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Stereoisomerism- Definition, Types, Uses, Practice Problems and FAQs

Stereoisomerism- Definition, Types, Uses, Practice Problems and FAQs

You must have watched an athlete participating in the 100-meter race. Just imagine the posture of the athlete just before the whistle is blown to start a race and the time when an individual tries to run as hard as possible. Different postures have different performances.

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But how it is related to chemistry? There are compounds which have the same molecular formula, and same functional groups but differ in the orientations in space of their attached groups and exhibit different properties. Such substances,.which exhibits difference in orientation in space and by that differ in properties are known as stereoisomers.

In the year 1942 French chemist, Louis Pasteur discovered that tartaric acid salts collected from a wine production vessel have the ability to rotate plane-polarized light, whereas the same salts from different sources do not rotate the light and this phenomenon was explained using the optical nature of the compound. Let’s have a tour of this article to learn more about stereoisomerism.

Table of content

  • Definition
  • Types of stereoisomerism
  • Use of stereoisomers
  • Practice problems
  • Frequently asked questions-FAQs

Definition

Stereoisomers are a type of isomers (compounds having the same chemical formula) obtained by changing the orientation of atom or groups attached to carbon in 3-dimensional space without changing the bonding pattern. and the phenomenon is known as stereoisomerism.

For example:-

Let us consider the two different orientations of the But-2-ene molecule, Molecular formula is C4H8. In this molecule, we can position the two methyl groups on the double-bonded carbon atoms either on the same side or on opposite sides. Both structures have the same bonding of atoms and differ only in the position of the methyl groups.

In the left molecular orientation, both the CH3 groups are present on the same side along the double bond (i.e rotation restricted system) and are known as cis form. Whereas in the second case the CH3 group are present diagonally opposite to each other and is known as trans form respectively. Both the molecule has the same formula but differ in the spatial arrangement of atoms in 3- dimensional space and therefore both pairs represent the stereoisomers.

Types of stereoisomerism

Stereoisomerism or different space orientation can arise out of either rotation around single sigma bonds between two atoms or by the different directional positions of attached atoms/groups on a single atom. Thus stereoisomers are broadly classified into two types- Conformational isomers and Configurational isomers

Conformational isomers:

Across singly bonded atoms. one atom and its attached group can be kept fixed, and the other atom can be rotated about the single sigma bond through 360°. The rotating atoms and their attached groups now can have different orientations relative to the fixed atoms and their attached groups. These different orientations obtained by rotation around a single bond without changing the constitutional bond position of the atom are called conformers or rotamers. The energy required for bond rotation is very small which will be the energy of room temperature. So, rotation can easily cross the barrier and rotate alongside the single bond. For example: Let us consider the ethane molecule-

For easy visualization, the molecules across the bond of rotation can be represented by two ways- Newman projection and Seahorse projection.

The above image represents the Newman projection of the ethane molecule when the dihedral angle between the hydrogen atom is 0 degrees and is known as eclipsed form. When the valency of the back carbon atom is rotated at an angle of 60 degrees it results in a change in orientation of the atom and is known as staggered form.

Both eclipsed and staggered forms together are examples of stereoisomerism.

  • Configurational isomers: The configuration of any molecule is the distinct arrangement of atoms around a central atom that cannot easily be interconverted. Stereoisomerism is shown by molecules without elements of symmetry, like planes or centres of symmetry.

Configurational isomers are broadly classified into two types:

  • Geometrical isomerism: Geometrical isomerism is a type of stereoisomerism in which the molecular formula and structure are the same but the relative arrangement of atoms differs due to restricted rotation like across a double bond. Geometrical Isomerism is a common occurrence in heteroleptic complexes. This type of isomerism results from the ligand's various geometric arrangements.

Rotation across double bonds cannot take place. In the above example, ‘the two carbon atoms are connected by a double bond and hence rotation between the two carbon atoms is not possible Such compounds can show stereoisomerism. Each carbon is attached to one ‘R’ and one ‘H’ each.

When the atom/group ‘R’ and ‘H’ are present along the same side of the rotation restricted system it represents the cis form and the atom/group ‘R and H’ present diagonally opposite to each other along the rotation restricted system is said to be the trans-form.

Both cis and trans form represents the stereoisomers coming out of stereoisomerism..

Optical isomers:

Optical isomerism occurs when all the valency of an atom is satisfied by different atoms /groups.resulting in a lack of any symmetry. These isomers have the same molecular formula as well the chemical and physical properties. They differ, however, in their effect on the direction of rotation of a plane-polarised light. For example, let us consider a molecule with the formula C2H4ClBr which is represented by two different images as shown below,

Both molecules will rotate the plane-polarized light when allowed to pass through and therefore there are classified as optical isomers.

The molecule which exhibits the optical isomerism generally contains the chiral centre or stereo centre.

A chiral centre is an atom with all different groups bonded to it in such a way that its mirror image is not superimposable. Nowadays, the term chirality centre is used interchangeably with the chiral centre.

Chirality is defined as an asymmetric object that cannot be superimposed over its mirror image. It is also known as a stereocenter. This is referred to as chirality. For example, our hands, legs, and so on.

An achiral object is one that is symmetric in nature and can be superimposed over its mirror image. For example, a cube, a cone, and so on.

