Difference Between Hyperconjugation and Resonance

Key Difference – Hyperconjugation vs Resonance

Hyperconjugation and resonance can stabilise polyatomic molecules or ions in two different ways. The requirements for these two processes are different.If a molecule can have more than one resonance structure, that molecule possesses the resonance stabilisation. But, hyperconjugation occurs in the presence of aσ-bondwith an adjacent empty or partially filled p-orbital or a π-orbital.This is thekey difference超结合和共振

What is Hyperconjugation?

电子在σ键（通常C-H或C-C键）与相邻的空或部分填充的P轨道或π轨道的相互作用通过增加系统的稳定性而导致扩展的分子轨道。这种稳定相互作用称为“超轭”。根据valence bond theory, this interaction is described as ‘double bond no bond resonance’.

What is Resonance?

谐振is the method of describing delocalized electrons in a molecule or polyatomic ion when it can have more than one Lewis structure to express the bonding pattern. Several contributing structures can be used to represent these delocalized electrons in a molecule or an ion, and those structures are calledresonance structures. All the contributing structures can be illustrated using a Lewis structure with a countable number ofcovalent bondsby distributing the electron pair between two atoms in the bond. Since several Lewis structures can be used to represent the molecular structure. The actual molecular structure is an intermediate of all those possible Lewis structures. It is called a共振混合动力. All the contributing structures have the nuclei in the same position, but the distribution of electrons can be different.

What is the difference between Hyperconjugation and Resonance?

超结合和共振的特征

Hyperconjugation

• Hyperconjugation affects the bond length, and it results in the shortening of sigma bonds (σ bonds).

Molecule	碳碳键的长度	原因
1,3-Butadiene	1.46 a	Normal conjugation between two alkenyl parts.
Methylacetylene	1.46 a	Hyperconjugation between the alkyl and alkynyl parts
甲烷	1.54 A	It is a saturated hydrocarbon with no hyperconjugation

• Molecules with hyperconjugation have higher values for the heat of formation compared to the sum of their bond energies. But, the heat of hydrogenation perdouble bondis less than that of in ethylene.

• The stability of碳正离子svaries depending on the number of C-H bonds attached to the positively charged carbon atom. The hyperconjugation stabilisation is greater when many C-H bonds are attached.

(CH₃)₃C⁺>（ch₃)₂CH⁺>（ch₃)CH₂⁺> CH₃⁺

• Relative hyperconjugation strength depends on the isotope type of the Hydrogen. Hydrogen has a greater strength compared toDeuterium (D)和tritium（t）. Tritium has the least ability to show hyperconjugation among them. The energy required to break C-T bond > C-D bond > C-H bond, and this makes it easier for H to hyperconjugation.

谐振

• Several Lewis structures can be used to represent the structure, but the actual structure is an intermediate of these contributing structures, and it is represented by a resonance hybrid.

• The resonance structures are not isomers. These resonance structures differ only in the position of electrons, but not in the position of nuclei.

• 每个路易斯结构的价值和未配对的电子都相等，这导致每个结构中的电荷相同。

• The actual structure possesses a lower total potential energy compared to the estimated value of the contributing structures. Therefore, molecules/ions having a resonance hybrid provides an extra stabilisation to the respective molecule/ion.

参考：

“What is Hyperconjugation ?”– Department of Chemistry, University of Calgary

Hyperconjugation– Wikipedia.org

“Chapter 4: Alcohols and Alkyl Halides”– Department of Chemistry, University of Calgary

“Hyperconjugation” –TutorVista.com

Image Courtesy:

“Phenol resonance” By Smallman12q – Own work (Public Domain) viaCommons Wikimedia

“Schreiner Hyperconjugation” By Chem540grp7f08 – Own work (Public Domain) viaCommons Wikimedia