The optically active compounds exist in two or more isomeric form and they differ in their optical activity. The isomers which have same chemical reactions and almost same physical properties but differ in their behaviour towards polarized light are optical isomers and the phenomenon is known as optical isomerism.
The two main types of optical isomerism are:
- Enantiomerism, and
- Diastereomerism.
There are other two forms in which an optical isomer can exist in a solution:
Meso Compounds: These compounds have more than one asymmetric carbon which is superimposable on its mirror image and hence are optically inactive.
Racemic Mixtures: These isomers are optically inactive as these are the mixture of equal parts of enantiomers.
Enantimerism
Pasteur in 1948 while carrying out an experiment observed that an aqueous solution of sodium ammonium tartrate was found to be optically active. He separated two different types of crystals from the solution of this compound and observed that each crystal showed optical activity equal in magnitude but in opposite direction. The crystals were found to possess two types of shape and both of them were non-superimposable.
The optical isomers which are nonsuperimposable mirror images of each other and which rotate the plane polarised light in equal magnitude but in opposite directions are known as enantiomers or enantiomorphs and the phenomenon is called as enantiomerism. As the molecule and its mirror image isomers differ only in the spatial arrangement of atoms, enantiomerism is only a particular type of stereoisomerism.
For example, lactic acid has two optical isomers, (figure. (a)) is the mirror image of (b). Thus, they are a pair of enantiomers. Enantiomers are stable, isolable compounds having different three-dimensional spatial arrangements. Under ordinary conditions, their interconversion is not possible.
Both enantiomers rotate the plane polarised light exactly to same extent (same angle), but one may rotate the plan to the left (anticlockwise; called laevorotatory), while the other rotates the plane to the right (clockwise; called dextrorotatory).
A mixture of two enantiomers in equal amount is called a racemic mixture. Racemic mixtures are optically inactive, i.e., they do not rotate the plane polarised light, because the two enantiomers rotate the plane polarised light in opposite directions, thus, cancelling each other.
Essential Condition for Enantiomerism
Enantiomer molecules are non-superimposable mirror images of each other. The non-superimposable mirror images are found due to chiral nature of the molecules. A chiral molecule has no plane of symmetry, therefore nonsuperimposable on its mirror image.
Chirality is mostly due to the presence of atleast one chiral carbon in the molecules. For example, lactic acid [CH3∗CH(OH)COOH] and 2-methyl-1-butanol [C2H5∗CH(CH3)CH2OH] contain one chiral carbon each (on the C atom marked with ∗ ) and therefore these molecules exist in enantiomeric forms.
On the other hand there are some compounds (e.g., 2,3-pentadiene ) which do not have any chiral carbon but possess the property of chirality and therefore exhibit enantiomerism. With the above mentioned examples it can be concluded that the chirality is the reason and the only condition of enantiomerism.
Characteristics of Enantiomers
The enantiomer molecules possess following characteristics:
They have similar physical properties like melting points, boiling points, densities, solubilities and refractive indices however the only difference is that they rotate plane polarised light in opposite direction, although the magnitude of specific rotation is same.
Their chemical properties are identical except in the reactions with other optically active compounds. For example, the (+)-lactic acid chemically reacts exactly the same as that of (−) lactic acid, however, there may be difference in the rates of reaction at which two enantiomers react with other optically active compounds. For example, the rate of esterification of (+) lactic acid with (+) sec-butyl alcohol [CH3CH2CH(OH)CH3] would differ from the rate of esterification of (−) lactic acid with the same alcohol.
They differ in biological properties. Unlike physical and usual chemical properties, enantiomers show different biological properties. For example, (+) sugar plays an important role in animal metabolism while (-) sugar is not metabolised at all. In the same way, (+) tartaric acid is readily consumed by the mould Penicillium glaucum whereas (-) tartaric acid is not.
When equal quantities of enantiomers are fused together in a solution, an optically inactive form called racemic modification or racemic mixture or racemate is formed. The racemic modification is generally denoted by the prefix ( \pm ). For example, if equal quantities of (+) lactic acid and (-) lactic acid are mixed with each other, the resulting racemic mixture obtained is (±) lactic acid, which is optically inactive.
Diastereomerism
Diastereomers are the stereoisomers that are not mirror images of each other and they are not linked with reflection operation unlike of enantiomers and the phenomenon is known as diastereomerism. They possess same physical properties. Thus, it can be concluded that diastereomers are stereoisomers that are not enantiomers, i.e., they are distinct molecule with the same structural arrangement of atoms that are non-superimposable, and not mirror images of each other.
For example; 2;3-dicholorpentane has two chiral centres.
As the four groups attached to one chiral carbon differ from those groups attached to the other, hence the two chiral carbon atoms are different from each other. So, there can be four stereoisomers (I, II, III and IV) for this compound.
Molecular analysis of the structures I and II concludes that they are nonsuperimposable mirror images of each other and therefore represent a pair of enantiomers. In the same way, III and IV are also non-superimposable mirror images of each other thus represent another pair of enantiomers.
When structures I and III were compared, the observation made was that the two structures have identical configurations at one carbon (C2) and the mirror image ‘configuration at carbon (C3). Thus the two forms (I and III) are neither identical nor mirror images of each other.
Such stereoisomers of a substance are called as diastereomers. In the same way structures of I and IV, II and III, and II and IV are the pair of diastereomers, as they are neither identical nor mirror images.
Another example for the structure containing two dissimilar chiral carbon atoms or the pairs of diastereomers, is 3-Chloro-2-butanol. Its four stereoisomeric forms are given below in structures V. and VI, V and VIII, VI and VII, and VI and VIII represent pairs of diastereomers.
Characteristics of Diastereomers
Unlike enantiomers, physical properties of diastereomers are different, thus their melting points, boiling points, solubilities in a given solvent, densities, refractive indices, etc. differ from each other. Due to this they can be separated by physical methods of separation like fractional distillation, fractional crystallisation, chromatography, etc.
The chemical properties of diastereomers are similar but not identical. This indicates their different reaction rates with chiral or achiral reagents. This is because neither the reagents nor the transition states are mirror images of each other. This results into diastereomers of different activation energies for a particular reaction and hence reacts at different rates.
Difference between Diastereomers and Enantiomers
| Diastereomers | Enantiomers |
| Their physical properties are different. | Their physical properties are identical. |
| Their chemical properties are similar but not identical. | Their chemical properties are identical. |
| Their separation by physical methods, like fractional distillation, chromatography, etc., is easy. | Their separation by physical methods is not possible. |
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