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Organic Chemistry
Table of Contents
Unit 30 Overview: Organic Chemistry: Pericyclic Reactions
30.1 Molecular Orbitals of Conjugated Pi Systems
30.2 Electrocyclic Reactions
30.3 Stereochemistry of Thermal Electrocyclic Reactions
30.4 Photochemical Electrocyclic Reactions
30.5 Cycloaddition Reactions
30.6 Stereochemistry of Cycloadditions
30.7 Sigmatropic Rearrangements
30.8 Some Examples of Sigmatropic Rearrangements
30.9 A Summary of Rules for Pericyclic Reactions

UV light can flip the script on electrocyclic reactions. It excites electrons, swapping HOMO and LUMO symmetries. This changes how molecules dance together, leading to different products than thermal reactions would.

Photochemical electrocyclic reactions follow a simple rule: even electron pairs go disrotatory, odd pairs go conrotatory. This flips the thermal reaction outcomes, giving us trans products where we'd expect cis, and vice versa.

Photochemical Electrocyclic Reactions

Effects of UV on orbital symmetries

  • Ultraviolet (UV) irradiation promotes an electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) changes the symmetry of the frontier molecular orbitals involved in the reaction
    • The HOMO and LUMO switch symmetries upon UV irradiation alters the stereochemical outcome of the electrocyclic reaction
      • Thermal reactions involve the HOMO, while photochemical reactions involve the LUMO (1,3-butadiene, 1,3,5-hexatriene)
    • The HOMO and LUMO have opposite symmetry properties
      • If the HOMO is symmetric, the LUMO will be antisymmetric, and vice versa (cyclobutene, cyclohexadiene)
    • The excited state resulting from UV irradiation plays a crucial role in determining the reaction pathway

Stereochemistry in photochemical electrocyclizations

  • For polyenes with an even number of electron pairs (4n electrons):
    • Photochemical electrocyclic reactions proceed in a disrotatory manner
      • Disrotatory: the terminal substituents rotate outward in opposite directions (1,3-butadiene)
    • The product will have a trans configuration at the newly formed sigma bond (cyclobutene)
  • For polyenes with an odd number of electron pairs (4n+2 electrons):
    • Photochemical electrocyclic reactions proceed in a conrotatory manner
      • Conrotatory: the terminal substituents rotate in the same direction, either clockwise or counterclockwise (1,3,5-hexatriene)
    • The product will have a cis configuration at the newly formed sigma bond (cyclohexadiene)

Thermal vs photochemical electrocyclic paths

  • For polyenes with 4n electrons (1,3-butadiene):
    1. Thermal electrocyclic reactions proceed in a conrotatory manner, leading to a cis product (cis-cyclobutene)
    2. Photochemical electrocyclic reactions proceed in a disrotatory manner, leading to a trans product (trans-cyclobutene)
  • For polyenes with 4n+2 electrons (1,3,5-hexatriene):
    1. Thermal electrocyclic reactions proceed in a disrotatory manner, leading to a trans product (trans-cyclohexadiene)
    2. Photochemical electrocyclic reactions proceed in a conrotatory manner, leading to a cis product (cis-cyclohexadiene)
  • The stereochemical outcomes of thermal and photochemical electrocyclic reactions are opposite for a given polyene system due to the involvement of the HOMO in thermal reactions and the LUMO in photochemical reactions
  • The Woodward-Hoffmann rules can be used to predict the stereochemical outcome of electrocyclic reactions based on the number of electron pairs and the reaction conditions (thermal or photochemical)
  • Photochemical electrocyclic reactions are a subset of pericyclic reactions, which involve the concerted rearrangement of electrons in a cyclic transition state
  • Photoisomerization can occur as a result of these reactions, leading to structural changes in the molecule
  • The quantum yield of a photochemical reaction measures its efficiency in terms of the number of product molecules formed per photon absorbed

Key Terms to Review (21)

