Dr. Shalini S.

Electrocyclic reactions are a class of pericyclic reactions in organic chemistry where a conjugated polyene undergoes a reversible ring-closure or ring-opening process under the influence of heat or light. These reactions are characterized by the conversion of π-electrons into σ-electrons and vice versa.

Key features of electrocyclic reactions include:

1. Concerted Mechanism: The reaction proceeds in a single step without intermediates, involving a cyclic redistribution of electrons.
2. Conjugated System: The reactants are usually conjugated polyenes, such as dienes or trienes.
3. Thermal and Photochemical Control: The reaction can be driven by either thermal energy (heat) or photochemical energy (light), leading to different stereochemical outcomes.

The stereochemical outcome of electrocyclic reactions is governed by the Woodward-Hoffmann rules, which are based on the conservation of orbital symmetry:

• Thermal Reactions: For an electrocyclic reaction to be thermally allowed, the process must conserve the symmetry of the highest occupied molecular orbital (HOMO). In thermal ring closures, if the polyene has an even number of π-electrons, it undergoes disrotatory closure; if it has an odd number of π-electrons, it undergoes conrotatory closure.

  • Disrotatory Motion: The terminal groups rotate in opposite directions.
  • Conrotatory Motion: The terminal groups rotate in the same direction.

• Photochemical Reactions: For photochemical reactions, the excitation of electrons changes the symmetry considerations. Typically, photochemical electrocyclic reactions proceed in the opposite stereochemistry to the thermal ones.

  • Even-numbered π-electrons: Conrotatory
  • Odd-numbered π-electrons: Disrotatory

Electrocyclic reactions are a class of pericyclic reactions in organic chemistry where a conjugated polyene undergoes a reversible ring-closure or ring-opening process under the influence of heat or light. These reactions are characterized by the conversion of π-electrons into σ-electrons and vice versa.

Key features of electrocyclic reactions include:

1. Concerted Mechanism: The reaction proceeds in a single step without intermediates, involving a cyclic redistribution of electrons.
2. Conjugated System: The reactants are usually conjugated polyenes, such as dienes or trienes.
3. Thermal and Photochemical Control: The reaction can be driven by either thermal energy (heat) or photochemical energy (light), leading to different stereochemical outcomes.

The stereochemical outcome of electrocyclic reactions is governed by the Woodward-Hoffmann rules, which are based on the conservation of orbital symmetry:

• Thermal Reactions: For an electrocyclic reaction to be thermally allowed, the process must conserve the symmetry of the highest occupied molecular orbital (HOMO). In thermal ring closures, if the polyene has an even number of π-electrons, it undergoes disrotatory closure; if it has an odd number of π-electrons, it undergoes conrotatory closure.

  • Disrotatory Motion: The terminal groups rotate in opposite directions.
  • Conrotatory Motion: The terminal groups rotate in the same direction.

• Photochemical Reactions: For photochemical reactions, the excitation of electrons changes the symmetry considerations. Typically, photochemical electrocyclic reactions proceed in the opposite stereochemistry to the thermal ones.

  • Even-numbered π-electrons: Conrotatory
  • Odd-numbered π-electrons: Disrotatory