Chapter 16: Conjugated Unsaturated Systems

Contents of Chapter 16

• Conjugated Diene
• Molecular Orbitals
• 1,2- and 1,4-Addition
• Kinetic vs. Thermodynamic Control
• Diels-Alder Reaction
• Aromatic Transition States
• UV Spectroscopy

Conjugation

• Definition: Conjugation is the sharing of electrons in a contiguous (conjugated) π system.   - Includes example systems:     - Allylic systems     - Conjugated dienes     - α,β-unsaturated carbonyls     - Aromatic systems

Molecular Orbitals (MO)

• Concept: Sideways overlap of p orbitals results in the formation of π Molecular Orbitals (MOs).   - p orbitals consist of 2 lobes.   - The symbols plus (+) and minus (-) indicate opposite phases of the wave function, not electrical charge.   - Bonding MO: Formed when lobes overlap constructively (+ and + or - and -).   - Antibonding MO: Formed when lobes overlap destructively (+ and -), resulting in a node.

Constructing Molecular Orbitals

• In linear systems, the number of p orbitals equals the number of MOs:   - Half bonding   - Half antibonding (*)   - If there is an odd number of MOs, the middle one is nonbonding.
• In a stable system:   - Bonding MOs are filled and antibonding MOs are empty.
• Bonding Process: Involves electrons from the highest occupied MO (HOMO) of the donor and the lowest unoccupied MO (LUMO) of the acceptor.

Allylic Systems

• Examples: Allylic cation, allylic radical, allylic anion     - Structures include allyl bromide and allyl alcohol.

Dienes and Their Classifications

• Dienes can be classified as:   - Isolated Dienes   - Conjugated Dienes   - Cumulated Dienes   - Linear Formula: (CH2)n
• Preferred Conformations:   - Coplanar conformations are preferred.   - Stability: s-trans conformations are more stable than s-cis due to steric strain.     - $ext{ΔG} ext{~} 3 ext{ kcal}$ for steric strain differences.     - $E_a ext{~} 4 ext{ kcal}$ represents the energy barrier to rotation.

Synthesis of Conjugated Dienes

• Synthesized via dehydrohalogenation or dehalogenation reactions, analogous to alkene synthesis.

MO Energy Diagram for Conjugated Dienes

• The average energy of electrons in a conjugated system is lower than that in isolated systems.   - Lowest Energy MO: π1 for 1,3-butadiene consists of all bonding interactions with electrons delocalized over four nuclei.

π Molecular Orbitals of 1,3-Butadiene

• π2 MO:   - Contains 2 bonding interactions and 1 antibonding interaction.   - Considered a bonding MO and represents the HOMO.
• π3* MO:   - Contains 1 bonding and 2 antibonding interactions.   - An antibonding MO with two nodes.
• π4* MO:   - Contains all antibonding interactions and is the highest energy MO, vacant in the ground state.

Electrophilic Additions to Conjugated Dienes

• Types of additions include 1,2-addition (direct addition) and 1,4-addition (conjugate addition).

Kinetic vs. Thermodynamic Control

• Kinetic Control:   - Product distribution determined by the relative rate of formation of products.   - For reactions involving HBr and Br2, 1,2-addition occurs faster than 1,4-addition.
• Thermodynamic Control:   - Product distribution determined by the relative stabilities of products.   - 1,4-addition products are more stable than 1,2-addition products.

Example Reaction Data

• Addition of 1 equivalent of HBr or Br2 to butadiene yields two constitutional isomers.   - Minor and major products vary with temperature (e.g., 40°C or 60°C).

Diels-Alder Reaction

• A pericyclic reaction involving cycloaddition of a conjugated diene and a dienophile (a compound with double or triple bonds).
• Features of Diels-Alder Reaction:   - Both a unique method of forming six-membered rings.   - Concurrent formation of two carbon-carbon bonds, stereospecific nature of the products.
• Diene Requirements: Must adopt s-cis conformation for reactivity.
• The Diels-Alder reaction is facilitated by the presence of electron-withdrawing substituents on the dienophile and electron-donating substituents on the diene.
• Positioning of Dienophile: Endo orientation is favored under kinetic control due to secondary orbital overlaps.
• Regiospecificity: Bicyclic Diels-Alder products show specific substituent orientations (1,2 or 1,4 placements).

UV Spectroscopy

• Wavelength Range: Ultraviolet light at 120 - 400 nm.
• Fundamental Absorption: All organic molecules absorb UV light, exciting electrons from the HOMO to the LUMO.
• Beer’s Law: $A = ext{εcl}$ where:   - A = Absorbance   - ε = Molar absorptivity   - c = Concentration (mol/L)
    - l = Path length of light (cm)
• Method of obtaining UV spectrum involves measuring intensity through solvent only vs. through the solution.

Analytical Applications of UV-Vis Spectroscopy

• Used for:   - Elucidating molecular structures and presence of conjugation.   - Quantitative analysis of unknown samples, especially in biochemical contexts.  

Conclusion

• The chapter summarizes key features and mechanisms of conjugated systems, their reactions including Diels-Alder and electrophilic additions, as well as essential spectroscopy techniques for studying these systems.