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.
Ultraviolet (UV): Ultraviolet spectroscopy is a technique used in organic chemistry to measure the absorption of ultraviolet light by a chemical substance, providing information about the structure and electronic transitions of molecules. It is particularly useful for studying conjugated systems and detecting functional groups within a molecule.
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.
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.
Ultraviolet: Ultraviolet (UV) is a region of the electromagnetic spectrum with wavelengths shorter than visible light, but longer than X-rays. This high-energy radiation is invisible to the human eye but plays a crucial role in various scientific and technological applications.
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.
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.
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.
Frontier Molecular Orbitals: Frontier molecular orbitals refer to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a molecule. These orbitals play a crucial role in understanding and predicting the reactivity and behavior of molecules, particularly in the context of electrophilic additions to conjugated dienes, the Diels-Alder cycloaddition reaction, electrocyclic reactions, and photochemical electrocyclic reactions.
Pericyclic Reactions: Pericyclic reactions are a class of organic reactions that involve the concerted rearrangement of pi-electrons within a cyclic transition state. These reactions are characterized by their unique mechanism, which allows for the formation or cleavage of cyclic structures through the simultaneous breaking and forming of chemical bonds.
Photoisomerization: Photoisomerization is a photochemical process in which a molecule undergoes a structural change, typically involving the rotation around a carbon-carbon double bond, in response to the absorption of light energy. This phenomenon is central to understanding the chemistry of vision and various photochemical reactions.
Woodward-Hoffmann Rules: The Woodward-Hoffmann rules are a set of principles that describe the stereochemical outcomes of pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. These rules provide a framework for predicting the feasibility and stereochemistry of these types of organic reactions based on the topology of the molecular orbitals involved.
Disrotatory: Disrotatory refers to the stereochemical outcome of an electrocyclic reaction where the substituents on the reacting system rotate in opposite directions during the cyclic interconversion. This term is particularly relevant in the context of understanding the stereochemistry of thermal and photochemical electrocyclic reactions.
Conrotatory: Conrotatory refers to the stereochemical outcome of a pericyclic reaction, specifically an electrocyclic reaction, where the substituents on the reacting system rotate in the same direction during the cyclization or cycloreversion process.
Photochemical Electrocyclic Reactions: Photochemical electrocyclic reactions are a class of organic reactions that involve the formation or cleavage of a cyclic structure upon exposure to light. These reactions are characterized by the rearrangement of electrons within a conjugated system, leading to the interconversion of isomeric species.
4n Electrons: 4n electrons refer to the number of electrons involved in certain photochemical electrocyclic reactions. These reactions involve the rearrangement of a cyclic molecule's structure through the breaking and forming of new bonds, driven by the absorption of light energy.
Cyclobutene: Cyclobutene is a cyclic organic compound with a four-membered ring consisting of three carbon atoms and one double bond. This structural feature makes cyclobutene an important molecule in the context of electrocyclic reactions, which involve the interconversion between cyclic and acyclic (open-chain) compounds.
4n+2 Electrons: The 4n+2 rule, also known as Hückel's rule, is a guideline used in organic chemistry to predict the stability and aromaticity of cyclic, planar, conjugated systems. It states that a cyclic, planar, conjugated system with (4n+2) $\pi$ electrons is aromatic and therefore more stable than a non-aromatic system.
Excited State: An excited state refers to a higher energy level that an atom or molecule can occupy temporarily after absorbing energy, such as in the form of light or heat. This elevated energy state is unstable and the system will typically return to its ground state, or lowest energy level, through various relaxation processes.
Quantum Yield: Quantum yield is a measure of the efficiency of a photochemical process, specifically the ratio of the number of molecules undergoing a particular photochemical reaction to the number of photons absorbed by the system. It is a crucial parameter in understanding and analyzing photochemical reactions, including those involved in photochemical electrocyclic reactions.
Cyclohexadiene: Cyclohexadiene is a cyclic organic compound with the molecular formula C$_6$H$_8$. It consists of a six-membered carbon ring with two carbon-carbon double bonds. This compound is an important intermediate in various photochemical electrocyclic reactions.