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Enantiomers are stereoisomers of a compound that are non-superimposable mirror images of each other.
- They are a pair of molecules that exist in two forms which are mirror images of each other but can not be superimposed on each other.
- Enantiomers exist in two forms and have chiral carbon which forms the center and has 4 atoms or groups attached to it.
- Two molecules can be called an enantiomer only if the two chiral carbons have different configurations.
- Enantiomers are optical isomers where the compounds differ in terms of their optical activity but are mirror images of each other.
Enantiomers are thus an important aspect of stereochemistry where the three-dimensional spatial arrangement of atoms and molecules, gives compounds special properties.
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Key Terms: Enantiomers, Optical Isomers, Stereochemistry, Isomerism, Chiral Carbon, Chirality, Stereoisomers, Chiral Atom, Enantiomorphism
What are Enantiomers?
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Enantiomers are optical isomers that are non-superimposable mirror copies of one another. A single chiral atom (or a structural characteristic in a compound that results in something similar) might cause the molecule to have two distinct potential structures, each of which is a non-superimposable mirror copy of the other.
- Enantiomers have several chiral features.
- Multiple geometric shapes exist in such molecules, however, some of these forms may still be perfect mirror images of each other.
- When enantiomers are placed in a symmetric environment, their physical and chemical characteristics are almost similar.
- Enantiomers differ only in the directions in which plane-polarized light is rotated.
- If one enantiomer spins the plane of polarized light to the left, the other enantiomer will rotate it to the right.
- Enantiomers can be dextrorotatory or laevorotatory, thus, the term optical isomers are used for them.
- Enantiomorphism is the structural feature of a chemical molecule that permits it to have two distinct potential enantiomorphs.
The following is the spatial arrangement of a pair of molecules that are enantiomers:
(+)Lactic Acid (-) Lactic Acid
Enantiomers
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Properties of Enantiomers
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The properties of enantiomers are listed as follows:
- Physical characteristics such as melting and boiling point, infrared absorptions, and NMR spectra are similar in enantiomers.
- The melting temperature of a combination of two enantiomers may differ.
- The intermolecular interactions between opposed enantiomers (R and S enantiomers) may differ from those between similar enantiomers.
- Chiroptical methods, like optical rotation, are the only physical techniques that can differentiate between the two enantiomers of a compound.
- The sign and magnitude of the torsional angles, as well as the bond lengths and angles, define the chiroptical characteristics of a molecule.
Chemical Nature of Enantiomers
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Some interesting features of the chemical nature of enantiomers are listed as follows:
- Chemical substances having stereoisomerism and enantiomeric structures often interact with other enantiomeric molecules in chemical processes.
- Enantiomers are known to exist in many biological compounds.
- Two distinct enantiomers of the same chemical substance can have drastically different impacts on a variety of species.
- This phenomenon is noted in the human reactions to many medicines.
- Only one of a drug's enantiomers will be capable of causing the required physiological effects.
- The other enantiomer of the drug is frequently inactive.
- One of a drug's enantiomers can have a negative impact on the organism's health.
- To limit this, enantiomerically "pure" medicines are frequently produced.
Structure of Enantiomers
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The structure of enantiomers is characterized by the following features:
- Stereoisomers, the non-superimposable mirror copies of one another, were first presented as enantiomers.
- Any molecule that cannot be superimposable on its mirror counterpart and so exists as a pair of enantiomers is said to be chiral.
- Any molecule that may be superimposable on its mirror counterpart, on the other hand, is achiral.
- Two enantiomers are possible if a molecule includes a single atom that is tetrahedrally linked to four distinct substituents.
- When the four substituents are distinct from one another, the structure will become superimposable on its mirror image, and hence achiral.
- A stereogenic center, or a stereocenter, is an atom that is linked to four distinct atoms.
- Chirality is the characteristic property of the molecule as a whole that cannot be localized around one atom or a group of atoms.
- The existence of a stereocenter is not required for chirality in a molecule, rather, it is the most prevalent source of chirality.
Representation of Enantiomers
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Chiral Carbon is a carbon atom that is bonded to four different atoms or groups in the molecule.
- It is also referred to as a stereocentre, a stereogenic center, or an asymmetric carbon atom (Chirality center).
- A molecule with asymmetric carbon atoms lacks a plane of symmetry.
- Such types of molecules represent enantiomer pairs.
Example: In Lactic acid, the middle carbon atom has four different groups, thus this carbon is known as Chiral Carbon.
Enantiomers and Chirality
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Chiral recognition is the process of distinguishing between the enantiomers of a chiral molecule. Since their properties are similar, it is difficult to distinguish them. Physical distinctions can only be seen through interactions with a discriminating secondary species. Some important features are as follows:
- Chirality is the structural foundation of enantiomerism.
- Enantiomers are molecules that exist in two forms that are mirror copies of one another but cannot be overlaid.
- The direction in which enantiomers rotate polarized light when dissolved in solution determines the optical isomer type- Dextro (d or +) or Levo (l or -) rotatory.
- As the optical activity of one enantiomer is negated by the other, a racemic mixture is formed when two enantiomers are present in equal quantities and do not spin-polarize light.
Enantiomers vs Diastereomers
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Stereoisomers are classified into two broad categories namely Diastereomers and Enantiomers. The difference between diastereomers and enantiomers is tabulated below:
| Enantiomers | Diastereomers |
|---|---|
| Enantiomers are mirror images of each other that have a similar molecular shape. | Diastereomers are non-mirrored images of each other with different molecular shapes. |
| Enantiomers have similar physical properties except for the ability to rotate plane-polarized light. | Diastereomers have different physical properties. |
| Enantiomers are present in pairs. | Diastereomers have several molecules. |
Diastereomers
Things to Remember
- Enantiomers are optical isomers with a pair of molecules existing in two forms that are non-superimposable mirror images of each other.
