Conjugated Diene Stability

• Heats of hydrogenation show conjugated dienes are more stable than two alkenes that are not conjugated

  • Approximate stabilization energy: $riangle H_{ ext{hydrogenation}} ext{ for conjugated dienes} \, \approx -15 \,\text{kJ/mol}$ relative to two isolated alkenes

• This stabilization arises from delocalization of pi electrons across the conjugated system

• Conceptual takeaway: conjugation lowers the overall energy of the system, making it more thermodynamically stable

Valence Bond Theory vs Molecular Orbital Theory

• Valence Bond (VB) Theory

  • Describes bonds as localized between pairs of atoms

  • Uses resonance (multiple Lewis structures) to describe delocalization indirectly

  • Bonds are drawn as line structures; e.g., localized σ and π bonds

• Molecular Orbital (MO) Theory

  • Describes electrons as delocalized over the entire molecule

  • Bonding and antibonding interactions arise from overlap of atomic orbitals (AOs) and/or hybrid orbitals

  • AOs combine to form molecular orbitals (σ, σ, π, π)

  • Produces delocalization built into the description of the molecule

• Major contrasts

  • VB: localized bonds + resonance forms; MO: delocalized electrons in MOs

  • VB emphasizes resonance between discrete structures; MO emphasizes continuous electron distribution in MOs

  • VB uses hybridization concepts to explain geometry; MO uses energy-level diagrams to explain bonding patterns

Valence Bond Theory vs Molecular Orbital Theory (Summary of Key Differences)

• VB bonds are localized; MO bonds are delocalized over the molecule

• VB relies on resonance forms to describe electron distribution; MO relies on constructive/destructive overlap of AOs to form bonding/antibonding MOs

• VB uses overlap of atomic/hybrid orbitals to explain bond formation; MO uses linear combinations of AOs to form molecular orbitals

• VB predicts molecular shape from electron pairs; MO predicts electron arrangement via MO occupancy

• Resonance in VB is needed to describe delocalization; in MO, delocalization is built-in

Molecular Orbital Theory: Ethylene

• Two atomic p orbitals on adjacent carbons overlap to form two new MOs

• The MO extends over the entire molecule

• The π bond in ethylene arises from the overlap of two unhybridized p orbitals

• MO description: one bonding π MO and one antibonding π* MO

• Relationship between MOs and atoms: number of MOs equals number of AOs (here, two p orbitals → two MOs)

• Electron occupancy: both electrons occupy the bonding π MO

Molecular Orbital Theory: Butadiene

• Butadiene is a conjugated diene (two ethylenes separated by a single bond)

• The π-bonding MOs involve the overlap of four unhybridized p orbitals

• The lowest energy MOs are filled first; electrons fill bonding MOs two at a time

• Nodes increase as energy increases, leading to higher-energy anti-bonding character in higher MOs

• The MO diagram provides an explanation for properties like the shorter, stronger central C–C bond due to delocalization across four atoms

Molecular Orbital Theory: Hexatriene

• Hexatriene involves six p orbitals (three conjugated double bonds)

• Construct an energy diagram with six MOs corresponding to the six p orbitals

• The pi system uses six pi electrons (assuming all π bonds contribute six electrons in total)

• The MO diagram shows progressive nodes and energy levels across the six-atom conjugated system

• When building the diagram, fill the lowest-energy bonding MOs first, pairwise, then place electrons in higher MOs as needed

Frontier MO Theory

• Frontier Orbitals: Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)

• These frontier orbitals are the most important for predicting reactivity, especially in photochemistry and cycloadditions

• Conjugated systems can interact with light via HOMO → LUMO transitions

• The energy gap between HOMO and LUMO often dictates absorption properties and color

• In many reactions, the interaction between the HOMO of one reactant and the LUMO of another governs whether a reaction is favorable

UV-Vis Spectroscopy

• Conjugated pi systems absorb UV or visible light

• Electronic transition: from HOMO to LUMO (π → π*) requires energy that falls in the UV/visible region

• UV-Vis provides structural information: absorbance depends on wavelength and electronic transitions

• A = log(I0/I), where A is absorbance, I0 is incident intensity, and I is transmitted intensity

