The word isomer comes from the Greek word isomerès , which means “equal parts”. Contrary to what might 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 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 in inorganic substances (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 constituted a very important advance in the field of chemistry at the time of its discovery.
Types of Isomers
As stated before, two types of isomers are presented that differ by the arrangement of their atoms. The types of isomers are as follows:
Constitutional (structural) isomers
These are 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: position isomers, chain or skeleton isomers, and functional group isomers, sometimes called functional isomers.
They have the same functional groups, but these are found in a different place on each molecule.
Chain or backbone isomers
They are distinguished by the distribution of carbon substituents in the compound, that is, by the way they are distributed linearly or branched.
Functional group isomers
Also called functional isomers, they consist of the same atoms but form different functional groups on each molecule.
There is an exceptional class of isomerism called tautomy, in which there is an interconversion of one substance into another that is usually given by the transfer of an atom between the isomers, causing an equilibrium between these species.
Stereoisomers (spatial isomers)
This is the name of substances that have exactly the same molecular formula and whose atoms are arranged in the same order, but whose orientation in space differs between them. Therefore, to ensure their correct visualization, they must be represented in a three-dimensional way.
Broadly speaking, there are two types of stereoisomers: geometric isomers and optical isomers.
They are formed by breaking a chemical bond in the compound. These molecules are presented in pairs that differ in their chemical properties, 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 superimposed on each other, each with its own characteristics. There are also conformational isomers, formed by the rotation of a substituent around a chemical bond.
They are the ones 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 match exactly. However, they have the same characteristics but differ in 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 dextrogyric (if the polarization of light is in the correct direction of the plane) or levorotary (if the polarization is in the left direction) from the plane).
When there is the same amount of both enantiomers (dyl), the net or resultant polarization is zero, which is known as racemic mixture.
Examples of isomers
The first example presented is that of structural position isomers, in which there are two structures with the same molecular formula (C 3 H 8 O), but whose substituent -OHit is in two different positions, forming 1-propanol (I) and 2-propanol (II)
In this second example, two structural chain or backbone 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).
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 alcohol and ether, whose physical and chemical properties vary. a lot from one functional group to another.
Furthermore, an example of tautomeria is the balance between some structures with functional groups C = O (ketones) and OH (alcohols), also called keto-enolic balance.
Next, two cis and trans-geometric isomers are presented, noting that the one on the left is the cis isomer, which is indicated with the letter Z in its nomenclature, and the one on the right is the trans isomer, indicated by the letter E.
Now two diastereomers are shown, where similarities in their structures are noted, but you can see that they cannot overlap.
Finally, two carbohydrate structures that are optical isomers called enantiomers are observed. The one on the left is right-handed because it polarizes the plane of light to the right. On the other hand, the one on the right is levorotatory, because it polarizes the light plane to the left.