What is optical isomerism?

optical isomer is a type of spatial isomerism whose main objective is to study the behavior of molecules that present asymmetry, that is, that do not overlap each other.

left hand mirror image

Molecules that are not superimposable (asymmetric molecules) can be compared to the right hand and its mirror image. In the image above, we have that the mirror image of the left hand is the right hand, which do not overlap.

The main differences between molecules that present optical isomerism are in the influence that they cause or not in the polarized light, being able to deflect or not light, and in relation to their physical and chemical properties.

To claim that a substance has optical isomerism, we must look for in its molecules the presence of at least one asymmetric carbon (also called chiral carbon), which is a carbon that has four different ligands, as in the following example:

Carbon with four different binders

The carbon above has the hydrogen (H), methyl (CH) ligands₃), bromine (Br) and ethyl (H₃C-CH₂) and therefore is a chiral carbon.

See now the types of isomers studied in Optical Isomerism:

Active optical isomers

Also called optical antipodes, enantiomorphs or chiral molecules, are organic molecules that have the ability to shift the direction of polarized light to the left or right.

The molecule of an organic compound that is able to bend light to the right is called a right-handed. The molecule of the same organic compound that is capable of shifting light to the left is called levorotary.

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Every organic compound that has a chiral carbon automatically has a right-handed molecule and a left-handed molecule. However, if a certain organic compound has more than one chiral carbon, we have to use the formula proposed by scientist Van't Hoff to determine the number of right-handed molecules and levoreiras. Look:

IOA = 2^Ç

• IOA = optically active isomers;

• C = number of chiral carbons in the molecule.

Inactive optical isomers

Also called racemic mixture, in the inactive optical isomers, we have the presence of the same number of right-hand and left-hand molecules in a container. This mixture does not have the ability to shift the direction of polarized light either to the left or to the right.

According to the rule proposed by scientist Van't Hoff, to determine the number of racemic mixtures formed by an organic compound, just use the following expression:

IOI = 2^Ç
2

• IOI = optically inactive isomers;

• C = number of chiral carbons in the molecule.

meso isomer

meso isomer is the molecule that has two equal chiral carbons, that is, both carbons have the same four ligands, as in the following example:

Tartaric acid has a meso isomer

The ligands for carbons 2 and 3 are exactly the same: a hydroxyl (OH), a carboxyl (COOH), a hydrogen (H) and the carbo-hydroxy-carboxyl group (CHOHCOOH).

Since the meso isomer has two equal chiral carbons, the effect that one of these carbons has on polarized light is canceled out by the effect of the other carbon. Therefore, it is an optically inactive isomer.

By Me. Diogo Lopes Dias

Would you like to reference this text in a school or academic work? Look:

DAYS, Diogo Lopes. "What is optical isomerism?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/quimica/o-que-e-isomeria-optica.htm. Accessed on June 28, 2021.