Chemical Concept

 I.      Introduction

Isomers are compounds having the same molecular formula but different chemical structures. There are mainly two types classes of isomerism – structural isomerism and stereoisomerism.

 II.      Structural Isomerism

Structural isomerism consists of isomers with atoms bonded differently to one another. There are three types of structural isomerism: chain isomerism, functional isomerism, and position isomersim.

Chain isomers are compounds having the same molecular formula but different branching of carbon chain, eg. linear vs branches.

Functional isomers are isomers that have different functional groups.

Position isomers are isomers having the same functional group but positioned on different carbon atom in the compound.

 III.      Stereroisomerism

Stereoisomerism consists of isomers with different spatial arrangement of atoms. There are mainly two kinds of stereoisomers: enantiomers and diastereomers.

Enantiomers, also known as optical isomers, are molecules that are mirror images but non-superimposable to each other. These compounds usually contain a chiral carbon, which is a carbon atom bonded to four different types of atoms/groups.  Chiral compounds are asymmetric and do not have a plane of symmetry.

As enantiomers are not identical, we use the Cahn-Ingold-Prelog system to designate the isomers either R or S configuration. Depending on the direction the priorities of the four groups (attached to the chiral carbon) are arranged, the compound is assigned R-configuration if it is clockwise, and S-configuration if it is anti-clockwise.

Fun fact : R = “Refus” (which means right in Latin) and S = “Sinister” (which means left in Latin).

Diastereomers are stereoisomers that are not mirror images and are non-superimposable to each other. Typically, enantiomers have the same physical and chemical properties while diastereomers have different physical and chemical properties.

Here is a video explaining optical isomerism:

One popular concept of optical isomerism is racemism. A racemic mixture is one with equal amounts of R- and S-enantiomers (1:1 ratio) such that the optical activity is cancelled out. As such, the mixture is optically inactive.


Clark, J. (2000). STRUCTURAL ISOMERISM. Retrieved March 28, 2017, from

Chiral drugs. (n.d.). Retrieved from

Patel, E. (2015, August 7). Absolute Configuration: R-S Sequence Rules. Retrieved March 28, 2017, from,_R-S_Sequence_Rules

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