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Question
Explain the statement "RS and SR are optically active enantiomers if at least one group is different".
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Solution
The molecules of enantiomers are not superposable on each other and, on this basis, we have concluded that enantiomers are different compounds. How are they different? Do enantiomers resemble constitutional isomers and diastereomers in having different melting and boiling points? The answer is no. Enantiomers have identical melting and boiling points. Do enantiomers have different indexes of refraction, different solubilities in common solvents, different infrared spectra, and different rates of reaction with achiral reagents? The answer to each of these questions is also no. Many of these properties (e.g., boiling points, melting points, and solubilities) are dependent on the magnitude of the intermolecular forces operating between the molecules, and for molecules that are mirror images of each other, these forces will be identical. We can see an example if we examine table where some of the physical properties of the 2-butanol enantiomers are listed. Enantiomers show different behavior only when they interact with other chiral substances. Enantiomers show different rates of reaction towards other chiral molecules—that is, toward reagents that consist of a single enantiomer or an excess of a single enantiomer. Enantiomers also show different solubilities in solvents that consist of a single enantiomer or an excess of a single enantiomer.
Physical Properties of (R) -and (S) -2 -Butanol
Physical Property(R)−2−Butanol(S)−2−ButanolBoiling point(1 atm)95.5∘C99.5∘CDensity (g mL−1at 20∘C)0.8080.808Index of refraction(20∘C)1.3971.397
The molecules of enantiomers are not superposable on each other and, on this basis, we have concluded that enantiomers are different compounds. How are they different? Do enantiomers resemble constitutional isomers and diastereomers in having different melting and boiling points? The answer is no. Enantiomers have identical melting and boiling points. Do enantiomers have different indexes of refraction, different solubilities in common solvents, different infrared spectra, and different rates of reaction with achiral reagents? The answer to each of these questions is also no. Many of these properties (e.g., boiling points, melting points, and solubilities) are dependent on the magnitude of the intermolecular forces operating between the molecules, and for molecules that are mirror images of each other, these forces will be identical.
We can see an example if we examine table where some of the physical properties of the 2-butanol enantiomers are listed. Enantiomers show different behavior only when they interact with other chiral substances. Enantiomers show different rates of reaction towards other chiral molecules—that is, toward reagents that consist of a single enantiomer or an excess of a single enantiomer. Enantiomers also show different solubilities in solvents that consist of a single enantiomer or an excess of a single enantiomer.Physical Properties of (R) -and (S) -2 -Butanol
Physical Property(R)−2−Butanol(S)−2−ButanolBoiling point(1 atm)95.5∘C99.5∘CDensity (g mL−1at 20∘C)0.8080.808Index of refraction(20∘C)1.3971.397
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