Advanced Topic 844 of 1102 Organic Chemistry
Woodward-Hoffmann Rules
Rules predicting stereochemistry of pericyclic reactions from orbital symmetry.
In-depth explanation of Woodward-Hoffmann Rules
The Woodward-Hoffmann rules predict whether a pericyclic reaction is allowed or forbidden under thermal or photochemical conditions by comparing the symmetry of the frontier molecular orbitals. A thermal reaction is allowed when the number of electron pairs involved is 4n+2 and the components react suprafacially, while a thermal reaction with 4n electrons is forbidden. Photochemical conditions reverse these rules. The rules rely on the conservation of orbital symmetry throughout the reaction pathway.
Going deeper: Woodward-Hoffmann Rules
Key ideas: A thermal pericyclic reaction is allowed if the number of (4q+2) suprafacial plus (4r) antarafacial components is odd. In practice: thermal 4n+2 electron cycloadditions (such as Diels-Alder, 6 electrons) are suprafacial allowed; thermal [2+2] is forbidden but photochemical [2+2] is allowed. Electrocyclisation: 4n electrons are conrotatory when thermal and disrotatory when photochemical; 4n+2 electrons are the reverse.
Common mistakes and connections: Students mix up conrotatory and disrotatory, or apply thermal rules to photochemical conditions. Another error is forgetting that count includes only electrons in the reacting pi bonds. Links to Diels-Alder, HOMO-LUMO symmetry and vitamin D photochemistry.
Visualization of Woodward-Hoffmann Rules
Orbital symmetry of the butadiene HOMO and LUMO decides whether thermal or photochemical cyclizations are allowed. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A thermal [4+2] cycloaddition is allowed, while a thermal [2+2] cycloaddition is forbidden.
Frequently asked questions about Woodward-Hoffmann Rules
What is Woodward-Hoffmann Rules?
Rules predicting stereochemistry of pericyclic reactions from orbital symmetry.
Is Woodward-Hoffmann Rules a beginner, intermediate or advanced topic?
Woodward-Hoffmann Rules is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Woodward-Hoffmann Rules?
Study these earlier topics in Organic Chemistry first: Claisen Condensation, Michael Addition, Pericyclic Reactions.
What comes after Woodward-Hoffmann Rules?
The next topics in Organic Chemistry are Cross-Coupling Reactions and Olefin Metathesis.
Can I practise Woodward-Hoffmann Rules with quizzes?
Yes. The interactive lesson includes a self-check question on Woodward-Hoffmann Rules, and the Organic Chemistry quiz draws on every topic in this area.
Where Woodward-Hoffmann Rules fits in Organic Chemistry
Woodward-Hoffmann Rules is topic 116 of 148 in Organic Chemistry.