Role Of Steric Hindrance

The reactivity and shape of molecules and ions are influenced by the steric effect, also known as non-bonding interactions. The consequence of this steric effect is called steric hindrance. The steric hindrance definition is extensive yet comprehensive. It can be defined as the slowing of chemical reactions due to the presence of steric bulk. In a simpler way, it can be explained how the physical structure of a molecule can have an effect on its ability to react. 

The above steric hindrance definition could be explained by the example that if a molecule has multiple bonds to groups or compounds other than hydrogen, meaning, if that molecule is bulky, it could slow down or could even stop a different molecule from finding its desired bond site, in a given chemical reaction. So, this steric hindrance effect at a particular atom or even a molecule could prevent or slow down the reactions at the atom. This steric effect in organic chemistry is nothing but the congestion caused by the physical presence of ligands. 

Steric hindrance affects the properties of the substituents, and thus, these steric properties have had numerous methods to be assessed. Through rate data, and A-value, both of which somewhat measure the bulk of the substituents, Ceiling temperatures and Ligand cone angles, we could assess the steric properties of the substituents. 

After understanding the steric hindrance definition, let us now move to the steric hindrance effect. The effect of steric hindrance could be largely classified into steric strain and reaction sensitivity. 

Steric strain

When the molecule of a chemical structure undergoes some stress, it raises its internal energy when compared to a referenced strain-free compound. This is when a molecule is said to have experienced strain. This strained molecule will have some extra amount of internal energy or strained energy. Now, usually, a molecule with the lowest energy formed is favoured, as it is in its natural structure. Hence there is very little repulsion between groups, and the angle strains present between the bonds are comparatively less too. Although when there are numerous bulky groups in close vicinity of one another, statin strain can occur. Thus, even when the electrons are repelling each other, the energy is used to force these bond angles to stay in a particular way. 

Reaction Selectivity

This steric hindrance effect of a molecule could be used in order to favour a specific reaction. Let’s support this with the help of an example. We are well aware that there exist two kinds of substitution reactions, being Sn1 and Sn2. Though both of these reactions can very easily occur with simple molecules, Sn2 cannot occur with bulky molecules, whereas the Sn1 reaction can. This means that only if Sn1 is desired a bulky molecule could be used to favour that reaction. 

Let’s take an example to explain the steric effect in organic chemistry,

The reaction between an electrophile and a nucleophile is a simple and easy way to portray the effects of steric hindrance. Let us take OH- as our nucleophile. We could change the bulk of the electrophile in place by adding more CH3  or methyl groups. What we observe is that when more and more methyl groups are added to this molecule, the space needed to form the required covalent bond to the electrophile decreases. This shows that when the bulk of the electrophile or the steric bulk increases, it hinders the molecule from performing various reactions. 

Applications of steric hindrance 

This steric effect in organic chemistry is vital for its application in various fields, including pharmacology, chemistry and biochemistry. Be it to predict the rate at which a drug might interact with the target molecule, to exploit the naturally occurring enzymes in biochemistry or to determine the activation energies and rates of chemical reactions occurring in organic chemistry, the application of steric effect in Organic chemistry are plenty. 

Conclusion

Thus, to conclude, after knowing about the steric hindrance definition, we go to know that due to the steric bulk caused by the surrounding ligands, steric hindrance could slow or prevent reactions in the atoms or molecules. It has a vast number of applications in the field of organic chemistry and can be useful even in reaction selectivity.