Optical isomers are broadly classified into two types:

  • Enantiomers: Enantiomerism occurs when two isomers are mirror images of each other, and non-superimposable on each other. Such pairs of isomers are referred to as enantiomers. Enantiomers are compounds that are stable and isolable but differ in their spatial arrangements in 3-D space. They are usually found in pairs. Enantiomers have identical properties. Their interaction with plane polarized light is different. The direction in which they rotate the plane-polarized light differs. if the compound rotates the plane-polarized light to the right or clockwise direction is known as a dextrorotatory optically active compound. and other rotates the plane polarized light towards the left or anticlockwise direction and is known as a laevorotatory optically active compound. For example,

In the above image, two molecules are shown which are non-superimposable mirror images of each other and are known as enantiomers.

  • Diastereomers: Diastereomers are isomers that have the same molecular formula and functional groups but do not behave as mirror images of each other. A molecule with 'n' asymmetric carbon atoms may contain up to '2n' diastereomers.
  • Epimers: When two diastereomers differ only at one stereocenter, they are called epimers. The physical properties and chemical reactivity of these isomers are different. It is generally obtained the changing the valency along only one stereogenic centre when more than one stereogenic centre is present in the given molecule.

Let us consider the below example,

In the given examples, the first molecule has two chiral carbon centres and is optically active. The second molecule is a mirror image of the first molecule and is non-superimposable on the first. And hence first and second molecules are known as enantiomers.

Similarly third and fourth molecules are enantiomers.

A positional change in one of the chiral carbon results in a molecule with a different orientation. Such molecules are called diastereoisomers. The first and third molecules which are not the mirror image of each other are diastereomers.

Uses of stereochemistry

Stereochemistry was critical in resolving the thalidomide crisis in the 1960s. Thalidomide was first manufactured in Germany in 1957. It was used by doctors to treat morning sickness in pregnant women. Later, it was discovered that the drug caused deformations in babies. While one isomer of the drug was not harmful, the other caused serious genetic damage to the embryos. Thalidomide undergoes racemization in the human body. Even if only one of the two stereoisomers enters the body, the body converts some of it to the other. The thalidomide disaster prompted governments to conduct more drug testing. Before a drug is made available to the general public, it is first tested on a small group of people in a clinical trial. Thalidomide is now used to treat leprosy. To prevent pregnancy, women must use it in conjunction with contraception. Thalidomide is now used as a leprosy treatment. To avoid pregnancy, women must use it in conjunction with contraception.

Practice problems

Q1. Select the correct option for the compound which contains a chiral centre.

Answer: (D)

Solution: A chiral centre is an atom with four different groups bonded to it in such a way that its mirror image is not superimposable. Nowadays, the term chirality centre is used interchangeably with the chiral centre. In the case of option A, B and C two valency of the carbon is Hydrogen, therefore, it is not a chiral centre in the case of option D the four valencies of carbon is H, Cl, CH3 and CH2Br respectively and therefore it represents the chiral centre or chiral carbon.

Q2. Which of the following compound can exhibit geometrical isomerism?

  1. Both A and B are correct

Answer: (C)

Solution: Geometrical isomerism is a type of stereoisomerism in which the molecular formula and structure are the same but the relative arrangement of atoms differs along the rotation restricted system. In options A and C the valency of the carbon is the same(i.e it contains two hydrogen atoms) along the rotation restricted system and therefore it cannot exhibit geometrical isomerism. Whereas, in the case of option (D) valency along the carbon present in the rotation restricted system is CH3 and H atom respectively whose orientation can be changed to obtain the cis and trans form.

Q3. Select the correct option in which the pair of compounds does not represent the conformational isomers.

Answer: (C)

Solution: When compounds with the same molecular formula are rotated along a single bond to form different orientations without changing the constitutional bond position of the atom it results in the formation of conformers or rotamers of an alkane. Option (A), (B) and (D) does not represent the conformational isomers because the molecular formula for the compound is different. In the case of option (C), the molecular formula is the same and the 2nd compound is obtained just by rotating the molecule and therefore represents the conformational isomerism.

Q4. Select the correct option for the number of stereogenic centres present in the given molecule.

  1. 2
  2. 1
  3. 3
  4. 4

Answer: (A)

Solution: Stereogenic centre includes both the chiral centre as well as geometrical centre. In the given compound there are two stereogenic1 centres which include one chiral centre also known as chiral carbon as all the four valencies along that carbon are different. It also contains one geometrical centre as there are two different groups attached along the carbon that constitutes the rotation restricted system.

Frequently asked question-FAQs

Q1. Does a compound which contains a chiral centre are always optically active in nature?
Answer:
A chiral centre is an atom with four different groups bonded to it in such a way that its mirror image is not superimposable. Nowadays, the term chirality centre is used interchangeably with the chiral centre. If a compound contains only one chiral centre it is certainly optically active in nature but if more than one chiral centre is present in the compound then the compound may be optically inactive due to internal compensation (i.e half molecule rotates in one direction and the other half rotate in opposite direction) making overall molecules as optically inactive in nature which can be identified by the symmetry present in the molecule.

Q2. What is the important condition for the compound to be optically active in nature?
Answer:
Optical activity is certainly an important condition but not the sole condition for a compound to be optically active in nature. For example, in the case of biphenyl compounds, spiranes and cumulene can be optically active though the molecule does not contain a chiral centre.

Q3. What are meso compounds?
Answer:
It is defined as a compound which contains two or more chiral centres but contains a plane of symmetry or centre of symmetry and makes the overall molecule optically inactive as the half molecule rotates in a clockwise direction while the other half part rotates anticlockwise direction respectively cancels the rotation of plane polarised light internally is known as meso compounds. For example,

Q4. What is the method to separate cis and trans isomers?
Answer:
Cis and trans form are two different geometrical isomers of the compound which exist individually. Both cis and trans forms can be isolated by using a technique known as fractional distillation which is used to separate a molecule having a difference in the boiling point.

 

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