Overview
This lecture introduces the concept of conjugation in organic chemistry, focusing on the stability provided by delocalized electrons in conjugated systems and defining key related terms.
What is Conjugation?
- Conjugation occurs when there are continuous, overlapping p orbitals across three or more adjacent sp2-hybridized atoms.
- This overlap allows pi electrons to delocalize, spreading over multiple atoms, which stabilizes the molecule.
- Conjugated systems are typically lower in energy due to this electron delocalization.
Hybridization and Pi Bonds Review
- A pi (ฯ) bond forms from the side-by-side overlap of unhybridized p orbitals on sp2-hybridized carbons.
- An sp2-hybridized carbon has three bonding areas and one unhybridized p orbital.
Stability Through Resonance
- Resonance is the movement of electrons within a molecule, allowing charge to be spread out (delocalized).
- Delocalization through resonance stabilizes molecules by lowering their overall energy.
Identifying Conjugated Structures
- A structure is conjugated if there are at least three adjacent sp2-hybridized atoms, each with p orbitals that overlap.
- Simple carbon chains with alternating double bonds (like 1,3-butadiene) are classic examples.
Conjugation Beyond Double Bonds
- Carbocations (positively charged carbons), carbanions (negatively charged carbons), and radicals (atoms with an unpaired electron) can participate in conjugation if they are sp2-hybridized.
- Lone pairs and charges can align their orbitals to participate in conjugation, even if not originally sp2-hybridized.
The Allylic Position
- The allylic position refers to atoms or charges (carbocation, carbanion, or radical) directly adjacent to a double bond but not part of it.
- Allylic charges/radicals benefit from conjugation and are stabilized by resonance.
Key Terms & Definitions
- Conjugation โ overlapping p orbitals across three or more adjacent atoms, allowing electron delocalization and increased stability.
- sp2 hybridization โ a type of carbon orbital arrangement with three bonds and one unhybridized p orbital.
- Pi (ฯ) bond โ bond formed by sideways overlap of two p orbitals.
- Resonance โ distribution of electrons across multiple structures, leading to stabilization.
- Allylic position โ position directly next to a double bond, capable of conjugation if containing a charge or radical.
Action Items / Next Steps
- Complete the worksheet on identifying conjugated structures versus non-conjugated ones.