- They exhibit the same physical and chemical properties in a symmetric environment.
- Enantiomers have the same properties like melting point, boiling point, infrared absorption, and NMR spectra.
- They are different in their way of rotating in the plane of polarised light.
- Acting distinctively in the chiral environment is a notable feature of enantiomers.
- Intermolecular forces and torsional angles vary between the enantiomeric forms giving the compounds unique properties.
- Chirality is a characteristic property of the molecule as a whole that cannot be localized around one atom or a group of atoms.
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Previous Year Questions
- The d and l enantiomers of an optically active compound differ in… (JKCET 2010)
- In a mixture, two enantiomers are found to be present in…
- The number of enantiomers of the compound… (BITSAT 2013)
- Which of the following will give a pair of enantiomers… (NEET 2007)
- Which of the following has two stereoisomers… (BITSAT 2016)
- The process of separation of a racemic modification into d and l-enantiomers… (NEET 1994)
- Among the following statements about the molecules… (WBJEE 2014)
- If there is no rotation of plane polarised light by a compound… (NEET 2007)
- In an SN1 reaction on chiral centers, there is… (NEET 2015)
- Which of the following compounds is not chiral… (NEET 1998)
Sample Questions
Ques. What are Enantiomers? (3 Marks)
Ans. Enantiomers are one of two different stereoisomers of a compound which are mirror images of each other. They are classified as a specific type of optical isomers. Thus, enantiomers can be defined as optical isomers that are non-superimposable mirror images of each other. The presence of a single chiral atom can result in the compound having two different possible structures, each of which is a non-superimposable mirror image of the other.
Ques. Is it true that all the Enantiomers are Chiral? (1 Mark)
Ans. Enantiomers explain the two stereoisomer relationships, whereas chiral describes an atom with four distinct groups connected to it. Although all molecular stereoisomers are mutually enantiomers, enantiomers also contain chiral centers in their molecules.
Ques. Are Enantiomers Chiral or Achiral? (2 Marks)
Ans. For example, two sheets of paper are achiral. Chiral molecules, on the other hand, are non-superimposable mirror copies of each other, similar to human faces. The mirror image of a Chiral molecule can't be perfectly aligned with it; therefore, the mirror images can't be overlaid. Enantiomers are the photographs in the mirror.
Ques. What are Diastereomers? Give an example. (3 Marks)
Ans. Diastereomers are stereoisomers that are not superimposable on each other and do not replicate each other's pictures. Stereoisomers of two or more stereocenters can be diastereomers. It can be difficult to tell whether two molecules are diastereomers. A classic example is the structures of cis-2-butene and trans-2-butene. When the CH3 group is on the same side of the center, the compound is termed cis whereas, when one of the CH3 atoms is switched with the hydrogen atom, it is called trans. The diastereomers have other structures apart from the cis and trans structures.
Diastereomers
Ques. Are enantiomers chemically different? (1 Mark)
Ans. Apart from their influence on plane-polarized light (they twist the polarization axis in equal and opposite amounts) and interactions with other chiral compounds, enantiomers have the same chemical and physical properties.
Ques. How are enantiomers different from other isomers? (2 Mark)
Ans. The structure of enantiomers differs in the stereogenic core (R or S). The enantiomers have identical chemical and physical properties in an achiral environment. Enantiomers interact differently with other chiral compounds by rotating planar polarized light at equal but opposite angles.
Ques. What is the difference between R and S enantiomers? (1 Mark)
Ans. The "right hand" and "left hand" nomenclature are used to name the enantiomers of a chiral chemical. The stereocenters are denoted by the letters R or S. If the arrow points in a counterclockwise direction (left after leaving the 12 o'clock position), the stereocenter arrangement is termed S ('Sinister' Latin='left'), while R symbolizes a clockwise direction.
Ques. What is the chemical nature of the enantiomers? (2 Marks)
Ans. It is well known that stereoisomeric chemical substances with differing enantiomeric structures regularly participate in chemical reactions with other enantiomeric chemicals. Enantiomers are now thought to be specific biological compounds. It's also worth remembering that two different enantiomers of the same chemical substance might have radically different effects on numerous species. This phenomenon is frequently observed in the effects of numerous drugs on humans. Only one of a drug's enantiomers may be able to provide the physiological advantages required in some circumstances.
Ques. Give examples of enantiomers. (2 Marks)
Ans. Two examples of enantiomers are mecoprop and lactic acid which is also known as milk acid. It has two enantiomers, the left enantiomer is (S)(+) and the second one is (R)(-).
Ques. State the difference between enantiomers and diastereomers. (5 Marks)
Ans. The difference between enantiomers and diastereomers are:
| Enantiomers | Diastereomers |
|---|---|
| They are each other’s non-superimposable mirror image structures | They are non-superimposable as well as non-mirror image structures. |
| They have similar chemical and physical characteristics. | They have different chemical and physical characteristics. |
| They are optically active. | They may or may not be optically active |
| They are racemic mixture formations. | They are not racemic mixture formation. |
Ques. Name the four types of stereoisomers. (2 Marks)
Ans. The four types of stereoisomers are:
- Conformational isomers
- Cis-trans isomers
- Diastereomers
- Enantiomers
Ques. What are R and S Enantiomers? (2 Marks)
Ans. In order to label the enantiomers of a chiral compound, the "right hand" and "left hand" nomenclature are used in which stereocenters are labeled with R or S. The stereocenter configuration is considered S ('Sinister' meaning “left” in Latin) if the arrow points in a counterclockwise direction while R represents a clockwise direction.
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