ISOMERISM: TYPES AND EXAMPLES OF ISOMERS - PHYSICAL - 2025

The isomerism relates to the existence of two or more substances having the same molecular formula, but whose structure is different in each of the compounds. In these substances, known as isomers, all the elements are present in the same proportion, but forming a structure of the atoms that is different in each molecule.

The word isomer comes from the Greek word isomerès, which means "equal parts." Contrary to what may be assumed, and despite containing the same atoms, isomers may or may not have similar characteristics depending on the functional groups present in their structure.

Two main classes of isomerism are known: constitutional (or structural) isomerism and stereoisomerism (or spatial isomerism). Isomerism occurs both in organic substances (alcohols, ketones, among others) and inorganic substances (coordination compounds).

Sometimes they appear spontaneously; In these cases, the isomers of a molecule are stable and occur under standard conditions (25 ° C, 1 atm), which was a very important advance in the field of chemistry at the time of its discovery.

Types of isomers

As mentioned above, there are two types of isomers that differ by the ordering of their atoms. The types of isomers are as follows:

Constitutional (structural) isomers

They are those compounds that have the same atoms and functional groups but arranged in a different order; that is, the bonds that make up their structures have a different arrangement in each compound.

They are divided into three types: positional isomers, chain or backbone isomers, and functional group isomers, sometimes referred to as functional isomers.

Position isomers

They have the same functional groups, but these are found in a different place in each molecule.

Chain or skeleton isomers

They are distinguished by the distribution of carbon substituents in the compound, that is, by how they are distributed in a linear or branched way.

Functional group isomers

There is an exceptional class of isomerism called tautomerism, in which there is an interconversion of one substance into another that generally occurs by the transfer of an atom between the isomers, causing an equilibrium between these species.

Stereoisomers (space isomers)

This is the name given to substances that have exactly the same molecular formula and whose atoms are arranged in the same order, but whose orientation in space differs from one to the other. Therefore, to ensure their correct visualization, they must be represented in a three-dimensional way.

In general terms, there are two classes of stereoisomers: the geometric isomers and the optical isomers.

Geometric isomers

They are formed by breaking a chemical bond in the compound. These molecules are presented in pairs that differ in their chemical properties, so to differentiate them the terms cis (specific substituents in adjacent positions) and trans (specific substituents in opposite positions of their structural formula) were established.

In this case, the diastereomers stand out, having different configurations and are not superimposable with each other, each with its own characteristics. There are also conformational isomers, formed by the rotation of a substituent around a chemical bond.

Optical isomers

They are those that constitute mirror images that cannot be superimposed; that is, if the image of one isomer is placed on the image of the other, the position of its atoms does not agree exactly. However, they do have the same characteristics, but differ by their interaction with polarized light.

In this group, the enantiomers stand out, which generate the polarization of light according to their molecular arrangement and are distinguished as dextrorotatory (if the polarization of the light is in the right direction of the plane) or left-handed (if the polarization is in the left direction of the plane).

When there is the same amount of both enantiomers (dyl), the net or resulting polarization is zero, which is known as a racemic mixture.

Examples of isomers

First example

The first example presented is that of structural positional isomers, in which there are two structures with the same molecular formula (C ₃ H ₈ O) but whose -OH substituent is found in two different positions, forming 1-propanol (I) and 2-propanol (II).

Second example

In this second example two structural chain or skeleton isomers are observed; both have the same formula (C ₄ H ₁₀ O) and the same substituent (OH), but the isomer on the left is straight-chain (1-butanol), while the one on the right has a branched structure (2-methyl-2 -propanol).

Third example

Two structural functional group isomers are also shown below, where both molecules have exactly the same atoms (with molecular formula C ₂ H ₆ O) but their arrangement is different, resulting in an alcohol and an ether, whose physical and chemical properties they vary greatly from one functional group to another.

Fourth example

Also, an example of tautomerism is the equilibrium between some structures with functional groups C = O (ketones) and OH (alcohols), also called keto-enolic equilibrium.

Fifth example

Next, two geometric isomers cis- and trans- are presented, noting that the one on the left is the cis isomer, which is denoted by the letter Z in its nomenclature, and the one on the right is the trans isomer, denoted by the letter AND.

Sixth example

Now two diastereomers are shown, where the similarities in their structures are noted but it can be seen that they cannot overlap.

Seventh example

Finally, two carbohydrate structures are observed that are optical isomers called enantiomers. The one on the left is right-handed, since it polarizes the plane of light to the right. On the other hand, the one on the right is left-handed, because it polarizes the plane of light to the left.

References

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