S. Gomathiselvan

Electrocyclic reactions are a type of pericyclic reaction where a conjugated pi-electron system undergoes a cyclization (ring-closing) or ring-opening reaction in response to thermal or photochemical activation. These reactions involve the reorganization of pi bonds to form a sigma bond (in ring-closing) or the breaking of a sigma bond to form pi bonds (in ring-opening). Key characteristics of electrocyclic reactions include: 1. **Conservation of Orbital Symmetry**: The reaction follows the Woodward-Hoffmann rules, which predict the stereochemistry of the reaction based on the symmetry properties of the molecular orbitals involved. 2. **Thermal vs. Photochemical Conditions**: The stereochemistry of the products can differ depending on whether the reaction is driven by heat or light. Thermal reactions usually proceed via a conrotatory or disrotatory pathway depending on the number of pi electrons in the system, while photochemical reactions proceed in the opposite manner. 3. **Cyclization/Ring Opening**: These reactions are reversible, with ring-closing leading to the formation of a new ring and ring-opening breaking a ring to form a conjugated system.