Difference Between Hyperconjugation and Resonance

Main Difference – Hyperconjugation vs Resonance

In a covalent compound, two major types of chemical bonds can be observed between atoms. They are the sigma bond and the pi bond. A single bond is always a sigma bond. A double bond is composed of a sigma bond and a pi bond. However, both types of bonds are formed due to the overlapping between atomic orbitals. The term hyperconjugation and resonance are used to describe two methods involved in stabilizing a molecule. The main difference between hyperconjugation and resonance is that hyperconjugation involves the interaction between a sigma bond and a p orbital or a pi bond whereas resonance involves the interaction between pi bonds.  

Key Areas Covered

1. What is Hyperconjugation
      – Definition, Mechanism, and Examples
2. What is Resonance
      – Definition, Mechanism, and Examples
3. What is the Difference Between Hyperconjugation and Resonance
      – Comparison of Key Differences

Key Terms: Atomic Orbitals, Hyperconjugation, Pi Bond, Resonance, Sigma Bond

What is Hyperconjugation

Hyperconjugation is the stabilization effect on a molecule due to the interaction between a sigma bond and a pi bond. Here, a sigma orbital will interact with an adjacent empty p orbital, partially filled p orbital or a pi orbital. This interaction is an overlapping of these orbitals. This results in the formation of an extended molecular orbital which gives the bonding electron more space. Then, the repulsion forces between electrons are decreased. As a result, the molecule gets stabilized. Usually, hyperconjugation occurs through the overlapping of bonding electrons of C-H sigma bond with 2p or pi orbital of the adjacent carbon.

Figure 1: Overlapping of a bonding orbital (C-H) with an antibonding orbital (C-Cl)

Hyperconjugation affects the bond length of a chemical bond. Usually, a sigma bond between two atoms is longer than a pi bond between the same two atoms. Hyperconjugation causes the length of the sigma bond to be reduced and the length of the pi bond to be increased. Moreover, it helps to increase the stability of a carbocation.

What is Resonance

Resonance is the stabilizing of a molecule through delocalization of bonding electrons in the pi orbital. As electrons do not have a fixed position in an atom or a molecule, they can move here and there easily. Therefore, the lone electrons and pi bonding electrons can be moved from one position to another in order to obtain a stabilized state. This is called resonance. In order to determine the most stable form of a molecule, we use resonance structures that show all possible structures a particular molecule can have.

Resonance structures have the same number of electrons and the same molecular formula. The hybridization of atoms in the molecule should also be same in every resonance structure along with the same number of lone pairs.

Figure 2:  Resonance Structures of Phenol

The above image shows all possible resonance structures of phenol. At the end of the resonance structures, the original structure of the phenol molecule has been given. It indicates that the real molecule does not have pure double bonds. There is a pi electron cloud instead of three double bonds. Therefore, resonance gives an intermediate structure to resonance structures.

Difference Between Hyperconjugation and Resonance

Definition

Hyperconjugation: Hyperconjugation is the stabilization effect on a molecule due to the interaction between a sigma bond and a pi bond.

Resonance: Resonance is the stabilizing of a molecule through delocalization of bonding electrons in the pi orbital.

Orbitals Involved

Hyperconjugation: Hyperconjugation involves sigma bond orbitals and p orbitals or pi bond orbitals.

Resonance: Resonance involves only pi bond orbitals.

Bond Length

Hyperconjugation: Hyperconjugation causes the sigma bond length to be shortened.

Resonance: Resonance has no effect on the sigma bonds.

Conclusion

Hyperconjugation is an extension of resonance since both methods cause the stabilization of a molecule through delocalization of electrons; however, hyperconjugation involves delocalization of sigma bond electrons along with pi bond electrons whereas resonance causes the delocalization through interaction between pi orbitals. This is the difference between hyperconjugation and resonance.

References:

1.“Resonance.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here. Accessed 25 Aug. 2017.
2. Devyani Joshi, Trainee at SRS Pharmaceuticals Pvt. Ltd., India Follow. “Hyperconjugation – organic chemistry.” LinkedIn SlideShare, 10 Nov. 2016, Available here. Accessed 25 Aug. 2017.

Image Courtesy:

1. “C-H bonding orbital mixing with a C-X anti-bonding orbital through hyperconjugation” By Hafargher – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Phenol mesomeric structures” By Devon Fyson – Own work (CC BY-SA 3.0) via Commons Wikimedia