Core Concepts
In this tutorial, you will learn about how physical structure can affect the reactivity of organic molecules through steric hindrance. Additionally, you will be able to visualize this concept by walking through an example.
Topics Covered in Other Articles
- Electrophiles
- Nucleophiles
- Molecular Geometry
- Substitution Reactions
Vocabulary
- Electrophile- electron-rich molecule
- Nucleophile- electron-poor molecule
- Steric Strain- increase in potential energy of a molecule due to electron repulsion of large side groups
Definition of Steric Hindrance
Steric hindrance is a phrase used in organic chemistry to describe how a molecule’s physical structure can affect its ability to react. When a molecule is bulky, meaning it has multiple bonds to compounds or groups other than hydrogen, it can slow down or even prevent another molecule from efficiently finding the desired bond site in a reaction. Let’s go through an example below!
Example of Steric Hindrance
A simple way to see the effects of steric hindrance is in a reaction between a nucleophile and an electrophile. We can use the same nucleophile, HO–, and change the bulk of the electrophile by adding more methyls, or CH3 groups. As shown in the diagram below, as more methyl groups are added to the molecule, there is less space for the covalent bond to the electrophile to form. Therefore, as the steric bulk increases, a molecule can be hindered from performing different reactions.
The Effects of Steric Hindrance
Steric Strain
The lowest energy form of a molecule is usually favored because it is the most natural structure, meaning there is little to no repulsion within groups and angle strain between bonds. But steric strain can occur when there are multiple bulky groups near each other in a molecule. Energy is being used to force the bond angles to stay a certain way even though the electrons of the groups are repelling each other.
Reaction Selectivity
A molecule’s steric hindrance can be used to favor a specific reaction. For example, there are two types of substitution reactions, Sn1 and Sn2. Both of these reactions can be performed with simple molecules; however, Sn1 can occur with bulky molecules, while Sn2 cannot because it is too congested for the mechanism known as a ‘backside attack’. Because of this, if an Sn1 reaction is desired, a bulky molecule may be used to make sure the Sn2 reaction does not occur.