Overview
This lecture introduces the concept of conjugation in organic molecules, explores its impact on molecular stability, and discusses resonance and molecular orbital (MO) theory as models for understanding conjugated systems.
Introduction to Conjugation
- Conjugation occurs when three or more adjacent atoms each have unhybridized p orbitals that overlap, forming a stable, low-energy molecular orbital.
- Conjugation increases stability by allowing delocalization of electrons across multiple atoms.
- Earlier topics like configuration, conformation, enantiomers, and diastereomers are different concepts from conjugation.
Molecular Orbital Theory and Conjugation
- MO theory is a model describing how atomic orbitals combine to form bonding and antibonding molecular orbitals in conjugated systems.
- The number of molecular orbitals equals the number of atomic p orbitals involved (e.g., four atomic p orbitals yield four molecular orbitals).
- Fewer nodes in a molecular orbital mean lower energy; more nodes mean higher energy.
- Electrons occupying low-energy bonding orbitals stabilize the molecule.
Resonance Theory
- Resonance theory models the delocalization of electrons in conjugated systems through multiple resonance forms.
- The more resonance forms a molecule has, the lower its overall energy (with some exceptions).
- Resonance hybrids represent the actual electron distribution, which is spread over multiple atoms.
Stability of Conjugated Compounds
- Experimental evidence (e.g., heats of hydrogenation) shows conjugated systems are more stable (lower in energy) than non-conjugated analogs.
- Allylic carbocations and radicals are unusually stable due to conjugation and electron delocalization.
Common Resonance Systems
- Four types of resonance stabilization in molecules:
- Three-atom π-allyl (π-allyl) systems (X=Y–Z with charge or lone electron on Z).
- Conjugated double bonds (e.g., dienes).
- Positive charge adjacent to a lone pair.
- Double bonds involving a more electronegative atom (e.g., carbonyls).
- Recognize, but do not memorize, these classifications.
Resonance Effects on Acid-Base Properties
- Conjugation and resonance lower the pKa (increase acidity) by stabilizing the conjugate base through electron delocalization.
- Example: α-hydrogens next to carbonyl groups are much more acidic than those in alkanes due to resonance stabilization of the resulting anion.
- Carboxylic acids are more acidic than alcohols, and phenols are more acidic than alcohols, due to resonance stabilization of their conjugate bases.
Aromatic Stability
- Aromatic compounds (like benzene) are extremely stable due to extensive resonance and electron delocalization.
- Benzene’s stability makes it far less reactive than typical alkenes.
Key Terms & Definitions
- Conjugation — Overlap of unhybridized p orbitals on three or more adjacent atoms enabling electron delocalization.
- Molecular Orbital (MO) Theory — Model describing combination of atomic orbitals into molecular orbitals (bonding/antibonding).
- Resonance — Representation of electron delocalization by drawing multiple valid Lewis structures (resonance forms).
- Node — Region in a molecular orbital where the probability of finding an electron is zero.
- Bonding/Antibonding Orbitals — Bonding MOs have low energy and stabilize molecules; antibonding MOs have higher energy.
Action Items / Next Steps
- Review SP2 hybridization and formation of π bonds (Chapter 1 material).
- Practice drawing resonance forms for π-allyl systems and carbonyl-containing compounds.
- Memorize relevant pKa values for alkanes, alcohols, carboxylic acids, and phenols.
- Prepare for next lecture: structure and properties of benzene.