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Pericyclic Reactions: Introduction

Pericyclic Reactions: Introduction

Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
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.

Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.

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Updated: Jan 8, 2026

Designed for Molecular Recycling: A Lignin-Derived Semi-aromatic Biobased Polymer

Designed for Molecular Recycling: A Lignin-Derived Semi-aromatic Biobased Polymer

Published on: November 30, 2020

How Circular Can the Chemical Industry Sector Be(come)?

Leonie Barner¹

¹School of Chemistry and Physics Faculty of Science Centre for Environment and Society Queensland University of Technology 2 George St Brisbane QLD 4000 Australia.

Global Challenges (Hoboken, NJ)

|December 15, 2025

Summary

The chemical industry must adopt circular economy principles to restore planetary boundaries. New circularity indicators and waste reporting frameworks are proposed to improve material use, as current rates are below 7%.

Keywords:

SDG bioeconomy chemical industry circular economy circularity indicators planetary boundary system

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Last Updated: Jan 8, 2026

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Area of Science:

Environmental Science
Industrial Chemistry
Sustainable Development

Background:

The chemical industry faces challenges in adopting circular economy models due to product dispersion.
Chemical products often do not fit traditional resource recovery loops.
The industry is critical for achieving planetary boundary restoration.

Purpose of the Study:

To discuss the urgent need for circularity in the chemical industry.
To introduce new circularity indicators and a waste framework.
To enhance the circular material use rate.

Main Methods:

Discussion of existing ISO 59000 family circularity indicators.
Introduction of novel circularity indicators.
Brief discussion of a waste framework with Scope 1, 2, and 3 categories.

Main Results:

Current circular material use rate in the chemical industry is below 7%.
Defined circularity indicators (ISO 59000) are applicable but require enhancement.
A waste framework can improve focus on value chain waste.

Conclusions:

The chemical industry's transition to a circular economy is essential for environmental sustainability.
New indicators and waste reporting are vital for improving circular material use.
Addressing product dispersion and value chain waste is key to achieving circularity goals.