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Conformation is the shape adopted by a molecule. The rotation around one or more single bonds influences the shape of the molecule. Diverse conformations can occur for any molecule when a single covalent link connects two polyatomic groups. Each of these has at least one atom that does not lie along the axis of the single bond being referred to.
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Keyterms: Conformation, molecule, single bonds, polyatomic groups, atom, single bond, carbon, molecular orbital, C-C bond, alkanes, electron
What is Conformation?
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In alkanes, the electron distribution in the sigma molecular orbital is symmetrical around the C-C bond's internuclear axis. As a result, free rotation about the C-C single bond is possible. Different spatial configurations of carbon atoms in space are seen as a result of this rotation, and these spatial arrangements can transform into one another.
Conformer or rotamer refers to a spatial arrangement of carbon and hydrogen atoms that may be changed into one another by rotating around a C-C single bond. Rotation around C-C single bonds allows alkanes to have an unlimited variety of conformations. Yet, due to repulsive interactions between the electron clouds of C-H bonds, this rotation is not fully free. Torsion strain is the name given to this repulsive interaction.
Read More: Hydrocarbons Important Questions
Types of Conformational Isomers
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Let's use ethane as an example to learn about the fundamentals of conformation. In the ball and stick model of ethane, by rotating one carbon atom while keeping the other carbon atom fixed along the C-C axis, we can see that the rotations will produce an unlimited number of spatial configurations of hydrogen atoms.
These are connected to one carbon atom in relation to hydrogen atoms attached to the other carbon atom. Conformational isomers or conformers are the names given to these different configurations.
Commonly, these may be divided into two categories:
Eclipsed Conformation
Eclipsed conformation occurs in a conformation when hydrogen atoms are attached to two carbons areas nearest to each other as possible. To explain further, two substituents X and Y on adjacent atoms A and B are in the closest proximity, indicating that the torsion angle X–A–B–Y is 0°.
Any open chain with a single chemical bond joining two sp3-hybridized atoms shows an eclipsed conformation. Thus, it has the maximum conformational energy. The most common explanation for this maximum level is a steric hindrance, although it may also be traced back to hyperconjugation (as when the eclipsing interaction is of two hydrogen atoms).
Staggered Conformation
Staggered conformation is another category of conformation in which hydrogen atoms linked to two carbons are as far apart as possible. Since there are fewer repulsive forces and less energy due to the large division between the electron clouds of C-H bonds, the staggered conformation is more stable than the eclipse conformation.
These conformations may be found in any open chain single chemical bond linking two sp3-hybridized atoms, and their conformational energy is generally quite low. There are specific forms of staggered conformations called gauche and anti for some compounds, such as n-butane.
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Difference Between Staggered and Eclipsed Conformation
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| Parameters | Staggered Conformation | Eclipsed Conformation |
|---|---|---|
| Definition | Collection of atoms or groups of atoms in a molecule resulting in a 60o dihedral angle. | Collection of atoms or groups of atoms in a molecule resulting in a 0o dihedral angle. |
| Potential Energy | Has lower potential energy than eclipsed conformation. | Has higher potential energy. |
| Strain | It is a low strain structure. | It is a high strain structure. |
| Stability | The steadiness is high. | The steadiness is low. |
Representation of Eclipsed and Staggered Conformation:
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Sawhorse Projections
In the Sawhorse projection, the bond between carbon atoms is shown as a straight line. The front carbon atom is represented by the lower end of the line, while the rear carbon atom is represented by the upper end. Since each carbon atom in ethane is connected to three hydrogen atoms, each carbon atom has three lines linked to it, each denoting a C-H bond that is inclined at a 120° angle to each other.
Newman Projections
In Newman projection, out of the two carbon atoms present in ethane, the nearer is shown as a dot. Whereas, the rear carbon atom is shown as a circle. Three lines, either bursting out of the circle or diverging from the dotted lines, are used to depict the three hydrogen atoms linked to each carbon atom. These lines meet at a 120° angle.
Read More: Difference between Cations and Anions
Things to Remember
- Definition: Conformational isomers (conformers) or rotational isomers are various arrangements of atoms in a molecule that may be produced by rotating around a single carbon-carbon bond (rotamers). Alkanes and cycloalkanes, as well as their substituted derivatives, exhibit this form of isomerism.
- Conformation vs. Configuration: Configuration, which refers to the spatial arrangements of atoms in a molecule that can only be altered by breaking and forming bonds. The spatial arrangements in conformation can only be changed by rotating around a single bond.
- Representation of Conformations: Conformers are represented in two simple ways- Sawhorse representation and Newman projection.
- Sawhorse Representation: The molecule is seen at an oblique angle along the model's axis in this projection.
- Newman Projection: This is a simple way to express conformations. The molecule is viewed from the front along the carbon-carbon bond axis in this projection.
Read More: Sulphonation
Sample Questions
Ques. What do you understand by torsional angle? Which of the conformations of ethane has the maximum and the minimum torsional strain? (2 Marks)
Ans. The repulsive interaction between the electron clouds, which affects the stability of a conformation, is called torsional strain. The magnitude of torsional strain depends upon the angle of rotation about C-C bond. This angle is called dihedral angle or torsional angle. Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form has the maximum torsional strain.
