Optical Isomerism

Stereoisomers have the same bonds but differ in 3D arrangement. They are divided into geometric and optical isomers. In this article, learn optical isomers in detail along with its properties and how optical phenomena arise.

1.0What are Optical Isomers?

Compounds that share the same molecular and structural formulas but exhibit different optical activities are called optical isomers. This phenomenon arises due to the spatial arrangement of atoms or groups within the molecules, leading to mirror-image isomers known as enantiomers. Enantiomers rotate plane-polarized light in opposite directions, a property known as optical activity. This unique feature serves as a distinguishing factor between the two isomers, providing a nuanced understanding of their identical yet asymmetrical nature.

Note: Optical isomerism is also observed in coordination compounds.

One of the types of stereoisomers is an optical isomer, otherwise known as an enantiomer. What sets this kind of stereoisomer apart is that it exists in non-superimposable mirror images. The isomers have the same molecular formula and connectivity, but differ in spatial arrangement of atoms, thus they will interact differently with polarized light. They exhibit optical activity, rotating plane-polarized light in opposite directions. A molecule with a chiral center (asymmetric carbon) typically forms optical isomers.

An example would be the enantiomers of chiral molecules such as Lactic acid. Lactic acid had two optic isomers: L-lactic acid and D-lactic acid. These isomers have the same molecular formula and connectivity but differ in the spatial arrangement of atoms around the chiral carbon, making them non-superimposable mirror images from one another.

2.0Properties of Enantiomers 

Enantiomers are like twins with the same looks (molecular formula and connectivity), but they act differently, and they have distinct properties. Which we will discuss in brief-

1. Identical Physical Properties: 

• Enantiomers have identical physical properties including melting point, boiling point, and solubility. This is because these properties depend on the molecular formula and structure, which are the same for enantiomers.

2. Different Optical Activity:

• Enantiomers exhibit opposite optical activities. One enantiomer rotates plane-polarized light clockwise (dextrorotatory, +), while the other rotates it counterclockwise (levorotatory, -). The degree of rotation is equal but opposite.

One illustrative example of enantiomers and their opposite optical activities is found in the compound limonene. Limonene is a common terpene found in citrus fruits and exhibits optical isomerism.

• Dextrorotatory Limonene:

The dextrorotatory form of limonene (designated as (+)-limonene) rotates plane-polarized light in a clockwise direction.

• Levorotatory Limonene:

The levorotatory form of limonene (designated as (-)-limonene) rotates plane-polarized light in a counterclockwise direction.

3. Non-Superimposable Mirror Images:

• Enantiomers are mirror images that cannot be superimposed onto each other. This non-superimposability arises due to their chiral nature and different spatial arrangements.

4. Interaction with Chiral Environments:

• Enantiomers interact differently with other chiral molecules or environments. Biological systems, in particular, often respond differently to enantiomers, leading to variations in pharmacological or physiological effects.

5. Chirality Center Influence:

• Enantiomers arise when there is a chiral center (asymmetric carbon) in the molecule. The spatial arrangement of substituents around this chiral center defines the two enantiomers.
• For example - Optical activity of 2-Chlorobutane the two enantiomers are formed by arranging the substituents (H and Cl) differently around the chiral center. One is (R)-2-chlorobutane, and the other is (S)-2-chlorobutane. They are non-superimposable mirror images of each other with opposite optical activities. 

6. Racemic Mixture: 

• The equimolar mixture of enantiomer is called a racemic Mixture. This Mixture is optically inactive due to external compensation.

7. Different Biological Activities: 

• In biological systems, enantiomers often exhibit different biological activities. One enantiomer may be therapeutic, while the other might be inactive or even harmful. This phenomenon is known as the "chiral switch" and is crucial in drug development.

3.0How Optical Phenomena Arise

Optical Activity 

Optical activity refers to the strength of a substance to rotate the plane of polarized light as it passes through. This phenomenon is commonly observed in chiral compounds, which lack a plane of symmetry and exist as non-superimposable mirror images (enantiomers). One enantiomer will rotate plane-polarized light clockwise (dextrorotary), while the other will rotate it counterclockwise (levorotary). The degree of rotation is measured in degrees and is specific to the particular enantiomer and the experimental conditions.

Important Points to Remember

1. Condition of Optical Activity- For a compound to show optical activity, it must be asymmetrical, lacking a plane of symmetry (POS), center of symmetry (COS), and alternating axis of symmetry (AAOS). These symmetrical features allow the compound to be superimposable on its mirror image, preventing optical activity.

Let’s understand the above condition with few examples

Compound	Centre of Symmetry	Plane of Symmetry	Optical Activity
	Absent	Yes	No
	Absent	Yes	No
	Absent	Yes	No
	Absent	No	Yes

Optically Active Carbon :

An asymmetric carbon atom (*C_abed) is one where a carbon atom is bonded to four different functional groups or element, and it lacks any element of symmetry.

If a molecule contains only one asymmetric carbon atom, the entire molecule becomes chiral and optically active, displaying optical isomers.