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Enantiomers and Diastereomers are the different types of optical isomers. Enantiomers are non-superimposable chiral compounds that are mirror images of one another, whereas diastereomers are stereoisomer compounds that are not mirror images of each other. Both enantiomers and diastereomers are included in the category of stereoisomers but both are different from each other in terms of their configuration.
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Key Takeaways: Stereoisomers, Diastereomers, Enantiomers, Optical Isomers, Chiral Molecule, Cis-Trans Isomers
Enantiomers
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Enantiomers are chiral compounds that are not superimposable and are mirror reflections of one another. A chiral molecule has an image that is not the same as its mirror image, and it is usually distinguished by a carbon centre that is connected to four distinct atoms. For a molecule to be classified as chiral, and hence an enantiomer, these atoms must be chemically distinct. The stereocenter is a tetrahedral carbon molecule to which the various atoms are linked.
Types of Isomers
The Cahn-Ingold-Prelog naming system was developed to account for minor differences in the spatial arrangement of the atoms of enantiomer compounds. Because the two molecules have an identical formula and atom structure, we must designate one as S and the other as R, based on the clockwise orientation of the atoms from the lowest to the highest atomic mass.
Consider the structures of Bromo-chloro-fluoromethane below:
The hydrogen and fluorine have distinct orientations, but they are both parts of the same chemical molecule. The right molecule will never have the same orientation as the left molecule, no matter how many times it is rotated. If you try to swap the Fluorine and Hydrogen around, the Bromine and Chlorine will shift as well. This explains the notions of enantiomers being non-superimposable and mirror images.
Enantiomers have different spatial arrangements of atoms, but chemical and physical properties are identical. They do, however, share the same melting and boiling points, as well as a variety of other characteristics. Similar features can be explained by the fact that their intermolecular forces are equivalent. However, their optical properties differ because they twist polarised light in opposite directions, although having the same amount of polarisation. Enantiomer molecules are distinguished by their differences in optical characteristics.
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Diastereomers
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Diastereomers are stereoisomer compounds that do not have molecules that are mirror images of one another and are not superimposable. When you look at the cis and trans isomer structures, you can see how diastereomers work.
The compounds are identical, but their arrangements are not identical. They are not mirror reflections of one another. The compound is called cis when the CH3 atoms are on the same side, and trans when one of the CH3 atoms is switched with the Hydrogen atom. However, diastereomers are not limited to the cis and trans structures. There are a lot of these molecules, as long as they show spatial arrangements of atoms that are not mirror reflections of one another and can't be superimposed.
The structures of cis-2-butene and trans-2-butene are shown below:
Structures of cis-2-butene and trans-2-butene
Difference Between Enantiomers and Diastereomers
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The main differences between enantiomers and diastereomers are as follows:
| Enantiomers | Diastereomers |
|---|---|
| They are non-superimposable mirror reflections of one another. | They are not superimposable and are not mirror reflections of one another. |
| They have the same chemical and physical properties but different optical properties. | They have a variety of chemical and physical characteristics. |
| Their molecular structures are often designated with R and S to distinguish them. | One molecule has the same configuration as the enantiomer, whereas the other has the same structure as the enantiomer. As a result, there is no need to employ nomenclature to distinguish them. |
| Have one or more stereocenters. | Two stereocenters are required. |
Things to Remember
- Stereoisomers with the same chemical and structural formula but distinct arrangements/configurations of the atoms that make up their structures are called enantiomers and diastereomers.
- Enantiomer molecules are mirror images of one another, whereas diastereomers are not mirror images.
- Enantiomers are chemically and physically identical, but their optical characteristics differ because some rotate polarized light in different directions.
- Not all diastereomers have optical activity.
- The R and S naming scheme is assigned to enantiomers based on the atomic mass of the substituents connected to the chiral centre.
- Only one of the diastereomers has the R and S configurations, while the other has the same configurations. This is the difference between them and enantiomer mirror images.
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Sample Questions
Ques. Compounds have different atomic configurations in space, although the same atoms are referred to as? (2 Marks)
Ans. Stereoisomerism is a term that has been used to describe a group of people that have similar characteristics. Structural isomers have the same molecular formula but differ in chain links or sequencing. Stereoisomers have the same structural isomer by definition.
Ques. What is the link between mirror pictures and objects? (3 Marks)
Ans. Symmetric objects can be layered with their mirror images. It is one and the same. It is not possible to superpose of asymmetric items and their mirror counterparts. They are two distinct entities. In the case of molecules, chiral compounds and their mirror images are separate. Enantiomers are chiral substances and their mirror copies, which are a form of stereoisomer.
Ques. What do enantiomers have in common? (2 Marks)
Ans. Enantiomers have the same chemical and physical properties in an achiral environment. Because they spin the direction of plane polarised light at equal but opposing angles, enantiomers interact differently with other chiral substances.
Ques. Isn't it true that all enantiomers have the same optical properties? (2 Marks)
Ans. Chiral compounds are usually optically active, and two enantiomers will rotate light in opposite directions, thus, enantiomers are also referred to as optical isomers. A racemate is a 50:50 mixture of both enantiomers that does not spin the light plane.
Ques. What are the enantiomers' chemical properties? (5 Marks)
Ans. Stereoisomeric chemical substances having different enantiomeric structures are known to participate in chemical interactions with other enantiomeric chemicals on a regular basis. Enantiomers are currently understood to be biological molecules with unique properties. It is also worth noting that two different enantiomers of the same chemical compound can have drastically different impacts on a variety of animals. This phenomenon can be seen in the effects of a variety of medications on humans. In some cases, only one of a drug's enantiomers will be able to deliver the physiological benefits required.
Ques. What are the characteristics of diastereomers? (5 Marks)
Ans. Following are the characteristics of diastereomers:
- Melting points, boiling points, densities, solubilities, refractive indices, dielectric constants, and specific rotations are all different physical properties of diastereomers. Except for the sign of particular rotation, enantiomers have similar physical properties.
- In Addition to geometrical isomers, other diastereomers can be optically active or inactive.
- The chemical characteristics of diastereomers are similar but not identical.
- Diastereomers can be distinguished from one another using techniques such as fractional crystallization, fractional distillation, and chromatography, among others, due to differences in their physical properties.
Ques. What are Optical Isomers? (2 Marks)
Ans. Optical isomers are the compounds having equal number of atoms and bonds but their arrangements are different. They are non-superimposable images. Enantiomers and stereoisomers are the two sub-types of optical isomers.
Ques. What is Polarization? Also state the Fazan Rule. (3 Marks)
Ans. Polarization refers to the distortion of the electron cloud of a negatively-charged ion by another positively-charged ion.
According to the Fazan Rule;
- smaller the size of the ion, more is its ability to distort the electron cloud of others and
- anions with a larger size undergo distortion easily
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