What are Isomers?


Core Concepts

In this article you will be able to understand everything about isomers, their types and their importance. After reading, you will know the basis and lose all fear of organic chemistry!

Related Topics

What is an Isomer?

Isomers are similar molecules with different structural characteristics. Although having the same number of atoms in each element, isomers have different atomic arrangements. Even with the same molecular formula, molecules often have different physical characteristics, especially if their respective functional groups are unique.

Constitutional Isomers

Structural and constitutional isomers refer to the same type of molecules. These molecules share the same chemical formula but different atom arrangements.

structural isomers

Position Isomers

In position isomers, the carbon skeleton chain is the same, but the functional groups in these isomers are at a different position. Regioisomers are another name for these molecules. For instance, 1-propanol and 2-propanol.

Functional Group Isomers

Functional isomers are molecules with the same chemical formula but various functional groups. Alkene and cycloalkene are two examples. Functional group isomerism is the phenomena in question.

Chain Isomers

Chain isomers are molecules with the same chemical formula but a distinct carbon chain in their structure. Skeletal isomers are another name for these molecules. Chain or nuclear isomerism are other names for this phenomena.


In metamers, an atom within a functional group remains the same, but have different groups bound to it to generate these molecules. Ethers and ketones are two examples.


Tautomers are molecules that quickly interconvert between themselves. Tautomers cannot exist separately from one another but instead have a dynamic equilibrium. The Keto Enol Tautomerization is very common example in organic chemistry.


The connection of the atoms in stereoisomers is the same, but their arrangement in three-dimensional space differs. We can categorize stereoisomers in many ways depending on how their arrangements vary from one another. Be aware that the atom connections varied somewhat between the structural molecules. The atoms in a stereoisomer’s molecule are all connected in the same way. Stereoisomers divide into two categories: optical and geometric. We distinguish stereoisomers based on relative configuration and absolute configuration.

Optical Isomers

Firstly, optical isomers are stereoisomers that are not geometric molecules. The positioning of substituted groups around one or more atoms of the molecule varies amongst optical molecules. Due of their interactions with plane-polarized light, they received their name. We refer to optical stereoisomers as either diastereomers or enantiomers.


Enantiomers are mirror images that cannot be superimposed. Your hands are an everyday illustration of a pair of enantiomers. Even if your hands are mirror pictures of one another, they cannot be superimposed no matter which way you bend, twist, or spin them.

enantiomer isomers


Diastereomers, which don’t produce mirror images, are another category of optical isomer. While some of the atoms in diastereomers have the same configuration, others have a distinct arrangement around one or more of the atoms.

Diastereomer isomers

Geometric Isomers

Geometric stereoisomers are molecules with the same atom bonding order but a distinct spatial arrangement of the atoms.

cis and trans isomers

Cis and Trans

While the trans isomer has the two single fluorine atoms on different sides of the molecule, the cis isomer has the two single hydrogen atoms on the same side of the molecule. The atoms’ bonding arrangements are the same in both molecules. There must be a stiff structure in the molecule to prohibit free rotation around a link in order for geometric isomers to occur. This happens when there is a ring or a double bond. For geometric molecules to exist, we need to connect two distinct groups to each of the two carbon atoms.

Why Isomers are Important?

Currently, understanding isomerism has aided in the introduction of safer and more effective drug substitutes for both newer and older medications. Many currently available medications have undergone a chiral switch, or a change from a racemic mixture to one of its isomers. The most well-known case is thalidomide.

Thalidomide was first promoted as an over-the-counter drug for treating morning sickness in pregnant women in. 10,000 newborns worldwide were born with phocomelia, or limb deformity, in the years that followed. Only half of the newborns made it to adulthood, and some of those who did had additional defects.

Thalidomide is a racemic combination of the (R)- and (S)-enantiomers and occurs in two mirror-image forms. The (S)-isomer harms fetal tissue, but the (R)-enantiomer has sedative effects. Separating these molecules before usage is useless since they interconvert under biological circumstances.

The thalidomide catastrophe led to stricter rules for medication approval in numerous nations.

thalidomide enantiomer isomers

Further Reading

> More about Thalidomide

> To learn more about isomerism

> Cannizzaro reaction