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Enantiomers are chemical isomers that are non-superimposable mirror reflections of one another. As a result, two enantiomorphs of a chemical substance will have the same chemical bonds but radically dissimilar three-dimensional structures.
- It is vital to note that enantiomers are isomers that are not exactly the same as each other and cannot be overlaid on each other.
- They are similar to one's right and left hands; without mirroring one of them, hands cannot be superposed with another.
- Enantiomers are also known as optical isomers, antipodes or optical antipodes.
- It has chiral carbon located at the centre and four other atoms attached to it.
- They are a pair of molecules with different optical activities and interact uniquely with polarized light.
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Key Terms: Enantiomers, Examples of Enantiomers, Isomers, Chemical bonds, Stereoisomers, Atoms, Molecules, Polarised light, Dextro Lactic Acid, Laevo Lactic Acid, Limonene
What are Enantiomers?
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Enantiomers are chemical isomers that are non-superimposable mirror reflections of one another. This sort of organic material is chemically and physically comparable.
- Furthermore, the number of atoms in the chemical formula is the same.
- The only difference is their orientation.
- They share the same features and are chemically identical.
- It's merely that the same molecule has been reversed laterally to generate a new one.
A pair of such substances can only be distinguished by throwing a polarised light on them. The light will be turned in one direction. Based on that orientation, one enantiomer is known as Dextro (+ or d), while the other, which rotates the polarised light in the opposite way, is known as Levo (+ or l).
- To identify these optical isomers, the term 'rotatory' is used after Dextro or Levo.
- A racemic mixture occurs when both enantiomers are in the same solution concentration.
- This mixture has no optical activity because the ability of both chemicals to spin polarised light is negated by each other.
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Examples of Enantiomers
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A basic example of a pair of enantiomers is dextro lactic acid and laevo lactic acid, whose chemical structures are given below.
Another important example of an enantiomer pair is provided below.
- These isomers are known as S- and R-methylchlorophenoxypropionic acid (often abbreviated to MCPP and referred to as mecoprop).
- This molecule is known to be a combination of S- and R- enantiomers, with the R- enantiomer being herbicidal in nature.
- As a result, this chemical is commonly employed as a herbicide.
- It is worth noting that, in contrast to cis and trans isomers, practically all pairings of enantiomers have similar physical qualities, such as solubility and melting point.
- They are known, however, to rotate light in different directions (both the enantiomers of a compound must be optically active).
Other examples of Enantiomers include
Carvone
Carvone tends to have two types of enantiomers: S—and R-. According to earlier studies, carvone is famous for its anti-inflammatory and antiallergic effects. However, its antiallergic effects are still under observation.
Limonene
Another common example of enantiomers is Limonene, which is a compound that is found in citrus fruits. Unlike other compounds, it has two enantiomers where one enantiomer has an orange-like smell, and the other has a lemon-like smell.
Thalidomide
Thalidomide is found in a racemic mixture of R- and S-enantiomers. The R-enantiomer has exhibited sedative effects. Similarly, the (S)-isomer exhibits a teratogenic effect.
Things to Remember
- Enantiomers are chemical isomers that are non-superimposable mirror reflections of one another.
- They have identical chemical and physical properties in an achiral environment.
- The chemical properties of two enantiomers are the same, yet they differ when they interact with different chiral substances.
- By looking at their (R, S) designations, you can tell if molecules are enantiomers or diastereomers.
- A basic example of a pair of enantiomers is dextro lactic acid and laevo lactic acid.
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Sample Questions
Ques: What is the distinction between enantiomers and diastereomers? (2 marks)
Ans: Enantiomers are stereoisomers that exist as mirror images of one other and are not superimposable on each other. Diastereomers, on the other hand, are stereoisomers with at least two stereocenters. It is vital to notice that these isomers are not mirrored images of one another.
Ques: Are enantiomers chiral? (2 marks)
Ans: Enantiomers are chiral pairings of stereoisomers. A chiral molecule and its mirror image are not superimposable, implying that the mirror image is a distinct molecule. The center atom in such compounds is known as the chiral or stereocenter.
Ques: What are the similarities between enantiomers? (2 marks)
Ans: In an achiral environment, enantiomers have identical chemical and physical properties. Enantiomers interact with other chiral compounds differently because they spin the direction of plane polarised light to equal but opposite angles.
Ques: What are R and S enantiomers? (1 mark)
Ans: The enantiomers of a chiral chemical are named using the "right hand" and "left hand" nomenclature. The stereocenters are denoted by the letters R or S.
Ques: Is it true that all enantiomers are optically active? (2 marks)
Ans: Chiral molecules are frequently optically active, and two enantiomers will rotate light in different directions; consequently, enantiomers are also known as optical isomers. A racemate or racemic mixture is a 50:50 mixture of both enantiomers that does not rotate the light plane.
Ques: What are the different kinds of enantiomers? (2 marks)
Ans: Stereoisomers are isomers in which the atoms in two molecules have the same order but are arranged differently in space. Diastereomerism (including 'cis-trans isomerism') and optical isomerism (also known as 'enantiomerism' and 'cis-trans isomerism') are the two primary kinds of stereoisomerism.
Ques: How do you identify enantiomers? (2 marks)
Ans: The (R, S) designations of molecules indicate whether they are enantiomers or diastereomers. Enantiomers are mirror representations of each other that cannot be superimposed.
Ques: Is it true that diastereomers are optically active? (2 marks)
Ans: The answer is that both enantiomers are diastereomers. The meso molecule in each example is not optically active, although its diastereomeric partner is. It is even conceivable to have diastereomeric pairings with no optically active members. They're both meso compounds that are optically inactive.
Ques: What are chemical properties of enantiomers? (3 marks)
Ans: The chemical properties of two enantiomers are the same, yet they differ when they interact with different chiral substances. Consider two persons shaking hands with their right hands. The gesture would be considerably different if the other person shook hands with his left hand and the other person shook hands with his right. Similarly, when a pair of enantiomers combines with another chiral substance, the chemical reactions are different.
Ques: What are the physical properties of enantiomers? (5 marks)
Ans: The physical properties of enantiomers are as follows:
- They exist in pairs and are physically and chemically similar compounds.
- They are classified as dextrorotatory or levorotatory chemicals based on how they bend polarised light.
- All physical properties are the same, including melting point, condensation point, boiling point, NMR spectra, and infrared absorptions.
- Even if the melting points of the constituent compounds are comparable, the melting point of their mixture may differ.
- Only chiroptical techniques can tell the difference between two enantiomers.
- The optical rotation of polarised photons is one such approach.
- These chiroptical properties are affected by bond angles, bond lengths, and the magnitude and sign of torsional angles.
Ques: Explain the structure of enantiomers? (4 marks)
Ans: The structure of enantiomers are as follows:
- The non-superimposable mirror copies of one another, known as stereoisomers, were initially introduced as enantiomers.
- Chiral molecules are those that exist as a pair of enantiomers and cannot be superimposable on their mirror counterpart.
- Conversely, an achiral molecule is any one that can be superimposable on its mirror counterpart.
- A molecule with a single atom tetrahedrally bonded to four different substituents can have two enantiomers.
- The structure becomes achiral when all four substituents are different from each other and can be superimposed on its mirror image.
- The distinctive quality of a molecule that cannot be centered on a single atom or a set of atoms is known as chirality.
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