➢ Structural isomer
➢ Conformational isomer
➢ Configurational isomer
➢ Geometrical isomer
➢ Optical isomers
➢ L and D isomers
➢ R and S racemic mixtures
➢ The importance of isomers in drug prescription
➢ The importance of isomers in biological processes
Definition of isomers
Isomers is a term which call in two compounds that have the same
molecular formula .
There are two major classes (types) of isomers, and under these
major classes there are further classifications of isomers.
Structural Isomer (Constitutional)
Definition: Structural isomers are molecules with the same molecular
formula, but have different structures or arrangements.
*Let's take a look at a molecular formula: C4 H10. While the molecular
formula can't tell us how the atoms are arranged, it can help us find
some of the possible arrangements. In this case, there are two different
arrangements for this molecular formula: butane and isobutane.
Butane and isobutane have the same number of carbon (C) atoms and
hydrogen (H) atoms, so their molecular formulas are the same.
However, each one has a different structural formula, which shows how
the atoms are arranged. So, we can say that butane and isobutane are
Types of Structural Isomerism
1 - Chain Isomerism.
2 - Position Isomerism.
3 - Functional Group Isomerism.
There are two isomers of butane, C4H10. In one of them, the carbon
atoms lie in a "straight chain" whereas in the other the chain is
For example, there are two structural isomers with the molecular
formula C3H7Br. In one of them the bromine atom is on the end of the
chain, whereas in the other it's attached in the middle.
Functional Group Isomerism
In this type, the isomers contain different functional groups. For
example, a molecular formula C3H6O could be either propanal (an
aldehyde) or propanone (a ketone).
Stereoisomers : Isomers that have the same connectivity of their atoms
but a different orientation of their atoms in space .
Conformational isomers are the type of stereoisomerism
Stereoisomers have the same functional groups and connectivities,
they differ only in the arrangement of atoms and bonds in space
The different spatial arrangements that a molecule can adopt due to
rotation about carbon-carbon single bonds are known as conformations
• Different conformations also are called conformational isomers or
A convenient way to describe conformation isomers is to look at the
molecule along the axis of the bond of interest
• A Newman projection is a graphical representation of such a view
Conformations of Alkanes: Rotation About C-C Single Bonds
When ethane molecules rotate about the carbon-carbon bond there
are two extremes:
● staggered conformation and eclipsed conformation
Configurational isomers are stereoisomers that cannot be converted at
room temperature and can be separated.
Types of Configurational isomers:
differ in the spatial position around a bond with restricted rotation or
across a ring system.
Optical isomers: differ in the 3D relationship of the substituents about
one or more atoms.
Enantiomers: Optical isomers that image of each other and have the
same physical and chemical properties
Diastereomers:stereoisomers that is not a mirror image and have
different physical and chemical properties.
Geometric isomerism concerns the type of isomer where the individual
atoms are in the same order, but manage to arrange themselves
different spatially. The prefixes cis- and trans- are used in chemistry to
describe geometric isomerism.
Geometric isomers occur when atoms are restricted from rotating
around a bond.
Divisions of geometrical isomers are:
The cis- prefix is from the Latin meaning "on this side". In this case, the
chlorine atoms are on the same side of the carbon-carbon double bond.
This isomer is called cis-1,2-dichloroethene.
The trans- prefix is from the Latin meaning "across". In this case, the
chlorine atoms are across the double bond from each other. This isomer
is called trans-1,2-dichloroethene.
Solutions of some organic compound have the ability to rotate the
plane of polarized light. These compounds are said to be optically
active. This property of a compound is called Optical Activity
Optical activity in a compound is detected and measured by means
of a Polarimeter
· The compound which rotate the plane of polarized light to
the right [clockwise] is said to be Dextrorotatory. It is
indicated by the sign (+).
The compound which rotates the plane of polarized light to the
left [anticlockwise] is said to be Laevorotatory. It is indicated
by the sign (-)
An optically active compound can exist in two isomeric forms
that polarize the plane of light on opposite directions. These
are called optical isomers and this phenomenon as optical
Dextrorotatory isomer or (+) isomer
Levorotatory isomer or (-) isomer
Optical Isomerism of Tartaric Acid
Tartaric acid (2,3-Dihydroxybutanedioic acid) contains two
asymmetrical carbon atoms.
Four forms of tartaric acid are known. Two are optically active
and two optically inactive. The optically active forms are
(+) Tartaric acid
(-) Tartaric acid
(±) Tartaric acid, racemic or equimolar mixture of (+) and (-)
The specific rotation is the number of degrees of rotation
caused by a solution of 1.0 g of the compound per mL of
solution in a sample tube 1.0 dm long at a specified
temperature and wavelength.
Diastereomers are stereoisomers that are not mirror images of one
another and are non-superimposable on one another. Stereoisomers
with two or more stereocenters can be diastereomers.
For example, consider the following molecules.
These molecules are not mirror images of one another. Additionally,
these molecules are non-superimposable because if one of these
molecules is flipped 180 degrees the stereochemistry is different at one
carbon (the alcohols) and the same at another carbon (the methyls).
Therefore, these molecules are diastereomers.
Uses Of Diastereomers
Glucose and galactose, two of the most common monosaccharides
found in the body, are diastereomers of each other:
Lactose, which is found in milk, is formed by a condensation reaction
between glucose and galactose.
Retinal is a molecule found in the photoreceptor cells of eyes that
normally exists in the cis form. When it absorbs light, it undergoes
Enantiomers have identical chemical
and physical properties except for
their ability to rotate plane-polarized
light (+/-) by equal amounts but in
opposite directions. Enantiomers
interact differently with other chiral
molecules i.e. biologically active
molecules as amino acids, sugars,
steroids etc. This means that some
molecules have, for example, different
odours. Limonene is just such a case.
Properties of enantiomers
Not every pair of molecules can be enantiomers. They have to
have specific properties. What are these properties?
A pair of enantiomers must be a chiral compound, which means it
has a chiral carbon. A chiral carbon, or a chiral center, is a
carbon that has four different groups attached to it.
D and L Isomers[enantiomers]
A special type of isomerism is found in the
pairs of structures that are mirror image of
These mirror images are called enantiomers.
D and L Isomers[enantiomers]
D and L isomers enantiomers
The two members of the pair are
designated as a D- and an L- sugar. In the
D isomeric form, the OH group on the
asymmetric carbon (a carbon linked to
four different atoms or groups) , while in
the L-isomer it is on the left.
IMPORTANCE OF ISOMERS IN
Stereoisomerism is the term applied to isomers that have the same
structural formula but different arrangements of atoms in space. The
level of priority groups refers to the molecular mass of a group attached
to the parent chain or central atom/ion; highest priority groups have the
highest molecular mass. An isomer will be deemed as ‘Cis’ if the
highest priority groups are on the same side of the molecule, an isomer
would be deemed as ‘Trans’ if the highest priority groups are on different
sides of the molecule. An example of cis – trans isomerism is shown
above. The structure and position of the groups is highly important to
chemistry and the manufacture of pharmaceuticals.
THE IMPORTANCE OF ISOMERS IN
Isomers are important because our entire biology, and that of every
organism on the planet, is built on them.
First, virtually every biologically important molecule has one or more
isomers. This is due, in part, to the necessary complexity of those
molecules, which require numerous different types of atoms (a
minimum of carbon, oxygen, hydrogen and often nitrogen,
phosphorus and others) and will commonly contain a mixture of
single and double bonds. Second, evolution, in almost all cases, has
favoured the use of one isomer for a given purpose over the use of
the others that exist.