• Molar absorptivity (ε) describes the intensity of absorption: $\varepsilon = \frac{A}{cl}$, where c is concentration and l is path length

• λmax (the maximum absorption wavelength) indicates the energy required for the HOMO→LUMO transition

• More conjugation lowers the π → π* energy gap, leading to larger λmax (red shift)

UV-Vis Spectroscopy: Practical Implications

• Increasing conjugation shifts absorption to longer wavelengths (smaller energy gap)

• Spectral data can inform about the degree of conjugation in a molecule

• Useful for structural assignments and monitoring electronic transitions in real-time

Looking Ahead (Course Planning)

• Continue Chapter 16, next session

• SI sessions and recitations scheduled in campus facilities

• Recitation times and locations provided for study support

Hydrohalogenation of Conjugated Dienes

• Hydrohalogenation: H–X adds across alkenes

• For conjugated dienes, hydrohalogenation can yield two regioisomeric products (Markovnikov regioselectivity) depending on where the proton adds and where the halide attaches

• Protonation of a conjugated diene forms the more stable, resonance-stabilized allylic carbocation

• Nucleophilic attack of the halide (X−) then occurs at either allylic position, generating two possible products

• In conjugated systems, electrophiles can add via 1,2- or 1,4-addition patterns depending on kinetics and thermodynamics

Electrophilic Addition to Conjugated Dienes (1,2- vs 1,4-Addition)

• 1,2-addition products form faster (kinetically favored) due to proximity of reactive centers

• 1,4-addition products are typically more stable (thermodynamically favored) due to greater substitution and overall stability of the product

• Temperature influences product distribution:

  • At lower temperatures, kinetic control dominates (more 1,2-adduct)

  • At higher temperatures, thermodynamic control dominates (more 1,4-adduct)

• Example data (illustrative): At 0 °C, 71% 1,2-adduct and 29% 1,4-adduct; At 40 °C, 15% 1,2-adduct and 85% 1,4-adduct

Electrophilic Addition to Conjugated Dienes: Temperature and Control

• Lower temperatures favor the kinetically faster 1,2-adduct due to a lower activation barrier

• Higher temperatures allow equilibration; the more stable, more substituted alkene (1,4-adduct) dominates under thermodynamic control

• Conceptual takeaway: temperature controls whether a reaction is under kinetic or thermodynamic control

Pericyclic Reactions: Introduction and Features

• Pericyclic reactions occur without ionic or radical intermediates

• Four characteristic features:
  1) Mechanism is concerted, proceeding without intermediates
  2) Involves a ring of electrons moving around a closed loop
  3) Transition state is cyclic
  4) Solvent polarity generally has no effect on the reaction rate

• Three main types:
  1) Cycloaddition reactions
  2) Electrocyclic reactions
  3) Sigmatropic rearrangements

The Diels–Alder Reaction

• A celebrated cycloaddition: conjugated diene reacts with a dienophile (alkene or alkyne)

• Forms a substituted cyclohexene from a diene and an alkene/alkyne

• Arrow-pushing can be drawn in a clockwise or counterclockwise fashion; no discrete intermediates in the energy diagram

• The Diels–Alder reaction is a [4+2] cycloaddition: the diene contributes 4 π electrons; the dienophile contributes 2 π electrons

• Schematic outcome: formation of a six-membered ring (cyclohexene derivative)

Diels–Alder Reaction: Practical Considerations

• Factors affecting reaction efficiency and outcome:

  • Substituents on the dienophile (electron-withdrawing groups accelerate the reaction and increase yield)

  • Conformation of the diene (must adopt s-cis conformation for reaction to occur readily)

  • Stereospecificity (the reaction preserves stereochemistry of the starting dienophile/diene)

  • Regioselectivity (alignment of substituents determines major regioisomer)

• Electron-withdrawing groups (EWG) on the dienophile accelerate the reaction by lowering the LUMO energy

• An alkyne can function as a dienophile as well

Diels–Alder Considerations: Substituents and Conformation

• Dienophile substituents: EWG on the dienophile increases reaction rate and yield

• Diene conformation: only s-cis dienes react readily; s-trans or other conformations are less reactive

