The key difference between isovalent and sacrificial hyperconjugation is their main form and canonical form. Isovalent hyperconjugation occurs in free radicals and carbocations where the canonical form displays no charge separation, but the main form has a charge separation, whereas sacrificial hyperconjugation is the state where canonical form involves no bond resonance, but the main form has no charge distribution.
Before understanding the difference between isovalent and sacrificial hyperconjugation, it’s important to understand what is hyperconjugation. Hyperconjugation is the interaction of σ-bonds with a pi bond network.
CONTENTS
1. Overview and Key Difference
2. What is Hyperconjugation
3. What is Isovalent Hyperconjugation
4. What is Sacrificial Hyperconjugation
5. Side by Side Comparison – Isovalent vs Sacrificial Hyperconjugation in Tabular Form
6. Summary
What is Hyperconjugation?
The term hyperconjugation refers to the interaction of σ-bonds with a pi network. In this interaction, electrons in a sigma bond interact with an adjacent partially (or completely) filled p orbital or with a pi orbital. This type of interactions takes place to increase the stability of a molecule.
Figure 01: Hyperconjugation
Generally, hyperconjugation happens because of the overlap of bonding electrons in the C-H sigma bond with a p orbital or a pi orbital of the adjacent carbon atom. Here, the hydrogen atom resides in close proximity as a proton. The negative charge that develops on the carbon atom is delocalized due to the overlap of p orbital or pi orbital.
What is Isovalent Hyperconjugation?
Isovalent hyperconjugation refers to the hyperconjugation that occurs in free radicals and carbocations where the canonical form displays no charge separation, but the main form has a charge separation. We can describe this type of hyperconjugation as an arrangement of chemical bonds in a hyperconjugated molecule where the number of bonds is similar to the two resonance structures while the second structure is less favourable by means of energy than the first structure. A good example of this type of hyperconjugation is H3C-CH2 and H3C-C+H2.
What is Sacrificial Hyperconjugation?
Sacrificial hyperconjugation refers to the hyperconjugation where canonical form involves no bond resonance but in the main form involves no charge distribution. This type of hyperconjugation is also known as “no bond hyperconjugation”. This is because, in the resonance structures of this hyperconjugation process, we can observe a bond is missing from the resonance structures ( the bond between a hydrogen atom and an alpha-carbon atom). Therefore, one of the hydrogen atoms is missing from the structure, but it still occurs nearby as a proton. This makes us able to give the alpha carbon atom its bond order as 1.5 approximately. Since there is one bond missing from the structure, it is known as sacrificial hyperconjugation.
What is the Difference Between Isovalent and Sacrificial Hyperconjugation?
The term hyperconjugation refers to the interaction of σ-bonds with a pi network. There are two major forms of hyperconjugation that we can discuss: isovalent and sacrificial hyperconjugation. The key difference between isovalent and sacrificial hyperconjugation is that the isovalent hyperconjugation occurs in free radicals and carbocations where the canonical form displays no charge separation, but the main form has. Meanwhile, the sacrificial hyperconjugation refers to the state where canonical form involves no bond resonance but in the main form involves no charge distribution.
Summary – Isovalent vs Sacrificial Hyperconjugation
The term hyperconjugation refers to the interaction of σ-bonds with a pi network. There are two major forms of hyperconjugation: isovalent and sacrificial hyperconjugation. The key difference between isovalent and sacrificial hyperconjugation is that isovalent hyperconjugation occurs in free radicals and carbocations where the canonical form displays no charge separation, but the main form does, whereas sacrificial hyperconjugation refers to the state where canonical form involves no bond resonance, but the main involves no charge distribution.
Reference:
1. “Nature of Bonding and Stereochemistry: Hyper Conjugation Part 3.” Examrace, Available here.
Image Courtesy:
1. “Alkene-hyperconjugation2” By Alsosaid1987 – Own work (CC BY-SA 4.0) via Commons Wikimedia