Isomerism is the existence of chemical compounds that have the same molecular formulae but different molecular structures or different arrangements of atoms in space.[1] Isomers are chemical compounds with the same molecular formula.[2] Isomers may be considered distinct compounds with different physical and/or chemical properties. The nature and significance of these differences depends on the isomers in question.

Isomerism is encountered frequently in organic chemistry because of the many ways carbon atoms can form chains and rings.

Structural Isomerism[]

Structural (or Constitutional) isomers differ in the order which their atoms are arranged (their molecular structure). There are three forms of structural isomerism.

  • Chain Isomerism, when a compound is rearranged accross different chains. eg. butane and methylpropane.
  • Positional Isomerism, when a functional group appears on a different part of a compound. eg. 1-chloropropane and 2-chloropropane.
  • Functional Isomerism, when isomers have different functional groups while sharing a molecular formula. eg. propan-1-ol (-OH) and methoxyethane (-O-).


When two molecules share a molecular formula and a molecular structure but take a different arrangement in space then they known as stereoisomers (spatial isomers).

E/Z isomerism or geometrical isomerism[]

E/Z isomerism occurs when there is restricted rotation around the central bond of a compound, as is the case around a double bond, ring or square complex. With some compounds this creates two stereoisomers of a compound. When two or more substituents are polarised you have a trans or E-isomer. When the same substituents lie on the same side of of the central bond you have a cis or Z-isomer. This is best illustrated by a diagram.

Organic E-isomers tend to have higher melting points than their Z-isomer counterparts.

Inorganic complexes can also display isomerism. Octahedral complexes of the molecular formula type MX3Y3 can display an even more complex form of isomerism, meridional isomerism (mer-isomer) and facial isomerism (fac-isomer) on top on their cis and trans forms.

Optical Isomerism[]



Keto-enol tautomerism.[1]


  1. 1.0 1.1 Isomerism: Oxford Dictionary of Chemistry (6th edition; 2008) OUP
  2. Went, C (1988). Chapter 2: Basic Stereochemistry - Organic Chemistry. London: Macmillan. p18