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Lecture Notes - Electrocyclic reactions & its stereochemistry @vchemicalsciences9396

🔥 Thermal vs. Photochemical Activation and Orbital Involvement: Under thermal conditions, the reaction proceeds via the ground-state HOMO, which is antisymmetric, leading to conrotatory motion in 4n π-electron systems and disrotatory in 4n+2 systems. Photochemical excitation promotes an electron to an excited state HOMO, which is symmetric, reversing the orbital symmetry rules and thus altering the stereochemical pathway. This fundamental difference explains why the same molecule may yield different stereochemical products under heat vs. light.

🔄 Conrotatory and Disrotatory Mechanisms Drive Stereochemical Outcomes: The direction of orbital rotation—whether both ends rotate in the same direction (conrotation) or opposite directions (disrotation)—controls whether the product is cis or trans and determines the relative stereochemistry. This is directly linked to the symmetry properties of the involved molecular orbitals and the π-electron count of the system. For example, a 4n system under thermal conditions undergoes conrotation, while a 4n+2 system undergoes disrotation, with the opposite occurring under photochemical conditions.

🌀 Orbital Overlap and Bond Formation/Breaking: Bond formation requires in-phase (same-sign lobe) overlap of p orbitals, while bond breaking occurs when orbitals of opposite phase overlap or when the overlap is disrupted. The lecture stresses that the stereochemical course depends on how these lobes rotate and overlap during the reaction, with terminal lobes playing a critical role in bond making and breaking. Understanding this orbital topology allows prediction of product stereochemistry and reaction feasibility.

⚖️ Electron Count (4n vs. 4n+2) and Reaction Pathway: The lecture highlights that π-electron count governs the electrocyclic reaction pathway. Systems with 4n π electrons (e.g., 4, 8) often follow conrotatory pathways thermally, while 4n+2 systems (e.g., 6, 10) follow disrotatory pathways. This electron counting rule is essential for predicting whether a ring closure or opening will proceed and under which stereochemical constraints, consistent with the Woodward–Hoffmann rules.

🔍 Symmetry Conservation Ensures Reaction Feasibility: The principle of conservation of orbital symmetry means that the symmetry elements (mirror planes, C2 axes) of reactants must be preserved in the products for the reaction to proceed. If the symmetry is not conserved, the reaction pathway is forbidden or highly unfavorable. This principle governs which stereochemical outcomes are possible and explains the selectivity observed experimentally.

🏗️ Ring Strain Influences Reaction Dynamics: Four-membered rings, such as cyclobutene derivatives, are prone to strain and thus exhibit reversible ring-opening and closing reactions with slower rates and less stable intermediates compared to larger rings. This strain affects the equilibrium position and the ease of ring transformations, which is crucial in synthetic applications and understanding reaction kinetics.

📚 Integration of FMO Theory in Electrocyclic Reactions: The lecture repeatedly applies Frontier Molecular Orbital (FMO) theory, focusing on HOMO and LUMO interactions. It clarifies that the HOMO of the reactant determines the stereochemical course in thermal reactions, while the excited state HOMO controls photochemical reactions. This approach provides a powerful predictive tool for organic chemists to design and analyze pericyclic reactions based on molecular orbital interactions.
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