Optical Isomerism

Optical Isomerism is a property of Organic Compounds in which they have the same molecular and structural formula but they can’t superimpose on each other. This is due to the different arrangement of carbon atoms in three-dimensional space. The structures exhibiting Optical isomerism are called Optical Isomers. Optical Isomerism is one of the most important concepts in Organic Chemistry and helps to understand various difficult reactions.

In this article, we will learn what is optical isomerism, its significance, types, examples and many more things.

What is Optical Isomerism?

Optical Isomerism is a kind of stereoisomerism which occurs when molecules are mirror images of one another but are not superimposable, despite sharing the same molecular and structural formulas. Because of their chiral centre, these molecules are also referred to as chiral molecules.

Optical Isomerism Definition

Optical Isomerism is defined as the property of organic compounds in which they have same molecular and structural formula but have non-superimposable images

Significance of Optical Isomerism in Chemistry

Optical isomerism is significant in substances that rotate the plane of polarized light, and the amount of rotation is dependent on various factors such as the concentration of the substance in the solution, the path length of the sample tube, the wavelength of the incident light, and the nature of the substance itself.

Optical Isomerism Example

One of the example of Optical isomerism is Butan-2-ol where the four carbons can be arranged in 3D space in two ways such as they are mirror image of each other and also non-superimposable.

Optical Isomer Definition

An optical isomer, is a stereoisomer that is non-superimposable onto its own mirror image. They are also known as antipodes or optical antipodes.

Optical Isomer Example

One of the best examples of Optical Isomer is D and L Glyceraldehyde which is discussed below:

Glyceraldehyde exists in two isomerism forms that are mirror images of each other. These isomers are referred to as D and L Glyceraldehydes. This absolute configuration is defined because of the OH group present on the second carbon. When the OH group is present on the right side of the molecules, it is considered D-Glyceraldehyde; when it’s present on the left side, it is considered L-Glyceraldehyde.

Chiral and Achiral Molecules

Chilarity is an important geometric property of a molecule’s symmetry. On this basis, molecules are divided into two types which are:

• Chiral Molecules
• Achiral Molecules

Chiral molecules

Chiral molecules are molecules that have a non-superimposable mirror image. They are frequently identified as “right-handed” or “left-handed” based on some other criterion or their absolute configuration.

One or more chiral centres can be found in chiral molecules, which are nearly always tetrahedral (sp3-hybridized) carbons with four distinct substituents. The presence of an asymmetric carbon atom is often the feature that causes chirality in molecules. Chiral molecules are optically active, meaning they rotate polarized light.

Achiral molecules

Achiral molecules are superimposable with their mirror images, meaning they can be rotated to the same shape. These molecules have a plane of symmetry or a centre of symmetry. They may have chiral conformations but are not chiral due to their symmetry. They do not rotate plane-polarized light. Examples of achiral molecules include meso compounds, which have a stereocenter but are not chiral due to their symmetry.

What are Enantiomers?

Enantiomers are non-superimposable mirror images of each other, much like a person’s right and left hands. They are often compared to hands because, without mirroring, one cannot be superimposed onto the other.

Relationship between Enantiomers

Enantiomers have distinct effects and interactions with other chiral molecules. They are distinguished by their ability to rotate plane-polarized light to equal but opposite angles, a property known as optical activity. When two enantiomers are present in equal proportions, they form a racemic mixture, which does not rotate polarized light because the optical activity of each enantiomer cancels out the other.

Enantiomers are chemically identical in every other respect, but they can have different effects on biological systems, making them significant in pharmaceuticals and other fields. The prefix “enantio-” designates the mirror-image relationship between enantiomers.

Examples of Enantiomers

The two examples of Enantiomers are:

• Dextro Lactic Acid and Laevo Lactic Acid
• Carvone and Limonene

Dextro Lactic Acid and Laevo Lactic Acid

Here in the first molecule the OH group is present on the right side of the carbon atom hence it is called D-Lactic acid and on the hand, the second molecule has the OH group present on the second carbon therefore it is called as L-Lactic acid.

Carvone and Limonene

Carvone can be synthesized from limonene and it was found to have a spearmint odor. It indicates that carvone is an optical isomer of limonene, and the synthesis of carvone from limonene demonstrates their relationship as enantiomers.

Types of Optical Isomers

Compounds showing optical isomers can be distinguished mainly into two types, which are:

• D-isomers
• L-isomers

D-Isomers

• D-isomers, also known as D-enantiomers, are a type of optical isomer. They are mirror images of L-isomers.
• In the case of monosaccharides, most naturally occurring sugars are D isomers, except for fucose. D-methamphetamine is an example of a chiral molecule that exists as a D-isomer (Dextro, or D-methamphetamine).
• D-isomers are a type of optical isomer that is important in pharmacology and are found in various molecules.

L-Isomers

• L-isomers, also known as L-enantiomers, are a type of optical isomer that spin plane-polarized light to the left (anticlockwise). They are denoted by the sign “L” and are often referred to as the “I-form.” In the context of carbohydrates
• In pharmacology, L-isomers are significant as chiral forms of molecules, and they have different properties compared to their D-isomeric counterparts.
• For instance, L-isomers of amino acids have been used as chiral selectors for direct enantioresolution in thin-layer chromatography (TLC)

Optical Isomers in Coordination Compounds

There are three types of optical isomers in coordination compounds:

• Octahedral Complexes
• Cis-Trans Isomers
• Square Planar Complexes

Octahedral Complexes

Octahedral complexes of type [M(xx)₃]n±, [M(xx)₂AB]n±, and [M(xx)₂B₂]n± exhibit optical isomerism. For example, the optical isomers of [Co(en)₃]³⁺ are shown below:

Cis-Trans Isomers

Cis-trans isomers exhibit optical isomerism when the whole molecule is asymmetric. For example, two geometrical isomers are possible in a coordination compound of type [Co(NH₃)₄(H₂0)₂]³⁺. They are cis and trans. The cis isomer shows optically active isomerism among these two isomers because the whole molecule is asymmetric.

