Steric strain is a term used in organic chemistry to describe the increase in potential energy of a molecule due to repulsion between electrons in atoms that are not directly bonded to each other. This repulsion occurs when bulky groups or atoms come too close to each other, causing an unfavorable interaction.
To understand steric strain, let’s consider an example. Imagine a molecule called 1,2-dibromoethane, which consists of two bromine atoms attached to a central carbon atom. In one conformation of this molecule, known as the staggered conformation, the bromine atoms are positioned in a way that they are further apart from each other. In another conformation, known as the eclipsed conformation, the bromine atoms are closer to each other.
The increase in potential energy in the eclipsed conformation compared to the staggered conformation is due to steric strain. The repulsion between the electron clouds of the bromine atoms causes this strain. In the eclipsed conformation, the bromine atoms are closer together, leading to increased repulsion and higher potential energy.
Steric strain can also be observed in other molecules, such as alkenes and ring systems. It is caused by van der Waals repulsions, which are the result of electron-electron repulsion between atoms or groups that are not directly bonded to each other. These repulsions cannot be reduced by rotating around a single bond.
The magnitude of steric strain depends on the size and shape of the bulky groups or atoms involved. Larger groups will experience stronger repulsion and therefore higher steric strain. The distance between these bulky groups also plays a role. The closer they are, the greater the steric strain.
It is important to note that steric strain is different from torsional strain. Torsional strain refers to the repulsion caused by the electrons in different groups when they pass by each other during molecular rotation. Torsional strain can be lessened by rotating the molecule around a sigma bond, unlike steric strain.
In conclusion, steric strain is the increase in potential energy of a molecule caused by repulsion between electrons in atoms that are not directly bonded to each other. It is a result of van der Waals repulsions and cannot be reduced by rotation around a single bond. Understanding steric strain is crucial in organic chemistry as it influences the stability and reactivity of molecules.