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 a σ-bond with an adjacent empty or partially filled p-orbital or a π-orbital. This is the key difference Hyperconjugation and Resonance
What is Hyperconjugation?
The interaction of electrons in a σ-bond (generally C-H or C-C bonds) with an adjacent empty or partially filled p-orbital or a π-orbital results in an extended molecular orbital by increasing the stability of the system. This stabilisation interaction is called the ‘hyperconjugation. According to the valence bond theory, this interaction is described as ‘double bond no bond resonance’.
What is Resonance?
Resonance 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 called resonance structures. All the contributing structures can be illustrated using a Lewis structure with a countable number of covalent bonds by 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 resonance hybrid. 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?
Characteristics of Hyperconjugation and Resonance
- Hyperconjugation affects the bond length, and it results in the shortening of sigma bonds (σ bonds).
|Molecule||C-C bond length||Reason|
|1,3-Butadiene||1.46 A||Normal conjugation between two alkenyl parts.|
|Methylacetylene||1.46 A||Hyperconjugation between the alkyl and alkynyl parts|
|Methane||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 per double bond is less than that of in ethylene.
- The stability of carbocations varies 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 to Deuterium (D) and 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.
- 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.
- Each Lewis structure has an equal number of valence and unpaired electrons, and this leads to having the same charge in each structure.
- 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.
References: “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) via Commons Wikimedia “Schreiner Hyperconjugation” By Chem540grp7f08 – Own work (Public Domain) via Commons Wikimedia