racemates into their component enantiomer is named optical resolution. There are three methods of resolution.

1) physical resolution: manual separation when enantiomers crystallize in different crystal forms.

2) chemical resolution: separation by conventional chemical separating methods of the diastereomer (Ch. 6) obtained by the reaction with chiral reagents.

3) biological resolution: use of a specific enzyme which selectively consumes one of the enantiomers.

▶go to S5.5 Racemates and optical resolution

▶(Back to Q5.10 as necessary)

A good example of chemical resolution is that of lactic acid CH₃CH(OH)COOH. Since lactic acid is a carboxylic acid, the chiral reagent of choice is a base which may form a crystalline salt with lactic acid. An alkaloid, (-)-brucine, is often employed. (-)-Brucine is a natural product.

The two salts are stereoisomers, or rather diastereomers. In principle these are separable by usual chemical method such as recrystallization from an appropriate solvent. For optical active carboxylic acid, an optical active base can be employed to prepare diastereomers. When the racemates belong to a different type of compounds, a different type of chiral reagent should be chosen. In Table 5.1, a variety of resolution reagents are listed.

Table 5.1 List of resolution reagents