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Optical Isomerism

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Optical isomerism is a kind of stereoisomerism. Now, before going to learn about optical isomerism, let us quickly recall what isomers and stereoisomers are. Isomers are the type of compounds having the same molecular formula but different bonding arrangements among the atoms. Whereas, in stereoisomers, both the molecular formula and bonding arrangement of atoms are similar. 

However, they have various spatial (three dimensional) atoms. It eliminates all various arrangements that are simply because of the molecule spinning in its entirety or revolving around the unique bonds.


What Is Optical Isomerism?

To define optical isomerism, it is a case where the isomers exhibit identical characteristics in terms of the molecular weight and the chemical and physical properties as well. However, they differ in their rotation effect of polarized light.

Optical isomerism mainly occurs in the substances that have similar molecular and structural formulas, but they can’t be superimposed on each other. To keep it simple, we can say that they mirror the images of each other. Alternately, it can also be found in the substances with an asymmetric carbon atom.

Typically, optical isomerism is exhibited by the stereoisomers that rotate the plane of polarized light. If the same plane of polarized light travelling through an enantiomer solution rotates in the clockwise direction, the enantiomer is then said to exist as (+) form, and if the plane of polarized light rotates in the anti-clockwise direction, the enantiomer is known to exist in (-).

For example, an enantiomer of alanine (otherwise called amino acid), which rotates the plane of polarized light in a clockwise and anti-clockwise direction, can be represented as (+) alanine and. (-) alanine respectively.

The rotation extent of plane-polarized light by the two enantiomeric forms is precisely the same whereas, the direction of rotation is opposite. Furthermore, if the two enantiomer pairs are present in an equal amount, then the resultant mixture is known as a racemic mixture. It means 50% of the mixture exists in (+) form, and the remaining 50% exist in (-) form.

Since the racemic mixture rotates the plane of polarized light equally towards the opposite direction, the net rotation remains zero. Therefore, the racemic mixture is optically inactive.

A chemical sample is considered as enantiopure (also termed as enantiomerically pure) when it has, within the detection limits, molecules of only one chirality.


Origin Of Optical Isomers

To define whether the compound is active optically or not, first, we have to see whether the carbon is attached to the four different groups or not. To understand optical isomerism in a better way, let us take an example of two models of organic compounds as represented below.

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These two models (A and B) have the same bonding arrangement of the atom, but it has a different spatial arrangement. From the above-given model, it is clear that the arrangement of the blue and orange groups in space is different. 

So, is it possible to align model A precisely like model B by rotating it? The answer is obviously no. Why because, if we rotate A, the arrangement of other groups gets disturbed as represented below.

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We cannot make the spatial arrangement of models A and B the same by rotating them in any direction. Both the models' A and B are said to be non-superimposable because we cannot make them look exactly.

Now, let us see what will happen if a molecule containing two similar groups attached to the central carbon atom is rotated, which is shown in the below figure.

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Rotating the molecule A by 180 degrees will give a similar atom arrangement as that of B, as given below.

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


Chiral and Achiral Molecules

The difference between the chiral and achiral molecules can be defined based on the symmetry plane. If all the attached groups to the central carbon atom are different, then there exists no plane of symmetry. Such a molecule is called a chiral molecule.

If all the groups attached to the central carbon atom are not completely different, then there exists a plane of symmetry. Such molecules are known as achiral molecules. It is also clear that only the molecule having a chiral centre will exhibit optical isomerism, which is shown below.

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Relationship Between The Enantiomers

Enantiomers are the type of stereoisomers where two molecules are non-superimposable mirror images of each other.

In other terms, one of the enantiomers is a mirror image of another, which cannot be superimposed. In other terms, if a mirror looks at one isomer, it would be able to see the other. The two isomers (the original ones and its mirror images) contain a different spatial arrangement.

FAQ (Frequently Asked Questions)

1. Does optically inactivity mean it cannot show Optical Isomerism?

Ans: No, there are nearly optical isomers always of optically active compounds, which are optically inactive. Perhaps, these are the compounds having both optically active and inactive stereoisomers. For example, in symmetrical molecules, there are 2 asymmetric carbons. 2, 4-dichloropentane is an example compound that has 2 asymmetric compounds. So we would expect 4 stereoisomers, but there exist only 3, 1 optically inactive isomer, called a meso-isomer, and 2 optically active stereoisomers, which are the mirror image of each other.

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As the meso form has an internal mirror plane (perpendicular to the screen via C3), it cannot be optically active isomers (those 2 things exclude each other: optical activity means the absence of a mirror plane). Therefore, the other 2 stereoisomers do not have such a mirror plane and are optically active.

2. What is the difference between Optical Stereoisomers and Isomers?

Ans: Stereoisomers are any kind of structural isomers differing in the manner by which their atoms are arranged. These include E-/Z- isomerism, cis-/trans-, optical isomerism, and many more.

Optical isomers (also called enantiomers) are a special class of stereoisomers that can rotate plane-polarized light while passing through the isomeric compound if one type of isomer is more prevalent to that of another. In Biochemistry, mainly, there are two types of optical isomers - D & L type isomers. The former rotates plane-polarized light clockwise and the latter rotates in counter-clockwise.

A racemic mixture exists when there is an equal proportion (50%) of each isomer type. So, no apparent rotation of the plane-polarized light occurs. In many optical isomeric organic compounds, the carbon atom that is responsible for causing the isomers to be formed is called chiral.