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AP Psychology

30.2 Electrocyclic Reactions

2 min readLast Updated on May 7, 2024

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

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  • Involve formation or breaking of a single σ\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 π\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 π\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 π\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 π\pi electron systems, occur in a disrotatory manner
    • For 4n+2 π\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 π\pi electron systems (thermal electrocyclic ring closure of 1,3,5-hexatriene) and photochemical 4n π\pi electron systems
  • Conrotatory motion has substituents on termini of polyene rotating in the same direction during the electrocyclic reaction
    • Observed in thermal 4n π\pi electron systems (thermal electrocyclic ring closure of 1,3-butadiene) and photochemical 4n+2 π\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

Term 1 of 29

4n
See definition

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.

Key Terms to Review (29)

Term 1 of 29

4n
See definition

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.

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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

Term 1 of 29

4n
See definition

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.



© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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