Difference Between Steric and Torsional Strain

Main Difference – Steric vs Torsional Strain

Strain is the repulsion between bond electrons of a molecule. The arrangement of a molecule depends on the strain since bond electron pairs are arranged in a manner that minimizes the strain. There are three main types of strains that can be found in a molecule. They are angular strain, torsional strain, and steric strain. The angular strain occurs when the bond angles of actual molecules are different from that of ideal molecules. The torsional strain arises when a molecule is rotated around a bond. Steric strain is formed when two or more bulky groups get close to each other. The main difference between steric and torsional strain is that steric strain cannot be lessened by rotating the molecule around a bond whereas torsional strain can be lessened by rotating the molecule around a bond.

Key Areas Covered

1. What is Steric Strain
      – Definition, Explanation with Examples
2. What is Torsional Strain
      – Definition, Explanation with Examples
3. What is the Difference Between Steric and Torsional Strain
      – Comparison of Key Differences

Key Terms: Angular Strain, Bond Electron Pair, Steric Strain, Torsional Strain

What is Steric Strain

The steric strain is the repulsion between two atoms or groups of atoms when the distance between them is decreased. This is also called steric hindrance. The steric strain is very important in determining the arrangement of a molecule since each and every molecule is arranged in such a way that the steric strain is minimized. When the steric strain is minimized, the potential energy of that molecule is decreased. Since matter is stable when it has a lower energy level, the lower energy level of a molecule makes it a stable molecule.

The concept of steric strain is very important in predicting the products of a chemical reaction. This is because groups of atoms are attached to a carbon atom in such a way that the steric hindrance is minimized. Therefore, a chemical reaction will give a mixture of molecules where stable products and unstable products are included. But the major constituent of this mixture will always be the stable product with a minimized steric hindrance.

Figure 1: Steric Strain in Organic Compounds

As shown in the above image, the potential energy of a molecule is increased according to the steric strain they have. When the distance between two methyl groups is decreased, the potential energy is increased.

Figure 2: Steric Strain Increases When Bulky Groups are Present

The above image shows that steric strain is increased when bulky groups are present. More sterically hindered molecules have a higher potential energy when compared to less sterically hindered molecules. Therefore, less sterically hindered molecules are more stable.

What is Torsional Strain

Torsional strain is the repulsion that arises between atoms or group of atoms when a molecule is rotated around a sigma bond. This is the repulsion that can be observed when bond electrons pass by each other. This type of strain is important in determining the stable conformations of organic compounds. These conformations can be represented by Newman projections. The Newman projection of a molecule is the conformation of that molecule when looked through C-C bond from front-back direction.

The torsional strain arises when the dihedral angle of bulky groups are low. The dihedral angle is the angle between two bonds of two different carbon atoms in a Newman projection. If the dihedral angle is high, then the torsional strain is low.

Newman projections can be found in two types as staggered conformation and eclipsed conformation. The eclipsed conformation show a high torsional strain than that of staggered conformation.

Figure 3: Two Types of Newman Projection

As shown in the above image, staggered conformation shows a dihedral angle of 60^o and eclipsed conformation shows a dihedral angle of 0^o. But when the molecule is rotated, the conformation is changed. The torsional strain in staggered conformation is lower than that of eclipsed conformation. When the molecule is rotated, the eclipsed conformation can become the staggered conformation; thus, the torsional strain is reduced.

Difference Between Steric and Torsional Strain

Definition

Steric Strain: Steric strain is the repulsion between two atoms or groups of atoms when the distance between them is decreased.

Torsional Strain: Torsional strain is the repulsion that arises between atoms or group of atoms when a molecule is rotated around a sigma bond.

Rotation of the Molecule

Steric Strain: Steric strain cannot be lessened by rotating the molecule around a sigma bond.

Torsional Strain: Torsional strain can be lessened by rotating the molecule around a sigma bond.

Cause for the Strain

Steric Strain: Steric strain occurs when the distance between bulky groups of a molecule is decreased.

Torsional Strain: Torsional strain occurs when bond electrons pass by each other when the molecule is rotating.

Conclusion

The strain of a molecule is the repulsion between bond electrons or lone electron pairs present in that molecule. This repulsion causes the potential energy of a molecule to be increased. Then, it makes the molecule unstable. The steric strain of a molecule is determined by the bulky groups present in a molecule and the distance between those bulky groups. Newman projection is a simple structure that shows the arrangement of atoms or groups of atoms in an organic molecule. It can be used to determine the torsional strain of a molecule. The main difference between steric and torsional strain is that steric strain cannot be lessened by rotating the molecule around a bond whereas torsional strain can be lessened by rotating the molecule around a bond.

References:

1. “Torsional Strain.” OChemPal, Available here. Accessed 28 Aug. 2017.
2. “Strain (Chemistry).” Wikipedia, Wikimedia Foundation, 25 July 2017, Available here. Accessed 28 Aug. 2017.
3. “Dihedral Angle.” OChemPal, Available here. Accessed 28 Aug. 2017.

Image Courtesy:

1. “Napthalene phenanthraene methyl-methyl strai” By DMacks – Own work (Public Domain) via Commons Wikimedia
2. “Steric hindrance disp” By Mwolf37 – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Escalonada e eclipsada” By Pauloquimico – Own work (CC BY-SA 3.0) via Commons Wikimedia