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Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

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Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.

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Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

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Published on: December 16, 2022

Do Xylylenes Isomerize in Pyrolysis?

Florian Hirsch¹, Kai Pachner¹, Ingo Fischer¹

¹Institute for Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, 97074, Würzburg, Germany.

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry

|June 6, 2020

Summary

Researchers captured infrared spectra of para-xylylene using free electron laser (FEL) radiation. Ortho- and meta-xylylene isomers isomerized to benzocyclobutene and styrene, explained by UV oscillator strength computations.

Keywords:

IR spectroscopy biradicals high-temperature chemistry pyrolysis xylylene

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Area of Science:

Physical Chemistry
Spectroscopy
Organic Chemistry

Background:

Xylylene isomers are important in organic chemistry.
Gas-phase spectroscopy provides detailed molecular information.
Isomerization reactions are fundamental chemical processes.

Purpose of the Study:

To obtain gas-phase infrared (IR) spectra of xylylene isomers.
To investigate the stability and reactivity of xylylene isomers.
To elucidate the mechanisms of xylylene isomerization.

Main Methods:

Gas-phase generation of xylylene isomers via flash pyrolysis.
Infrared (IR) spectroscopy using a free electron laser (FEL).
IR/UV double resonance detection scheme to monitor ion dip signals.

Main Results:

A gas-phase IR spectrum of para-xylylene was successfully recorded.
Ortho- and meta-xylylene isomers were observed to partially rearrange.
Isomerization products identified as benzocyclobutene and styrene.
UV oscillator strength computations supported the observed isomerization pathways.

Conclusions:

Para-xylylene can be spectroscopically characterized in the gas phase.
Ortho- and meta-xylylene isomers are unstable under the experimental conditions, undergoing isomerization.
Computational analysis provides a theoretical basis for the observed chemical transformations.