Isomerism: Types and Examples of Isomers

The isomería refers to the existence of two or more substances that have 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 an arrangement of atoms that is different in each molecule.

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

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

Sometimes they occur spontaneously; in these cases the isomers of a molecule are stable and present 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 stated above, two types of isomers are presented that differ by ordering their atoms. The types of isomers are the following:

Constitutional isomers (structural)

Are those compounds that have the same atoms and functional groups but arranged in a different order; that is, the links that make up its structures have a different arrangement in each compound.

They are divided into three types: position isomers, chain or skeletal isomers and isomers of functional groups, sometimes called functional isomers.

Position isomers

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

Chain or skeletal isomers

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

Functional group isomers

Also called functional isomers, they are made up of the same atoms, but these form different functional groups in each molecule.

Tautomería

There is an exceptional kind of isomerism called tautomería, in which there is an interconversion of one substance into another that is generally given by the transfer of an atom between the isomers, causing a balance between these species.

Stereoisomers (spatial isomers)

It is called this way to the substances that have exactly the same molecular formula and whose atoms are arranged in the same order, but whose orientation in space differs between one and 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: geometrical isomers and 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, which have different configurations and are not superimposable, each with its own characteristics. Also found are the conformational isomers, formed by the rotation of a substituent around a chemical bond.

Optical isomers

They are those that constitute mirror images that can not overlap; that is to say, that if the image of an 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 they are differentiated by their interaction with polarized light.

In this group the enantiomers stand out, which generate the polarization of the 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 levorotatory (if the polarization is in the left direction) of the plane).

When there is the same amount of both enantiomers (d and l) the net or resultant polarization is zero, which is known as the racemic mixture.

Examples of isomers

First example

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

Second example

In this second example, two structural chain or skeletal 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 isomers of functional group are also shown below, where both molecules have exactly the same atoms (with molecular formula C). ₂ H ₆ O) but its arrangement is different, resulting in an alcohol and an ether, whose physical and chemical properties vary greatly from one functional group to another.

Fourth example

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

Fifth example

Next, two geometrical cis- and trans- isomers are presented, observing 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 these can not overlap.

Seventh example

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

References

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