Stability of Conformations

In a sigma molecular orbital, the distribution of the electrons is balanced around the internuclear axis of the carbon-carbon (C-C) bond. Hence, it allows free rotation around the C-C single bond. The three-dimensional arrangements of carbon atoms in space are observed due to this rotation, and these atoms can be changed into one another.

Such a three-dimensional arrangement of hydrogen and carbon atoms, which can be easily transformed by each other by the process of rotation around the C-C bond, is known as conformations, conformers, or rotamers. However, due to repulsive interaction, these rotations are not free between the electrons of clouds of C-H bonds. This type of more repulsive interaction is known as torsional strain.

Conformation Isomerism

Let us understand the basics of conformation with the example of ethane.

Conformational isomerism is a type of stereoisomerism in which interconversions of isomers possibly take place by the process of rotation around a single bond. In this process, the rotational energy works as a barrier only in cases of the single bond type of rotation. It has to be overcome for interconverting one type of conformer to another. For conformational isomerism to occur, the barrier of energy should be small. Conformational isomers are of various types like ethane and butane.

We can classify these conformational isomers broadly into two main cases:

Eclipsed Conformation: This is a type of conformation where the hydrogen atoms are arranged in such a way that they are connected to the carbons that are as nearest to each other as possible. 

Staggered Conformation: In staggered conformations, the hydrogen atoms are attached to the carbons, and are placed as far away from one another as possible. This type of conformation is more stable compared with the eclipsed conformation because it consists of less repulsive forces and less energy due to many partings in the middle clouds of the energy i.e., C-H bonds. 

Factors Affecting Stability of Conformers

Some of the factors that affect the stability of the conformers are:

Angle Strain: This type of strain generally represents the cyclic structure but is not present in the cyclic structure. Any deviation from the normal type of angles goes together with the angle strain.

Van der Waals Strain: The group of non-bond atoms that are far away, and when they come close to each other there is repulsion, is known as the van der Waals strain.

Torsional Strain and Steric Strain

In Newman projection, we study the energy of anti, gauche, and other conformations. The gauche and eclipsing groups destabilise the conformation of steric strain or torsional strain. Steric hindrance is a broad concept, and it is obtained when the groups of atoms are very close to each other, and there is a sudden increase in energy due to the repulsion of the clouds of electrons.

The torsional strain results mainly from the eclipsing groups. The increase in the amount of energy from the eclipsing groups of the nearby carbons is known as the torsional rise of eclipsing groups. 

The existence of both the strains, torsional and steric, results with the eclipse of two large groups: 

The interactions of the two gauche groups are always in a steric hindrance. Gauche conformations are always less stable due to high energy in comparison with the anti-conformation due to the steric strain, which is related to the bigger groups close to each other. (60o vs 180o).

Conclusion

In a sigma molecular orbital, the distribution of the electrons is balanced around the internuclear axis of the C-C bond. Hence, it allows free rotation around the C-C single bond. The three-dimensional arrangements of the carbon atoms in space are observed due to this rotation, and these atoms can be changed into one another.

Conformational isomerism is a kind of stereoisomerism in which interconversions of isomers take place by rotations within a single bond. In this process, the rotational energy works as a barrier only in cases of the single bond type of rotation. Some factors that affect the stability of the conformers are van der Waals strain, steric strain, torsional strain, and angle strain.