Optical Isomerism: Definition, Occurrence, Molecules and Examples

Optical isomerism is basically a form of stereoisomerism. To understand the concept of optical Isomerism, we need to recall what isomers and stereoisomers are. Isomers are those compounds which have a similar subatomic equation but a unique holding course of action among molecules. While in stereoisomers, both molecular formula and bonding arrangements of atoms are the same. Optical isomers take place generally in substances that have identical molecular and structural formulae, but they can not be superimposed on each other. It can also be found in the substances that have an asymmetric carbon atom. Let’s have a closer look at the concept along with some important questions.

Key Terms: Organic chemistry, atoms, carbon, polarized light, stereoisomerism, Optical isomerism, Isomer, isomerism, Optical isomers

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What is Optical Isomerism?

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The compounds that contain an asymmetric carbon atom from stereoisomers, this type of isomerism is called optical isomerism. In other words, optical isomers is a form of stereoisomerism and occurs as a result of chirality of molecules, limited to molecules with a single chiral centre. Optical isomers are named like this due to their effect on plane polarized light. 

Basically, optical isomerism is shown by stereoisomers which rotate the plane of polarized light. If the plane of the polarized light that passes through enantiomer solution, rotates in a clockwise direction, then this enantiomer will exist as positive form and if the plane of the polarized light rotates in an anti-clockwise direction, then the enantiomer will exist in negative form.

For instance, an enantiomer of alanine (amino corrosive) which turns the plane of polarized light in clockwise and anticlockwise can be expressed as (+) alanine and. (- ) alanine respectively. The rotational extent of plane polarized light is exactly the same, however the direction of rotation is opposite. Furthermore, if two enantiomer pairs exist in equal amounts, then the resultant mixture is called the racemic mixture. By this, it is clear that 50% of the mixture is in positive form, while the other 50% is in negative form. As the plane of polarized light is made rotated by the racemic mixture equally in the opposite direction, the net rotation is zero. Thus, the racemic mixture is optically inactive.

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Optical Isomers Occurrence

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In order to determine if the compound is optically dynamic or not, we need to first see whether the carbon is connected to four distinct groups or not. We can understand the optical isomerism in a better way by an example of two models of organic compound as shown below:

These two models have similar holding arrangements of the atom however the spatial arrangement is different. From the above model of A and B, the spatial arrangements of blue and orange are different. This is because if we rotate A, the arrangement of the other group gets disturbed.

It is not possible to make the spatial arrangement of A and B exactly the same by rotating them in any direction. A and B are supposed to be non-superimposable on the grounds that we can't make them look similar. 

Now let us see if a molecule containing two same groups connected to a central carbon atom is rotated as shown below:

Turning particle A by 180 degrees will give the exact same arrangement of atom as that of B:

From the above explanation, we can presume that the compound will be optically active if and only if all the groups attached to the central carbon atom are different. 

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Chiral and Achiral Molecules

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We can explain the difference between chiral and achiral based on the plane of symmetry. If each attached group to the central carbon atom is different, then there will be no plane of symmetry. Such a molecule is called a chiral molecule.

On the other hand, if all the attached groups to the central carbon atom are not different, then there will be a plane of symmetry and these molecules are called achiral molecules. Unmistakably, only molecules having a chiral centre will show optical isomerism.

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Relation between the Enantiomers

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Enantiomers are a type of stereoisomers wherein two atoms are a non-superimposable identical representation of one another. All in all, one of the enantiomers is an identical representation of the other which can't be superimposed. All in all, if a mirror takes a gander at one isomer, it would see the other. The two isomers (the first and its identical representation) have an alternate spatial course of action.

The vital distinction among optical and mathematical isomerism is that optical isomers are sets of mixtures which show up as identical representations of one another while mathematical isomers are sets of mixtures containing the equivalent substituents joined to a carbon-carbon twofold bond in an unexpected way.

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Steps to Identify Optical Isomerism

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Optical isomerism is characterized as the kind of isomerism where the isomers have indistinguishable sub-atomic weight though the substance and actual properties are additionally the same. Be that as it may, they vary in their impact on the plane of polarized light. 

Optical isomerism is seen primarily on the substances that have a similar sub-atomic and underlying equation, yet they can't be superimposed on one another. As such, we can say that they are perfect representations of one another or non-very imposable perfect representations of one another. On the other hand, it can likewise be found in substances that have a hilter kilter carbon molecule.

Commonly, optical isomerism is shown by stereoisomers which pivot the plane of polarized light. On the off chance that the plane of enraptured light going through enantiomer arrangement pivots the clockwise way then the enantiomer is said to exist as (+) structure and assuming the plane of energized light turns in enemy of clockwise course, the enantiomer is said to exist in (- ). This implies that the optical isomers are optically dynamic. 

Optical isomers are two mixtures which contain similar numbers and sorts of particles, and bonds yet vary in the course of action of the molecules. Each non-superimposable identical representation structure is called an enantiomer. Atoms or particles that are available in optical isomers are called chiral.

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Things to Remember

• The topic “Optical Isomerism” is covered in Chapter 9 i.e, Coordination Compounds of NCERT Class 12 Chemistry.
• Candidates can expect 4-5 marks from this portion in the exam.
• Optical isomerism is a form of stereoisomerism.
• Isomers are the molecules that possess the same molecular formula but have different atomic arrangements in space.
• Optical isomers are named because of their effect on plane polarized light. 
• Simple substances that exhibit optical isomerism exist as two isomers are known as enantiomers.

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