Isomerism
What is Isomerism?
Isomerism is the phenomenon in which more than one compounds have the
same chemical formula but different chemical structures.
Chemical compounds that have identical chemical formulae but differ in
properties and the arrangement of atoms in the molecule are
called isomers. Therefore, the compounds that exhibit isomerism are known
as isomers.
The word “isomer” is derived from the Greek words “isos” and “meros”, which
mean “equal parts”. This term was coined by the Swedish chemist Jacob
Berzelius in the year 1830.
Table of Contents
• Isomerism Types
• Structural Isomerism
• Stereoisomerism
• Ionization Isomerism
• Frequently Asked Questions – FAQs
Isomerism Types
There are two primary types of isomerism, which can be further categorized
into different subtypes. These primary types are Structural
Isomerism and Stereoisomerism. The classification of different types of
isomers is illustrated below.
Structural Isomerism
Structural isomerism is commonly referred to as constitutional isomerism.
The functional groups and the atoms in the molecules of these isomers are
linked in different ways. Different structural isomers are assigned different
IUPAC names since they may or may not contain the same functional group.
The different types of structural isomerism are discussed in this subsection.
Chain Isomerism
• It is also known as skeletal isomerism.
• The components of these isomers display differently branched structures.
• Commonly, chain isomers differ in the branching of carbon
• An example of chain isomerism can be observed in the compound C5H12, as
illustrated below.
Position Isomerism
• The positions of the functional groups or substituent atoms are different in
position isomers.
• Typically, this isomerism involves the attachment of the functional groups to
different carbon atoms in the carbon chain.
• An example of this type of isomerism can be observed in the compounds having
the formula C3H7Cl.
Functional Isomerism
• It is also known as functional group isomerism.
• As the name suggests, it refers to the compounds that have the same chemical
formula but different functional groups attached to them.
• An example of functional isomerism can be observed in the compound C3H6O.
Metamerism
• This type of isomerism arises due to the presence of different alkyl chains on
each side of the functional group.
• It is a rare type of isomerism and is generally limited to molecules that contain a
divalent atom (such as sulphur or oxygen), surrounded by alkyl groups.
• Example: C4H10O can be represented as ethoxyethane (C2H5OC2H5) and methoxy-
propane (CH3OC3H7).
Tautomerism
• A tautomer of a compound refers to the isomer of the compound which only
differs in the position of protons and electrons.
• Typically, the tautomers of a compound exist together in equilibrium and easily
interchange.
• It occurs via an intramolecular proton transfer.
• An important example of this phenomenon is Keto-enol tautomerism.
Ring-Chain Isomerism
• In ring-chain isomerism, one of the isomers has an open-chain structure
whereas the other has a ring structure.
• They generally contain a different number of pi bonds.
• A great example of this type of isomerism can be observed in C3H6. Propene and
cyclopropane are the resulting isomers, as illustrated below.
Stereoisomerism
This type of isomerism arises in compounds having the same chemical
formula but different orientations of the atoms belonging to the molecule in
three-dimensional space. The compounds that exhibit stereoisomerism are
often referred to as stereoisomers. This phenomenon can be further
categorized into two subtypes. Both these subtypes are briefly described in
this subsection.
Geometric Isomerism
• It is popularly known as cis-trans isomerism.
• These isomers have different spatial arrangements of atoms in three-
dimensional space.
• An illustration describing the geometric isomerism observed in the acyclic But-
2-ene molecule is provided below.
Optical Isomerism
• Compounds that exhibit optical isomerism feature similar bonds but different
spatial arrangements of atoms forming non-superimposable mirror images.
• These optical isomers are also known as enantiomers.
• Enantiomers differ from each other in their optical activities.
• Dextro enantiomers rotate the plane of polarized light to the right whereas laevo
enantiomers rotate it to the left, as illustrated below.
Ionization Isomerism
The compound which gives different ions in the solution, although they
have same composition, is called ionization isomers and this property is
known as ionization isomerism. Compounds which gives different ions in
solution although they have same composition are called ionization
isomerism. This form of isomerism arises when the counter ion in a complex
salt is itself a potential ligand and can displace a ligand which can then
become the counter ion.
One example of ionisation isomerism is [Co(NH3)5SO4]Br and
[Co(NH3)5Br]SO4.
We can prepare these ionisation isomers in the following method.
[CoBr(NH3)5]SO4→ [CoBr(NH3)5]2+ + SO42− = Red−Violet
[CoSO4(NH3)5]Br → [CoSO42−(NH3)5]+ + Br− = Red
Frequently Asked Questions – FAQs
Q1
What is isomerism in organic chemistry?
Isomerism in organic chemistry is a phenomenon shown by two or more
organic compounds having the same molecular formula but different
properties due to difference in arrangement of atoms along the carbon
skeleton (structural isomerism) or in space (Stereo isomerism)
Q2
What is threo and erythro?
Erythro and threo are two configurations in which molecules are written
when the molecules have a chiral carbon atom. Erythro is the configuration
when the same groups are on the same side of the carbon atom and Threo
is the configuration when the same groups are on the opposite side of the
carbon atom.
Q3
What is Diastereoisomerism?
Diastereomers are defined as non-mirror image non-identical
stereoisomers. Hence, they occur when two or more stereoisomers of a
compound have different configurations at one or more (but not all) of the
equivalent (related) stereocenters and are not mirror images of each other.
Q4
What are the types of isomerism?
Two main forms of isomerism are structural or constitutional isomerism, in
which bonds between the atoms differ; and stereoisomerism or spatial
isomerism, in which the bonds are the same but the relative positions of the
atoms differ.
Q5
What are functional isomers?
Functional isomers are structural isomers that have the same molecular
formula (that is, the same number of atoms of the same elements), but the
atoms are connected in different ways so that the groupings are dissimilar.
These groups of atoms are called functional groups, functionalities.
