Lecture Notes on Conjugated Pi Systems and Molecular Orbital Theory

Announcements

Assignment 6: Open and available.
Quiz 6: To be posted today, covering concepts from Friday.
Exam 1 Grading: Expected by end of the week.
Next Lecture: Variations of Diels-Alder reactions; bring model kits to class.

Main Topic: Mechanism for halogenation and halohydrins in conjugated dienes.
Molecular Orbital Description: Dienes behave differently when conjugated.
- Applies to trienes, tetraenes, etc.
- Related to UV-Vis Spectroscopy.
Key Reaction: Diels-Alder Cycloaddition.

Reaction coordinate diagram:
- Thermodynamic Product: More stable, iodine adds to end carbon.
- Kinetic Product: Less stable, partial positive charge in more substituted position.
1,2 vs. 1,4 Addition: Kinetic vs. thermodynamic products.

Conjugated Pi System: Halogenation leads to 1,2 or 1,4 products.
Mechanism:
- Starts with bromination of 1,3-butadiene.
- Formation of bromonium ion.
- Allylic cation formation.
- Ends with halide ion or water addition (halohydrin formation).

Conjugated Pi Systems: More stable than other pi systems.
Bond Lengths: Single bonds in conjugated systems are shorter.
- SP2 Hybridization: Shorter, fatter than SP3.
Molecular Orbitals (MOs): Constructive and destructive overlap leads to bonding and antibonding orbitals.
- Phase: In-phase (bonding), out-of-phase (antibonding).
- Population of MOs: Start with lower energy orbitals.
HOMO and LUMO: Highest occupied and lowest unoccupied molecular orbitals.

Energy Levels: Related to HOMO-LUMO gap.
Conjugation and Wavelength: More conjugation lowers energy gap.
Lambda Max: Indicates maximum absorbance.
- Carotene and Lycopene: Examples showing visible color changes.

Topic: Pericyclic reactions, specifically Diels-Alder reaction.
Mechanism: Involves orbital overlap, no typical electrophiles or nucleophiles.

Bring molecular model kits to next class for visualizing three-dimensional structures.