14.1 Stability of Conjugated Dienes: Molecular Orbital Theory
3 min read•may 7, 2024
are special molecules with overlapping p orbitals that form a continuous π system. This unique structure allows electrons to spread out, making the molecule more stable than its non-conjugated counterparts.
helps explain this stability by showing how atomic orbitals combine to form molecular orbitals. The resulting energy levels and electron behavior give conjugated dienes their distinctive properties and reactivity.
Molecular Orbital Theory and Conjugated Dienes
Molecular orbital theory for conjugated dienes
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- Molecular orbital theory uses and energy levels to describe electron behavior in molecules
- Combines atomic orbitals into molecular orbitals (linear combination of atomic orbitals, LCAO)
- have lower energy and stabilize the molecule (σ and )
- have higher energy and destabilize the molecule (σ* and )
- Conjugated dienes have overlapping p orbitals that form a
- Allows over the entire conjugated system
- Delocalization lowers the molecule's overall energy, increasing stability ()
- Conjugated π system forms bonding and antibonding π molecular orbitals
- Bonding π molecular orbitals have lower energy than the original atomic p orbitals
- Antibonding π molecular orbitals have higher energy than the original atomic p orbitals
- Occupied bonding π molecular orbitals being lower in energy than the atomic p orbitals results in net stabilization of conjugated dienes
Conjugated vs nonconjugated diene properties
- Conjugated dienes have more uniform bond lengths compared to
- Conjugated dienes have shorter single bonds and longer double bonds
- Due to electron delocalization in the conjugated π system
- Partial double bond character shortens single bonds in conjugated dienes (1,3-butadiene)
- Partial single bond character lengthens double bonds in conjugated dienes (1,3-butadiene)
- Conjugated dienes have lower than nonconjugated dienes
- Heat of hydrogenation: energy released when a compound is completely hydrogenated
- Lower heat of hydrogenation in conjugated dienes indicates greater stability (1,3-butadiene vs )
- Stability attributed to electron delocalization in the conjugated π system
Electron delocalization in conjugated systems
- Delocalization: spreading of electrons over multiple atoms in a molecule
- Conjugated systems have overlapping p orbitals that allow electron delocalization
- Creates a conjugated π system extending over the entire conjugated region
- Electrons in the conjugated π system are spread out, not confined to a single bond ()
- Electron delocalization in conjugated systems increases molecular stability
- Delocalized electrons occupy lower-energy bonding π molecular orbitals
- Lowers energy and stabilizes the molecule compared to a nonconjugated counterpart (1,3-butadiene vs 1,4-pentadiene)
- More extensive conjugation leads to greater delocalization and stability ()
- represent electron delocalization in conjugated systems
- Each resonance structure shows a different π electron arrangement
- Actual structure is a hybrid of all resonance structures with delocalized electrons ()
Molecular Orbital Energy Levels and Hybridization
- Energy level diagrams illustrate the relative energies of molecular orbitals
- (Highest Occupied Molecular Orbital) is the highest energy occupied orbital
- (Lowest Unoccupied Molecular Orbital) is the lowest energy unoccupied orbital
- Nodes in molecular orbitals indicate regions of zero electron density
- of atomic orbitals affects the overall molecular orbital structure and energy levels
Key Terms to Review (27)
1,3-Butadiene: 1,3-Butadiene is a simple conjugated diene, composed of four carbon atoms with two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature gives 1,3-butadiene unique chemical properties and reactivity that are important in various organic chemistry topics.
1,3,5-hexatriene: 1,3,5-hexatriene is a conjugated polyene with six carbon atoms and three carbon-carbon double bonds arranged in an alternating pattern. It is a key term that is important in understanding the stability of conjugated dienes, the molecular orbitals of conjugated pi systems, the stereochemistry of thermal electrocyclic reactions, and the photochemical electrocyclic reactions.
1,3,5,7-octatetraene: 1,3,5,7-octatetraene is a conjugated polyene, a type of organic compound with a continuous sequence of alternating carbon-carbon single and double bonds. This structural feature is important in the context of understanding the stability of conjugated dienes through molecular orbital theory.
1,4-Pentadiene: 1,4-Pentadiene is a conjugated diene, a type of organic compound containing two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature makes 1,4-pentadiene particularly relevant to the discussion of the stability of conjugated dienes in the context of molecular orbital theory.
Antibonding Molecular Orbitals: Antibonding molecular orbitals are higher-energy molecular orbitals that are formed when atomic orbitals combine in a way that results in destructive interference, leading to a decrease in electron density between the bonded atoms. These orbitals are essential in understanding the stability and reactivity of molecules.
Benzene: Benzene is a planar, aromatic hydrocarbon compound with the chemical formula C6H6. It is a key building block in organic chemistry and has a unique resonance structure that contributes to its stability and reactivity.
Bonding Molecular Orbitals: Bonding molecular orbitals are the regions of high electron density that form between atoms when they share electrons to create a chemical bond. These orbitals are crucial in understanding the stability and properties of molecules, as they determine the strength and nature of the bonds that hold atoms together.
Conjugated Dienes: Conjugated dienes are organic compounds with two carbon-carbon double bonds that are separated by a single carbon-carbon bond. This arrangement of alternating double and single bonds creates a system of delocalized pi electrons, which gives conjugated dienes unique stability and reactivity properties.
