Conjugation:
* Involves: Overlap of p-orbitals in a system of alternating single and double bonds (or other pi systems).
* Mechanism: The p-orbitals align and overlap, forming a delocalized pi-electron system. This delocalization spreads the electron density over a larger area, increasing stability.
* Examples: Alkenes, alkynes, aromatic compounds, conjugated ketones.
* Effects:
* Increased stability: The delocalization of electrons lowers the energy of the system.
* Changes in chemical reactivity: Conjugation can alter the reactivity of functional groups.
* UV-Vis spectroscopy: Conjugated systems often absorb UV-Vis light due to the delocalized electrons.
Hyperconjugation:
* Involves: Overlap of the sigma bond (C-H or C-C) with an adjacent empty or partially filled p-orbital.
* Mechanism: The electrons in the sigma bond are "donated" to the empty or partially filled p-orbital, leading to a delocalization of electron density.
* Examples: Alkanes, carbocations, free radicals.
* Effects:
* Increased stability: Hyperconjugation stabilizes carbocations and radicals by dispersing the positive charge or unpaired electron.
* Alkanes: Hyperconjugation contributes to the stability of alkanes, even though they are not directly conjugated.
* NMR spectroscopy: Hyperconjugation can influence the chemical shift of protons in NMR spectra.
Here's a table summarizing the differences:
| Feature | Conjugation | Hyperconjugation |
|---|---|---|
| Orbitals involved | p-orbitals | sigma bonds and p-orbitals |
| Mechanism | Overlap of p-orbitals | Overlap of sigma bonds with p-orbitals |
| Examples | Alkenes, alkynes, aromatic compounds | Alkanes, carbocations, radicals |
| Stability | Increased stability due to delocalization | Increased stability by dispersing charge or unpaired electrons |
In short:
* Conjugation: Delocalization of pi electrons in p-orbitals.
* Hyperconjugation: Delocalization of sigma electrons into adjacent p-orbitals.
Both conjugation and hyperconjugation are important for understanding the structure, reactivity, and spectroscopic properties of molecules.