Molecules can vary by the way atoms are arranged in three-dimensional space. The same combination of atoms occupies space in different ways. Isomers are compounds having the same atomic composition, but differ either in connectivity or in spatial arrangement. ^[1-4]

Types of Isomers

There are two main types of isomers: constitutional isomers and stereoisomers. ^[1-8]

1. Constitutional Isomers

Constitutional isomers, also called structural isomers, differ in how atoms, subtituents, and functional groups are attached to the molecular chain, resulting in different connectivities. Structural isomers are divided into several groups. ^[1-8]

i. Skeletal Isomers

Also known as chain isomers, the arrangement of the carbon chain is different for different isomers. The simplest hydrocarbons like methane, ethane, and propane do not have structural isomers. The smallest hydrocarbon that can display such isomerism is butane (C₄H₁₀), which has four carbon atoms. Its two isomers are i. n-butane (straight chain) and ii. isobutane (branched chain).

Likewise, pentane has three isomers – i. n-pentane, ii. isopentane, and iii. neopentane.

ii. Positional Isomers

Positional isomers are also known as regioisomers. They keep the same carbon skeleton and change the location of a substituent or functional group. For example, 2-methylpentane and 3-methylpentane are positional isomers because the position of the methyl functional group (-CH₃) changes. The number in front of the compound’s name is the locant, i.e., the specific position of a substituent or functional group on a parent chain or ring.

Similarly, butanol shows both positional isomerism and skeletal isomerism. In the straight-chain structure, the –OH group can be on carbon 1 or carbon 2, giving rise to 1-butanol and 2-butanol. Butanol is not limited to only these two isomers, because it also has branched-chain isomers: 2-methyl-1-propanol and 2-methyl-2-propanol.

iii. Functional Isomers

These structural isomers have different functional groups and significantly different physical and chemical properties. An example of a pair of functional isomers is propanal and propanone (acetone). Both have the molecular formula C₃H₆O. Propanal is an aldehyde, while propanone is a ketone. Another example pair is glucose and fructose.

2. Stereoisomers

Stereoisomers have the same connectivity of atoms but are arranged differently in space. The geometric positionings of atoms and functional groups are different. Molecules that are stereoisomers of each other represent the same structural isomer. There are several groups of stereoisomers ^[1-8].

i. Geometric Isomers

Geometric isomers have the same order of bonding but different arrangements of atoms in space. For a geometric isomer to exist, the structure must be rigid and nonrotatable. Such properties are observed in alkenes and ringed structures. The two carbons in the double bond must have different groups attached to it. Geometric isomerism is also known as cis-trans isomerism or E-Z isomerism.

An example of geometric isomerism is 2-butene. There are two ways to draw the molecule, as shown below. The two isomers are known as cis-2-butene and trans-2-butene. In cis-2-butene, the two hydrogen atoms are on the same side of the molecule. In trans-2-butene, the two hydrogen atoms are on opposite sides.

The geometric isomers have different physical and chemical properties. Alkynes do not have geometric isomers since only one group is attached to the carbon atoms involved in the triple bond.

ii. Enantiomers

Enantiomers, also called optical isomers, are non-superimposable mirror images of each other. Each member of an enantiomeric pair is chiral. Many such molecules contain a chirality center, often a tetrahedral carbon bonded to four different substituents.

Lactic acid, for example, exists as (S)- and (R)-lactic acid enantiomers because its central carbon is attached to COOH, OH, CH₃, and H groups.

iii. Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other. They usually have different physical properties. Cis-trans isomers are a common type of diastereomer. Epimers are diastereomers that differ in configuration at only one of two or more tetrahedral stereogenic centres. Thus, D-glucose and L-glucose are enantiomers, while D-galactose is a diastereomer and specifically the C-4 epimer of D-glucose

iv. Conformational Isomers

Conformational isomers or conformers differ because of rotation about formally single bonds. This rotation is usually possible, though it may be hindered by an energy barrier. In ethane, the two limiting arrangements are staggered ethane, where the C-H bonds are as far apart as possible, and eclipsed ethane, where they line up with each other.

These conformations are commonly visualized by looking along the C-C bond using a Newman projection.

Isomerism highlights how molecules with the same formula can behave in very different ways because of differences in structure and spatial arrangement. This feature makes it a fundamental concept in chemistry, since it helps explain variations in physical properties, chemical reactivity, and biological effects. Its importance lies in medicine, materials science, and biochemistry, where even a small change in arrangement can lead to a major change in function.