scoresvideos
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

Electrocyclic reactions are a fascinating type of pericyclic reaction where a single bond forms or breaks between the ends of a conjugated system. These reactions happen all at once, with bonds changing simultaneously, and can turn open-chain molecules into rings or vice versa.

The way these reactions unfold depends on the number of electrons involved and whether heat or light is used. This determines if the molecule's ends rotate in the same or opposite directions, leading to different 3D shapes in the final product.

Electrocyclic Reactions

Concept of electrocyclic reactions

  • Involve formation or breaking of a single $\sigma$ bond between termini of a conjugated system through a cyclic transition state
  • Proceed in a concerted manner with all bond breaking and forming occurring simultaneously
  • Allow for interconversion between open-chain polyenes (molecules containing multiple conjugated double bonds) and cyclic structures
  • Subset of pericyclic reactions which involve concerted bond formation or breaking through a cyclic transition state (other types include cycloadditions and sigmatropic rearrangements)

Stereochemistry in electrocyclic reactions

  • Stereochemical outcome depends on number of $\pi$ electrons in the system and reaction conditions (thermal or photochemical)
  • Thermal electrocyclic reactions follow Woodward-Hoffmann rules based on orbital symmetry conservation
    • For 4n $\pi$ electron systems (butadiene), occur in a conrotatory manner with substituents on termini rotating in the same direction (both clockwise or counterclockwise)
    • For 4n+2 $\pi$ electron systems (hexatriene), occur in a disrotatory manner with substituents on termini rotating in opposite directions (one clockwise, one counterclockwise)
  • Photochemical electrocyclic reactions follow the opposite stereochemical course compared to thermal reactions
    • For 4n $\pi$ electron systems, occur in a disrotatory manner
    • For 4n+2 $\pi$ electron systems, occur in a conrotatory manner
  • Stereochemistry of products is determined by the concerted mechanism of the reaction

Disrotatory vs conrotatory motions

  • Disrotatory motion has substituents on termini of polyene rotating in opposite directions during the electrocyclic reaction
    • Observed in thermal 4n+2 $\pi$ electron systems (thermal electrocyclic ring closure of 1,3,5-hexatriene) and photochemical 4n $\pi$ electron systems
  • Conrotatory motion has substituents on termini of polyene rotating in the same direction during the electrocyclic reaction
    • Observed in thermal 4n $\pi$ electron systems (thermal electrocyclic ring closure of 1,3-butadiene) and photochemical 4n+2 $\pi$ electron systems
  • Disrotatory and conrotatory motions determine stereochemistry (spatial arrangement of atoms) of products formed
    • Specific motion (disrotatory or conrotatory) dictates relative stereochemistry of substituents in resulting cyclic product

Orbital considerations in electrocyclic reactions

  • Electrocyclic reactions are governed by orbital symmetry principles
  • Frontier molecular orbitals (highest occupied and lowest unoccupied molecular orbitals) play a crucial role in determining reaction feasibility and stereochemical outcome
  • The cyclic transition state allows for efficient orbital overlap during the reaction

Key Terms to Review (29)

Electrocyclic reaction: An electrocyclic reaction is a type of pericyclic reaction where a pi-bonded molecule cyclically closes to form a sigma bond, or vice versa, through a concerted process involving the cyclic movement of electrons. It is characterized by its dependence on the electronic configuration and often occurs under thermal or photochemical conditions.
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.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
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.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Stereoisomers: Stereoisomers are molecules that have the same molecular formula and connectivity, but differ in the three-dimensional arrangement of their atoms in space. This spatial arrangement of atoms leads to different physical and chemical properties, even though the atoms are connected in the same way.
Stereoselectivity: Stereoselectivity refers to the preference of a chemical reaction to form one stereoisomeric product over another. It is a crucial concept in organic chemistry that describes the ability of a reaction to control the spatial arrangement of atoms in the final product.
Concerted Mechanism: A concerted mechanism refers to a reaction that occurs in a single, continuous step without the formation of any discrete intermediates. In a concerted mechanism, the bonds that are being formed and broken happen simultaneously, leading to the product in a single, coordinated process.
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.
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.
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.
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.
Cyclic Transition State: A cyclic transition state is a high-energy, ring-like molecular structure that forms temporarily during certain organic reactions, such as the Diels-Alder cycloaddition reaction and electrocyclic reactions. This transient state plays a crucial role in the mechanism and stereochemistry of these important transformations.
Conjugated System: A conjugated system refers to a series of alternating single and double bonds within a molecule, typically in a linear or cyclic arrangement. This arrangement allows for the delocalization of electrons, which has important implications for the molecule's stability, reactivity, and spectroscopic properties.
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.
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.
Electrocyclic Reactions: Electrocyclic reactions are a class of pericyclic reactions in organic chemistry that involve the formation or cleavage of a cyclic structure through the concerted movement of $\pi$ electrons. These reactions play a crucial role in the synthesis and interconversion of various organic compounds.
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.
Photochemical: Photochemical refers to chemical reactions or processes that are initiated or driven by the absorption of light energy. These light-induced transformations are fundamental to various fields, including organic chemistry, biochemistry, and environmental science.
Thermal: Thermal refers to the transfer and exchange of heat energy, which is a fundamental aspect of chemical reactions and physical processes. It describes the temperature-dependent nature of various phenomena and the ways in which heat influences the behavior and properties of substances.
4n: The term '4n' refers to a fundamental concept in organic chemistry known as the 4n rule, which is closely associated with electrocyclic reactions. It describes the relationship between the number of π electrons involved in a cyclic system and the predicted outcome of the reaction.
4n+2: The term '4n+2' is a mathematical expression that describes a pattern in organic chemistry, particularly in the context of electrocyclic reactions. It refers to a series of compounds where the number of π-electrons is equal to 4n+2, where 'n' is an integer. This pattern is significant in understanding the stability and reactivity of certain organic molecules.
Roald Hoffmann: Roald Hoffmann is a Polish-American theoretical chemist who is known for his significant contributions to the field of organic chemistry, particularly in the area of pericyclic reactions and the development of the Woodward-Hoffmann rules.
Hexatriene: Hexatriene is a conjugated hydrocarbon molecule consisting of six carbon atoms and three double bonds. It is a key intermediate in various thermal electrocyclic reactions and plays a crucial role in understanding the stereochemistry of these processes.
Sigmatropic Rearrangements: Sigmatropic rearrangements are a class of pericyclic reactions in organic chemistry where atoms or groups migrate from one position in a molecule to another position within the same molecule. These rearrangements involve the simultaneous breaking and forming of sigma bonds, leading to the formation of a new product with a different connectivity compared to the starting material.
Polyene: A polyene is a type of organic compound characterized by the presence of multiple carbon-carbon double bonds in its structure. These compounds are particularly important in the context of electrocyclic reactions, as their unique electronic properties and reactivity patterns make them central to many pericyclic transformations.
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.
Orbital Symmetry: Orbital symmetry is a fundamental concept in organic chemistry that describes the spatial arrangement and interaction of molecular orbitals involved in pericyclic reactions, such as electrocyclic reactions, cycloadditions, and sigmatropic rearrangements. It helps predict the stereochemical outcomes and feasibility of these concerted reactions.
Robert Burns Woodward: Robert Burns Woodward was a renowned American organic chemist who made significant contributions to the field of organic synthesis and the elucidation of complex molecular structures. His pioneering work in the total synthesis of numerous natural products, including vitamins, alkaloids, and antibiotics, revolutionized the understanding and application of organic chemistry.