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Isomerism occurs when many compounds have the same chemical formula but distinct chemical structures. Isomers are, therefore, substances that show isomerism.
The name “isomer” is derived from the Greek terms “isos” and “meros,” which both mean “equal proportions.” There are a lot of carbon compounds that hold the same number of atoms in them but have entirely different structures. On the other hand, some compounds have the same number of atoms of each element, but in this case, they are arranged differently. This is the concept of isomers. Here, the term isomers basically tell us that the compounds have exactly the same number of atoms of each element; however, the arrangement of these atoms is entirely different.
To understand this in a much clearer manner, let’s assume there are two compounds of A, one atom of B and D, and three atoms of C. Once these atoms are in the structured form, the number of atoms will remain the same while the structure will differ. There can be many combinations made from the given number of atoms if we keep shuffling the atoms. All the different structures formed can be known as the isomers of each other. Therefore, isomers can be defined as compounds with the same molecular formula but different structural formula.
Isomerisation
Isomerisation is the process of converting one isomer into another. This is sometimes referred to as a rearrangement reaction. Because the isomers’ bond energies are equivalent, they exist in different ratios. As a result, they may be readily converted by overcoming the energy barrier that separates them.
Classification of Isomerism
Isomers are divided into two types, each with multiple subgroups:
Constitutional Isomers
The relationship between constitutional (or structural) isomers differs from the relation between stereoisomers. They have similar components but are linked in different ways. Constitutional isomers have varying chemical and physical properties. Examples of constitutional isomers include n-butane and isobutane and ethanol and dimethyl ether. Butane is composed of four carbon atoms connected in a continuous chain, and isobutane has a branching structure.
As the number of atoms rises, so does the number of possible constitutional isomers. Different functional groups can also exist in constitutional isomers. The quickest technique to tell if molecules are constitutional isomers is to count the number of carbons and degree of unsaturation, i.e., the Hydrogen Deficiency Index (HDI). They are constitutional isomers if all the atoms are the same and the molecules have the same HDI. Because they are superimposable mirror copies, they are thought to be identical molecules.
Stereoisomers
Stereoisomers are those isomers that have the same structural formula but differ in the arrangement of atoms in three-dimensional spaces; the phenomenon is called stereoisomerism.
As methane (CH4) is a perfect tetrahedron, isomerism is impossible. However, apparent isomerism can occur for a short time because of the bending and vibration of carbon-hydrogen bonds, so they are not considered isomers.
Types of Stereoisomers
Conformational Isomers
These isomers are formed when any molecule has a single bond-free rotation. Usually, the interconversion of conformations occurs at room temperature because, during the rotation, the energy barrier is low. Also, it is really difficult to separate these isomers from one another.
Configurational Isomers
In these isomers, the configuration is different. There cannot be rotation around the single bond in these isomers, which means different conformational isomers cannot be created. Hence, configurational isomers indicate that they differ in the spatial arrangement of the atoms, and in this case, it can only be interconverted by breaking bonds. In the case of carbon compounds, they can be separated and cannot be interconverted at room temperature.
Conclusion
One of the most important characteristics of organic compounds is their propensity towards isomerism. Isomers are two or more substances with the same molecular formula but different physical and chemical characteristics. This phenomenon is known as isomerism. A molecule’s composition relates to the number, kind, and arrangement of its atoms. The classification of isomerism is of two types: structural isomers and stereoisomers.
The connectivity of the atoms in structural isomers and constitutional isomers differs, and this phenomenon is known as constitutional isomerism or structural isomerism.
Stereoisomers are chemical and structurally identical molecules that differ in the relative arrangement of atoms or groups in space, a phenomenon known as stereoisomerism. So, when we have a situation where you have the same constituent atoms, where you have the same chemical formula, but you are still dealing with different molecules because of either how their bonds are made or what shape are they in, it is known as isomers.
