The molecules which have the same constitution but different arrangements of atoms in three-dimensional space are called stereoisomers and the study of these compounds is known as stereoisomerism.
Stereoisomers have the same bond connectivities but have different spatial arrangements of atoms, such that they have relatively different orientations in space. They also have different physical and chemical properties.
Moreover, they can not be distinguished by two-dimensional structures but only by three-dimensional structures.
Stereoisomerism is further classified into the following categories:
1. Conformational isomerism
This can be represented by:
- Sawhorse projections
- Newman projections
2. Configurational isomerism
- Cis/trans isomerism
- E/Z isomerism
- Syn/anti isomerism
Conformational isomers are interconvertible structures. They have different three-dimensional arrangements of atoms due to the free rotation of carbon-carbon single bonds. They simply interconvert at room temperature without bond breaking or making. They require very low energy for interconversions. They are also known as conformers.
Conformations are the momentary arrangements of atoms in space. Each of these arrangements has a different degree of intramolecular interactions. They also have the same carbon skeleton because free rotation does not affect the carbon skeleton.
Moreover, conformational isomers are also known as rotamer, and they are not separable.
Representation of conformations
Conformation can be represented in two ways; sawhorse and Newman projections.
1. Sawhorse projections of cycloalkane:
2. Newman projections of the chair and boat conformation of cycloalkane:
Ethane shows two conformations due to free rotations of the C-C bond; a staggered conformation and eclipse configuration. The staggered stable because the bonds are at a maximum distance and thus experience minimum repulsion whereas, in eclipsed conformation methyl and hydrogens are close to each other. This causes Van der Waals’s repulsion. Hence, the appearance of steric hindrance and torsional strain makes this conformation unstable.
For example, the conformations of ethane are given below:
Configurational isomers are not interconvertible without bond breaking or making. This type of stereoisomerism is found in the compound which contains a stereocenter. Such isomers are classified into two main types:
- Optical isomers
- Geometric isomers
Optical isomers have the same molecular formula but different optical activities, such that they differently rotate the plane-polarized light.
Initially, they were named by using the D/L nomenclature system, but later on, R/S nomenclature was introduced to classify them. The compounds that rotate plane-polarized light in a clockwise direction are called ‘R’ isomers whereas, the compounds that rotate plane-polarized light in an anticlockwise direction are called ‘S’ isomers.
There are two types of optical isomers:
Enantiomers are non-superimposable mirror images of each. Enantiomerism is shown by the chiral molecules. They have four different groups attached to the central chiral carbon. Enantiomers are asymmetrical molecules and have the tendency to rotate plane-polarized light in opposite directions.
For example, carvone has a pair of non-superimposable mirror images. They have the same physical properties yet behave differently in a chiral environment.
Diastereomers are non-superimposable, non-mirror images of each other. They have two or more chiral centers and are nonidentical to each other.
They have different configurations at stereogenic centers.
Geometrical isomers have relatively different positions of identical groups in space. It appears due to the restricted rotation of carbon-carbon double bonds. Such isomers same molecular formula but they have different arrangements of atoms or groups of atoms in space around double bonds.
Conditions for geometrical isomerism
- Restricted rotation
- Presence of double bond.
- There must be two different groups on either side of the double bond.
These structures can not be interconverted without bond breaking. Geometric isomers usually come in pairs. i.e, Cis/trans or E/Z isomer. Cis/ trans isomerism is a common example of geometrical isomerism.
For cis-trans isomerism, there must be two identical groups on two sides of a double bond.
When similar groups are on the same side across a double bond then these are called cis isomers.
On the other hand, if these similar groups are on the opposite side across a double bond then these are called trans isomers. They have different physical properties. In addition to this, cis isomers are less stable due to steric factors.
When non-similar groups are present on two sides of a double bond then E/Z isomerism is used. This is based on the priority rules. These priorities are assigned on the basis of reactivity, atomic number, and atomic mass.
For example, when higher priority groups are present on similar sides across the double bond then this is called a ‘Z’ isomer. Interestingly, Z comes from the German word Zusammen meaning together.
When higher priority groups are present on opposite sides across the double bond then this is called an E isomer. And, E also comes from the German word Entgegen, which means the enemy.
This type of geometrical isomerism is shown by compounds other than alkenes where only two or three groups are present across a double bond. The term syn is used when similar groups or groups of similar priorities are present on the same side across a double bond. For example,
Whereas, the term anti is used when similar groups or groups of similar priorities are present on the opposite side across a double bond.
Differentiate between isomers and stereoisomers.
Isomers are compounds having the same molecular formula but different structural formulas whereas stereoisomers have the same connectivities but different spatial arrangements of atoms.
How can we identify stereoisomers?
We can identify stereoisomers by their spatial arrangements of atoms.
Name the different types of stereoisomers.
Stereoisomers are classified into two main types,
- Conformational isomers.
- Configuration isomers.
What are chiral compounds?
The compounds having four different groups attached to the central atom are called chiral compounds.
What is the difference between identical and enantiomers?
Identical means totally same and are not differentiable whereas, enantiomers are mirror images of each other and different from one another.
Are all stereoisomers optically active?
All stereoisomers are not optically active.
What is the formula to determine the number of stereoisomers?
The number of stereoisomerism is calculated by using a formula,
Stereoisomers = 2n
Where n= number of stereocenters.
How many stereoisomers does a chiral center have?
One chiral center can have two stereoisomers.
Do all stereoisomers have a chiral center?
All stereoisomers do not have chiral centers.
Do functional groups have any effect on stereoisomerism?
No, functional groups do not affect stereoisomerism.
- Principle and Application of Stereochemistry by Micheal North.
- Eight Edition of Organic Chemistry by Paula Yurkanis Bruice.
- Stereoisomers by (MSU chemistry)