Properties of Enantiomers and Distreiomers
Isomerism of Class 11
Properties of Enantiomers & Distreiomers
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 DH¹ 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 racemic mixture 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.
Absolute and Relative Configuration
While discussing optical isomerism, we must distinguish between relative and absolute configuration (arrangement of atoms or groups) about the asymmetric carbon atom. Let us consider a pair of enantiomers, say (+)- and (–)- lactic acid.
We know that they differ from one another in the direction in which they rotate the plane of polarised light. In other words, we know their relative configuration in the sense that one is of opposite configuration to the other. But we have no knowledge of the absolute configuration of the either isomer. That is, we cannot tell as to which of the two possible configuration corresponds to (+) - acid and which to the (–) - acid.
D and L system
The sign of rotation of plane-polarized light by an enantiomer cannot be easily related to either its absolute or relative configuration. Compounds with similar configuration at the asymmetric carbon atom may have opposite sign of rotations and compounds with different configuration may have same sign of rotation. Thus d-lactic acid with a specific rotation + 3.82o gives l-methyl lactate with a specific rotation -8.25°, although the configuration (or arrangement) about the asymmetric carbon atom remains the same during the change.
Obviously there appears to be no relation between configuration and sign of rotation. Thus D/L system has been used to specify the configuration at the asymmetric carbon atom. In this system, the configuration of an enantiomer is related to a standard, glyceraldehyde. The two forms of glyceraldehyde were arbitrarily assigned the absolute configurations as shown below.
If the configuration at the asymmetric carbon atom of a compound can be related to D (+)-glyceraldehyde, it belongs to D-series; and if it can be related to L(–)-glyceraldehyde, the compound belongs to L-series. Thus many of the naturally occurring a-amino acids have been correlated with glyceraldehyde by chemical transformations. For example, natural alanine (2-aminopropanoic acid) has been related to L(+)-lactic acid which is related to L(–)-glyceraldehyde. Alanine, therefore, belongs to the L-series.
In general, the absolute configuration of a substituent (X) at the asymmetric centre is specified by writing the projection formula with the carbon chain vertical and the lowest number carbon at the top. The D configuration is then the one that has the substituent 'X' on the bond extending to the 'right' of the asymmetric carbon, whereas the L configuration has the substituent 'X' on the 'left'. Thus,
However, it must be clearly understood that lower case d and l (or + and -) refer to the direction of rotation of plane polarised light, which is a measured physical constant. It is not necessarily related to configuration around asymmetric carbon. Capital D and L are now used to refer to the absolute configuration around the asymmetric carbon.
R and S System :
This is a newer and more systematic method of specifying absolute configuration to optically active compounds. Since it has been proposed by R.S. Cahn, C.K. Ingold and V. Prelog, this system is also known as Cahn-Ingold-Prelog system. This system of designating configuration has been used increasingly since the early 1960's and may eventually replace the DL-system.
Cahn-Ingold-Prelog system is based on the actual three-dimensional or tetrahedral structure of the compound. In order to specify configuration about an asymmetric carbon *C abde, the groups a, b, d and e are first assigned an order of priority determined by the 'sequence rules'. These rules will be given later. For the present, let us assign priorities 1, 2, 3, 4 to the groups a, b, d, e. Thus the order of priorities may be stated as
a > b > d > e
(1) (2) (3) (4)
Now the tetrahedral model of the molecule is viewed from the direction opposite to the group 'e' of lowest priority (4). The 'conversion rule' says that :
i) If the eye while moving from a®b®d travels in a clockwise or right-hand direction, the configuration is designated R (Latin, Rectus = right).
ii) If the eye while moving from a®b®d travels in counterclockwise or left-hand direction, the configuration is designated S (Latin, Sinister = left).
The sequence rules to determine the order of priorities of groups are :
1) The atoms or groups directly bonded to the asymmetric carbon are arranged in order of decreasing atomic number and assigned priority 1, 2, 3, 4, accordingly.
Thus in chlorobromofluoromethane (CHClBrF), the substituents Br (at no = 35), Cl (at no = 17), F (at no = 9) and H (at no = 1) give the order of priorities.
Br > Cl > F > H
(1) (2) (3) (4)
2) When two or more groups have identical first atoms attached to asymmetric carbon, the priority order is determined by considering the atomic numbers of the second atoms; and if the second atoms are also identical the third atoms along the chain are examined.
Let us consider the three groups
In methyl and ethyl the first atom is carbon and, therefore, atomic numbers of the second atoms H (at no 1) and C (at no 6) decide the priority order, ethyl > methyl. While considering ethyl and n-propyl the second atom is also identical (carbon) and hence the third atoms (H, C) give the priority order n-propyl > ethyl.
3. If the first atoms of the two groups have same substituents of higher atomic number, the one with more substituents takes priority.
Thus ––CHCl2 has a higher priority than ––CH2Cl.
4. A doubly or triply bonded atom 'A' present in a group appended to assmmetric carbon, is considered equivalent to two or three singly bonded 'A's, respectively.
Let us now illustrate the above rules by assigning R and S configurations to enantiomers of some compounds.
If more than one asymmetric centre is present in a molecule, the configuration at each centre is specified by the symbol R or S together with the number of the asymmetric carbon. Thus L-lactic acid has the configurations (2R, 3R).
Optical Isomerism in Compounds With More Than One Asymmetric Carbon Atom
In the above discussion we have seen that an asymmetric carbon atom can produce molecular asymmetry. Thus the molecules containing an asymmetric carbon exist in two optically active forms, (+)-isomer and (–)-isomer, and an equimolar mixture of the two, (±)-mixture, which is optically inactive. When there are two or more asymmetric carbon atoms in a molecule, the problem is complicated considerably.
An organic compound which contains two dissimilar asymmetric carbons, can give four possible stereoisomeric forms. Thus 2-bromo-3-chlorobutane may be written as
The two asymmetric carbons in its molecular are dissimilar in the sense that the groups attached to each of these are different.
C2 has CH3, H, Br, CHClCH3
C3 has CH3, H, Cl, CHBrCH3
Such a substance can be represented in four configurational forms.
The forms I and II are optical enantiomers (related as object and mirror image) and so are forms III and IV. These two pairs of enantiomers will give rise to two possible racemic modifications.
It may be noted that forms I (2 S, 3R) and III (2 S, 3 R) are not mirror images or enantiomers, and yet they are optically active isomers. Similarly, the other two forms i.e., II (2 R, 3S) and IV (2 R, 3 S) are also not enantiomers but optically active isomers.