• Stereospecificity: cis/trans relationships in starting materials influence the stereochemistry of the product

• Regioselectivity: depends on substitution pattern; if both components are unsymmetrical, two regioisomeric products are possible; major product follows alignment of electron-rich and electron-poor regions

Regioselectivity in Diels–Alder Reactions

• Major product prediction involves aligning partial charges: regions of high electron density on the diene align with electron-poor areas on the dienophile

• Resonance structures can be used to predict the favored regiochemical outcome

Cyclic Dienes: Endo versus Exo

• When a cyclic diene undergoes a Diels–Alder reaction, two bicyclic products are possible: endo and exo

• The major product is typically the endo cycloadduct due to favorable secondary interactions between developing π bond and electron-withdrawing groups (EWG) in the transition state

Predicting Products and Practice Questions (Diels–Alder)

• Practice: predict products for given reactions and rank dienes by reactivity in Diels–Alder reactions

• Typical prompts involve identifying stereochemistry (cis/trans), endo/exo outcomes, and regiochemical major products based on substituent effects

Diels–Alder Thermodynamics and Conditions

• Most Diels–Alder reactions are thermodynamically favored at low to moderate temperatures

• At temperatures above roughly 200 °C, retro-Diels–Alder can predominate

• ΔG must be negative for the reaction to be favorable; ΔH is typically negative due to bond formation, and ΔS is also negative due to a decrease in the number of molecules and formation of a ring

• Diels–Alder thermodynamics: overall gain in bond strength often outweighs entropy loss at appropriate temperatures

Additional Diels–Alder Considerations

• Substituent effects on dienophile; s-cis conformations on diene; stereospecificity; regioselectivity are key

• Endo rule often governs major product due to secondary orbital interactions

Diels–Alder: Practical Look Ahead

• Most chapter-focused content emphasizes applying MO and frontier-orbital concepts to predict outcomes

• Consider HOMO of the diene interacting with LUMO of the dienophile for cycloaddition feasibility

• Symmetry-allowed vs symmetry-forbidden pathways depend on orbital phase matching

M.O. Theory and Cycloaddition Reactions: Core Rules

• In cycloadditions, the reaction proceeds when the HOMO of one component can interact with the LUMO of the other in a phase-matched way

• For a simple thermal [2+2] cycloaddition, the necessary HOMO-LUMO phase alignment is not satisfied, making the process symmetry-forbidden thermally

• Photochemical excitation can overcome symmetry restrictions: the excited-state HOMO can align in phase with the other molecule's LUMO to enable the reaction

• Key concept: orbital symmetry conservation governs whether a pericyclic reaction is allowed under given conditions

Practical Pericyclic Predictions: A Quick Guide

• If the reacting pair would require mismatched phases in the ground state, expect a symmetry-forbidden thermal process

• If photochemical activation is used, symmetry can be broken in a controlled way to allow otherwise forbidden processes

• Use HOMO/LUMO interactions and phase matching to predict outcomes for cycloadditions and rearrangements

Practice: Predicting Products and Synthetic Routes

• Typical exercises include proposing products for given dienes/dienophiles and designing routes to transform one molecule into another using pericyclic strategies

• Consider conformation, stereochemistry, and regiochemistry to determine major products and viable synthetic steps

Review: Key Concepts to Memorize

• Conjugation stabilizes dienes: $\Delta E_{ ext{stabilization}} \approx -15\ \text{kJ/mol}$ for conjugation

• VB vs MO: localized vs delocalized views of bonding; MOs explain delocalization and energy ordering

• Frontier Orbitals: HOMO and LUMO drive reactivity and light absorption

• UV-Vis: A = log(I0/I), $\varepsilon = \dfrac{A}{cl}$; λmax correlates with energy gap and degree of conjugation

• Diels–Alder: [4+2] cycloaddition; endo vs exo; regioselectivity; stereospecificity; dienophile EWGs accelerate reaction

• Pericyclic reactions: concerted, cyclic transition states; orbital symmetry conservation determines allowed vs forbidden pathways

• Temperature influences kinetic vs thermodynamic control in conjugated-diene additions

• [2+2] cycloadditions are symmetry-forbidden thermally but allowed photochemically via excited-state orbital interactions