Square Planar Complexes

Square planar complexes of type [MX₂L₂] exhibit optical isomerism when the ligands L are different. For example, the complex [NiCl2(en)₂] has two optical isomers.

Structural Isomers vs Optical Isomers

Structural and optical isomers are two types of stereoisomers that differ in their arrangement in three-dimensional space. Here are the main differences between them:

Difference between Structural Isomers and Optical Isomers
Structural Isomers	Optical Isomers
Also known as constitutional isomers	Also known as enantiomers or diastereomers
These isomers have the same molecular formula but different structures, resulting in different arrangements of atoms.	These isomers have the same molecular formula and the same arrangement of atoms, but they differ in the placement of substituted groups around one or more atoms of the molecule.
The chemical formula of a compound gives the chemical elements present in the compound and the number of atoms per chemical element.	Optical isomers rotate plane-polarized light in different directions and are related as non-superimposable mirror images.
Examples of structural isomers include 1-butene and 2-butene, which have the same chemical formula but different arrangements of atoms.	Examples of optical isomers include glyceraldehyde, which has D and L isomers, and alpha and beta glucose.

Learn, Geometric and Optical Isomerism

Explain Optical Isomerism in 2-Chlorobutane

Optical isomerism in 2-chlorobutane results from a chiral centre at the second carbon atom, which has four groups attached. Due to this, 2-chlorobutane exhibits optical isomerism and has two enantiomers. The spatial arrangement of the four different groups (chlorine, hydrogen, and methyl) around the chiral atom is different in the two enantiomers. The structure of 2-chlorobutane and its mirror image cannot superimpose perfectly, making the compound optically active.

Explain Optical Isomerism in Butan-2-ol

In butan-2-ol, the second carbon atom has a hydroxy group, an ethyl group, a methyl group, and a hydrogen attached, making it an asymmetric carbon atom. This results in the molecule having two optical isomers, also known as enantiomers. To identify the optical isomers of butan-2-ol, consider the following skeletal formula:

In this formula, the second carbon atom (the one with the -OH attached) has four different groups around it, making it a chiral centre

Optical Isomerism in Butan-2-ol

Applications of Optical Isomerism

Here are some of the critical applications:

• Pharmaceuticals: Optical isomerism is crucial in the pharmaceutical industry as enantiomers of a drug molecule can exhibit different pharmacological activities. For example, one enantiomer may be therapeutically effective while the other may be inactive or even show adverse effects. Separating and using only the desired enantiomer is essential to ensure the safety and adequacy of pharmaceuticals.
• Chemical Synthesis: In chemical synthesis, the production of single enantiomers is often a focus due to their specific properties. Modern chiral synthesis techniques are used to separate and produce single enantiomers, which are essential in drug discovery and development.
• Coordination Chemistry: Optical isomerism is also relevant in coordination chemistry, particularly in the study of metal complexes. Metal complexes can exhibit optical isomerism, which affects their reactivity and properties.
• Biological Systems: Optical isomerism is significant in biological systems, as enantiomers can have different interactions with enzymes and other biological molecules. It has implications for understanding biological processes and developing pharmaceuticals.

Optical Isomerism – FAQs

1. What is Optical Isomerism?

Optical isomerism occurs when molecules, having non-superimposable mirror images and identical formulas, are chiral due to a unique center. This chirality results in optical activity, allowing the rotation of polarized light.

2. How does Chirality contribute to the Formation of Optical Isomers?

Chirality, found in molecules with a tetrahedral chiral center, leads to optical isomers—non-superimposable mirror images. This asymmetry arises from distinct substituents, forming enantiomers with differing arrangements, crucial for optical isomerism.

3. How many Optical Isomers are possible for Glucose?

For glucose, there are 16 possible optical isomers because it has four chiral centers, and the formula 2n is used to calculate the number of optical isomers (n being the number of chiral centers).

4. What is the difference between Geometrical and Optical Isomerism?

Geometrical isomerism arises from different spatial arrangements due to double bonds or ring structures, while optical isomerism results from non-superimposable mirror images, mainly associated with chiral centers.

5. What is the formula for calculating Number of Optical Isomers?

The formula for calculating the number of optical isomers (N_opt) is 2ⁿ, where n is the number of chiral centers in the molecule.

6. How to Find the Optical Isomer of a Compound?

To find the optical isomer of a compound, identify chiral centers, determine their configurations (R or S), then create a mirror image with opposite configurations to obtain the enantiomer.

7. How many optical isomers are possible for glucose?

Glucose has four chiral carbon atoms, so it can have 2⁴ = 16 optical isomers. However, due to symmetry, only 2 of them are unique.

8. How many Optical isomers of Tartaric Acid are Possible?

Tartaric acid has two chiral carbon atoms, resulting in 2² = 4 possible optical isomers. These isomers exist as pairs of enantiomers.

9. What are the Condition for Optical Isomerism?

Optical isomerism occurs when a molecule has chiral centers (asymmetric carbon atoms) and lacks a plane of symmetry. In simple terms, it needs to be non-superimposable on its mirror image for optical isomerism to be possible.

10. What are two Types of Optical Isomerism?

The two types of Optical Isomerism are D and L Isomerism named as Dextrorotatry and Laevorotatry isomerism