Ques. Equal amounts of (e, e) and (a, a) conformers of trans - 1, 2 - dichloro cyclohexane exist in non-polar solvents but the (e, e) conformer exists in polar solvents. Explain. (3 Marks)
Ans. In (I), there is dipole-dipole repulsion between two equatorial Cl atoms. It partially destabilizes the sterically more favoured (e, e) conformer but not the (a, a) conformer (II) where Cl atoms are further apart.
Therefore, the conformers (I) and (II) coexist together because their energies are very close. But the polar solvent molecules surround the Cl atoms by dipole-dipole attraction and relieve the dipole-dipole repulsion between Cl atoms themselves. Hence, the lower energy (e, e) conformer (I) predominates in polar solvents.
Ques. Staggered conformation of ethane is most stable while eclipsed conformation is least stable. (5 Marks)
Reason: Staggered form has the least torsional strain and eclipsed form has the maximum torsional strain.
(A) Both assertion and reason are correct and the reason is the correct explanation for assertion.
(B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
Ans. We know that bonds are not stable. They rotate in the compound. For example: In ethane the centre atom is carbon and by the carbon one methane and three hydrogen atoms are attached. So ethane generally shows two conformations: one is staggered and the second is eclipsed.
Conformations: They are the isomers of the same alkane which are formed due to slight changes in the structure, specially due to the carbon-carbon bond changes i.e. bond angle, bond length, etc.
Eclipsed Form: The form in which the atoms are close to each other. They have high energy because repulsion is more between the atoms and the groups of atoms. This is not a stable state for molecules.
Staggered Form: The form in which the same or heavier atoms are opposite to each other. They have less energy because repulsion is less among the atoms or groups of atoms. This is more stable than the eclipsed form. In staggered form there are two types: Anti and gauche staggered.
Anti-staggered Form: In this form, the angle between the same atoms or the heavier atoms is 180° i.e. they are exactly placed opposite to each other.
Gauche-staggered Form: In this form, the angle between the same atoms or groups of atoms is 60°.
Torsional Strain: It is defined as the repulsion between electrons in bonds that do not share an atom.
Here we are given the compound ethane. And staggered conformation of ethane is most stable while eclipsed conformation is least stable because staggered form has the least torsional strain and eclipsed form has the maximum torsional strain. So, both assertion and reason are correct and reason is the correct explanation for assertion.
Hence option A is correct.
Ques. What are the differences between conformation and configuration? (5 Marks)
Ans.
| Parameters | Conformation of Molecules | Configuration of Molecules |
|---|---|---|
| Specificity | Conformation differs for every given molecule because there can be multiple conformations in which a single molecule exists. | The configuration of a specific molecule can be defined by permanent geometry of the structure. |
| Structure | Based on the rotatory character of the atoms and the conformation changes based on the single-bonded molecules. | Based on the spatial arrangement of the atoms which are involved in the formation of the molecules. |
| Number | The conformations of any molecule can be infinite because the rotational characters of the atoms can be infinite. | Configuration is the permanent geometry of the element and hence there can only be one configuration. |
| Conversion | There can be multiple conformations and they can be interconvertible among one another. | Configuration can never be converted to different forms as they are permanent three-dimensional structures based on the arrangement of atoms. |
| Energy | Conformations have a low energy barrier, which varies from 4.2 to 46 KJ per mole. | The difference in energy between the two configurations is more than 84 KJ per mole. |
| Flexibility | Conformations are very flexible. | Configurations are less flexible. |
| Separation | Conformations are inseparable. | Configurations are separable. |
Ques. Write Gauche conformation of the compound CH2Cl - CH2Cl. (6 Marks)
Ans. Gauche conformation of the given compound is:
(i) Mole fraction of anti and gauche form: Mole fraction of stable conformers (i.e., mole fraction of anti and gauche can be calculated if dipole moment of anti and gauche form is known.
μ0b = μ (anti) x Xaμ (gauche) x Xg
where xa = mole fraction of anti form and xb = mole fraction of gauche form.
Suppose,
µ0b = 1.00
µg = 5.55
Then xa can be calculated as follows:
µ0b = µa * Xa + µg * Xg
1 = 0* Xa * 5.55 Xg
Xg = 1⁄5.55 = 0.18
Sum of mole fraction of xa+xg=1
xa=1 - xg = 1 - 0.18 = 0.82
(ii) Relative amounts of anti and gauche conformers: The anti conformer of n-butane is more stable than the gauche at about 900 Kcal/mole (i.e., 0.9 Kcal/mole). This is an energy barrier between anti and gauche. Thus, gauche ⇔ anti, Δ H = - 900 cal/mole.
Suppose at room temp Δ G is negligible.
So AG = - RT In Keq
Keq= [anti]/[gauch]
![Keq= [anti]/[gauch]](https://images.collegedunia.com/public/image/5b25afa40afec6bca2172cbeaa6c7458.png?tr=w-389,h-41,c-force){kind=small}
The ratio of Keq is 4.57 4.6, which means that about 82% mole (4.6100/5.6) of the molecule is in the anti conformation and 18% in the gauche conformation at any one time.
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