Conjugated π System: A conjugated π system is a series of interconnected carbon-carbon double bonds where the π electrons are delocalized across the entire system. This delocalization of electrons is a key feature that contributes to the stability and reactivity of these types of organic compounds.
Electron Delocalization: Electron delocalization is the phenomenon where electrons in a molecule are not confined to a single bond or atom but are instead spread out or delocalized over multiple atoms or bonds. This concept is fundamental to understanding resonance, the stability of conjugated systems, and the behavior of aromatic compounds in organic chemistry.
Energy Level Diagram: An energy level diagram is a graphical representation of the quantized energy levels of an atom or molecule. It illustrates the discrete energy states that electrons can occupy within a system and the transitions between these levels that can occur through the absorption or emission of energy.
Heats of Hydrogenation: Heats of hydrogenation refer to the amount of energy released or absorbed when a compound undergoes hydrogenation, the chemical reaction where hydrogen gas is added to a compound. This term is particularly relevant in the context of understanding the stability of alkenes and conjugated dienes.
HOMO: HOMO, or Highest Occupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the highest energy level occupied by electrons in a molecule. This term is crucial in understanding the stability, reactivity, and spectroscopic properties of organic compounds, particularly in the context of conjugated systems, pericyclic reactions, and the chemistry of vision.
Homotopic: In the context of 1H NMR spectroscopy and proton equivalence, homotopic protons are those that can be interchanged by a symmetry operation without changing the molecule's overall spatial arrangement. These protons have identical chemical environments and therefore exhibit identical chemical shifts in NMR spectroscopy.
Hybridization: Hybridization is a fundamental concept in chemistry that describes the process of mixing atomic orbitals to form new hybrid orbitals, which are used to explain the geometry and bonding patterns of molecules. This term is closely related to the development of chemical bonding theory, valence bond theory, and molecular orbital theory, as well as the structure and properties of various organic compounds.
Linear Combination of Atomic Orbitals (LCAO): The linear combination of atomic orbitals (LCAO) is a fundamental concept in molecular orbital theory, which describes the formation of molecular orbitals by combining the wave functions of individual atomic orbitals. This approach is used to understand the electronic structure and bonding in molecules.
Lowest unoccupied molecular orbital (LUMO): The LUMO is the lowest energy molecular orbital that does not contain electrons but can accept them during chemical reactions or excitations. It plays a crucial role in determining the reactivity and properties of molecules, especially in conjugated systems analyzed by ultraviolet spectroscopy.
LUMO: LUMO, or Lowest Unoccupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the energy level of the highest-energy orbital that is not occupied by electrons in the ground state of a molecule. The LUMO is crucial in understanding the stability and reactivity of conjugated systems, as well as the behavior of molecules in various photochemical and pericyclic reactions.
Molecular Orbital Theory: Molecular Orbital Theory is a model that describes the behavior of electrons in a molecule by considering the formation of molecular orbitals from the combination of atomic orbitals. This theory provides a more comprehensive understanding of chemical bonding compared to the earlier Valence Bond Theory.
Node: A node is a fundamental concept in various fields, including atomic structure, molecular orbital theory, and the stability of conjugated dienes. It represents a specific point or location within a system, where important properties or interactions occur.
Nonconjugated Dienes: Nonconjugated dienes are organic compounds that contain two carbon-carbon double bonds that are not connected by a single carbon-carbon bond. Unlike conjugated dienes, the double bonds in nonconjugated dienes are separated by at least one single bond, resulting in a different electronic structure and reactivity compared to their conjugated counterparts.
Resonance Structures: Resonance structures are a set of contributing structures that describe the delocalization of electrons in a molecule. They represent the different ways in which the atoms in a molecule can be bonded to satisfy the octet rule and create the most stable arrangement of electrons.
Wave Functions: Wave functions are mathematical representations that describe the quantum state of an electron or other particle. They are fundamental in quantum mechanics, as they provide a complete description of the particle's behavior and properties.
π Bonding Orbitals: π Bonding Orbitals are a type of molecular orbital that arise from the sideways overlap of p-orbitals in conjugated systems. These orbitals are crucial in understanding the stability and reactivity of conjugated dienes, as described in the topic of 14.1 Stability of Conjugated Dienes: Molecular Orbital Theory.
π* Antibonding Orbitals: π* antibonding orbitals are a type of molecular orbital that arises from the constructive interference of atomic p-orbitals in a conjugated system. These orbitals have a higher energy than the corresponding bonding π orbitals and are characterized by electron density that is concentrated outside the internuclear region between the bonded atoms.
σ Bonding Orbitals: σ (sigma) bonding orbitals are the most stable type of covalent bonds formed between atoms, characterized by a high electron density concentrated along the internuclear axis. They are essential in understanding the stability and structure of conjugated dienes as described in Molecular Orbital Theory.
σ* Antibonding Orbitals: σ* antibonding orbitals are higher energy molecular orbitals that result from the constructive interference of atomic orbitals. They are important in understanding the stability and reactivity of conjugated dienes, as described in the context of 14.1 Stability of Conjugated Dienes: Molecular Orbital Theory.