Deep-Dive Guides · Pericyclic Reactions & Photochemistry
Pericyclic Reactions & Photochemistry — the complete deep dive
Pericyclic chemistry looks like a zoo of arrows until you see the single governing idea: orbital symmetry is conserved in a concerted, cyclic transition state.
Free extract — the opening section of the guide, in full.
0. How to use this guide — the pericyclic map
A pericyclic reaction proceeds through a single cyclic transition state with no intermediate, no ionic or radical species, and a continuous reorganisation of bonding along a closed loop of interacting orbitals. There are four families; nearly every examinable reaction is one of them:
| Family | What changes | Counting handle | Flagship examples |
|---|---|---|---|
| Electrocyclic | one σ-bond forms/breaks across the ends of a conjugated π-system (ring ⇌ open chain) | count the π-electrons in the open form; stereochem = con/disrotatory | butadiene⇌cyclobutene, hexatriene⇌cyclohexadiene, Nazarov |
| Cycloaddition | two π-components join, forming two new σ-bonds and a ring | [m+n] = electrons in each component; stereochem = supra/antarafacial | Diels–Alder [4+2], [2+2], 1,3-dipolar, cheletropic |
| Sigmatropic | a σ-bond migrates across a π-system to a new position | order [i,j]; count electrons in the cyclic TS | [1,5]-H shift, [1,7]-H shift, [3,3] Cope & Claisen, [2,3]-Wittig |
| Group transfer / ene | σ-bond + allylic H transfer to a π-component (concerted) | 6-electron, suprafacial (like a Diels–Alder with an allylic H) | ene reaction, diimide reduction, retro-ene |
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The remaining 11 sections — MO foundations & the Woodward–Hoffmann rules, Electrocyclic reactions, Cycloadditions, Sigmatropic rearrangements and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.