Enantiomers and Diastereomers

Jul 25, 2022, 16:45 IST

what is the difference between Enantiomers and Diastereomers

 Physical and chemical properties of enantiomers are as follows:

1. Enantiomers have identical physical properties like boiling point, melting point, solubility etc.

2. They rotate the plane of polarized light in opposite directions, though in equal amounts. The isomer that rotates the plane to the left (counterclockwise) is called the laevo isomer and is designated as (), while the one that rotates the plane to the right (clockwise) is called the dextro isomer and is designated as (+). Because they differ in this property they are often called optical antipodes.

3. The chemical properties of enantiomers are the same towards achiral reagents, solvents, catalysts and conditions. Towards chiral reagents, solvents, catalysts and conditions enantiomers react at different rates. The transition states produced from the chiral reactant and the individual enantiomers are not mirror images. 

They are diastereomeric and hence have different enthalpies. The delta H values are different for the two and hence the rates of reaction and the amounts of product formed. Their rates may be so far apart that one enantiomer undergoes the reaction at a convenient rate while the other does not react at all. This is the reason that many compounds are biologically active while their enantiomers are not.

Although pure compounds are always optically active, if they are composed of chiral molecules, mixtures of equimolar amounts of enantiomers are optically inactive since the equal and opposite rotations cancel. Such mixtures are called racemic mixtures or racemates. Their properties are not always the same as those of the individual enantiomers. The properties in the gaseous or liquid state or in solution usually are the same, since such a mixture is nearly ideal, but properties involving the solid state, such as melting points, solubilities and heats of fusion, are often different. Thus, racemic tartaric acid has a melting point of 204206°C and a solubility in water at 20°C of 206 g/litre, while for the (+) or the () enantiomers, the corresponding data are 170°C and 1390 g/litre. The separation of a racemic mixture into its two optically active components is called resolution.

The methods used for the resolution of a Enantiomers and Diastereomers are

(i) Mechanical Separation: In rare cases, the crystals of (+) enantiomer can be hand separated from those of the (-) enantiomer of the racemate.

(ii) Chemical Separation: Pasteur was the first investigator to resolve a racemate chemically and his method is used even today. For example, an optically pure compound, a (+) base, is reacted with a racemic acid, resulting in two salts: a (+) (+) salt and a () (+) salt. Since these are diastereomers, they have different solubilities and are separable by fractional crystallization, after which the enantiomers are recovered. If the diastereomers are liquids, they may be separable by fractional distillation, or chromatography.

(iii) Biochemical Separation: A third method, also used successfully by Pasteur, takes advantage of the fact that microorganisms usually can metabolize only one enantiomer of a racemate, while leaving behind a pure solution of the unused one. Now a days, the same result is obtained by using the enzyme that catalyzes the cell reaction rather than the whole microbe.

(iv) Separation Using Chromatography: Another technique is to pass a solution of a racemate through a chromatography column containing a chiral adsorbent. One of the enantiomer is preferentially adsorbed or may be preferentially eluted. A variant of this method to elute with a chiral solvent.

Diastereomers have different physical properties. e.g. melting and boiling points, refractive indices, solubilities in different solvents, crystalline structures and specific rotations. Because of their differences in solubility, they often can be separated from each other by fractional crystallization. Because of slight differences in molecular shape and polarity, they can also be separated from each other by chromatography. Diastereomers have different chemical properties towards both chiral and achiral reagents. Neither any two diastereomers nor their transition states are mirror images of each other and so will not necessarily have the same energies.