Electrocyclic Ring Closure and Ring Opening Reactions:
Electrocyclic ring closure reactions occur by the reorganization of pi electrons of an acyclic conjugated polyene system due to the interaction between the terminal carbons forming a sigma bond to a cyclic system. The reversal of the reaction, namely the ring opening also occurs in a concerted manner.
Cycloaddition and Cycloreversion Reactions:
Cycloaddition reactions involve the interaction of two or more pi systems forming a cyclic structure. Reorganization of the pi electrons leading to the formation of sigma bonds takes place during a cycloaddition reaction. When the interacting pi systems are part of the same molecule we have intramolecular cycloaddition reactions. The reversal of cycloaddition reactions known as the cycloreversion also occurs in a concerted manner. Although the above definition is very general it must be emphasized that not all cycloaddition reactions proceed in a concerted manner. There are many examples of cycloaddition reactions that proceed by a stepwise ionic or radical mechanism. These are not considered under the pericyclic type.
Sigmatropic Rearrangements:
Reactions involving the breaking and migration of a sigma bond (a group connected by a sigma bond) through a pi frame work with concomitant reorganization of the p framework and formation of a new sigma bond are known as sigmatropic rearrangements.
Frontier Orbital Method:
Frontier orbitals of a system are constituted by the pair of the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO). For example, the pi- MO of ethylene is the HOMO of ethylene and the pi* level is the LUMO in the ground state electronic configuration. In the case of excited state of butadiene, p3 is the HOMO and pi4 is the LUMO. Pericyclic reactions can be analyzed by the interaction of HOMO and LUMO of the reacting systems. When two reacting systems are separated by infinite distance their MOs are unperturbed by each other. When they approach sufficiently close to each other, their MOs begin to interact. MOs that are close in energy interact more strongly than others. Interaction of two filled MOs does not lead to any net stabilization of the system. It is the interaction of one filled MO with an unfilled MO that leads to the net stabilization of the system. Hence it is the interaction of the HOMO of one of the reacting system with the LUMO of the other reacting system that is considered in this method of analysis. If the interaction of the two MOs is of the bonding type in the ground state electronic configuration, then we call the reaction thermally allowed. If the HOMO-LUMO interaction leads to anti-bonding interaction in the ground state electronic configuration then the reaction is thermally disallowed.
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