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Pericyclic Reactions Solved Assignment in pdf

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Pericyclic Reactions
Definition:
1. Concerted reaction that proceed via a cyclic transition state
2. No distinct intermediates in the reaction
3. Bond forming and bond breaking steps are simultaneous but not necessarily
synchronous
Classification:
1. Electrocyclic ring closing and ring opening reaction
2. Cycloaddition and Cycloreversion reaction
3. Sigmatropic Rearrangements
4. Chelotropic Reaction
5. Group transfer Reaction
Methods of Analyzing Pericyclic Reaction
1. Orbital symmetry correlation method
(Woodward, Hoffmann, Longuet-Higgins and Abrahamson)
2. The frontier orbital method
(Woodward, Hoffmann and Fukui)
3. Transition state aromaticity method
(Dewar and Zimmerman)
Woodward-Hoffmann Rules: Predicts the allowedness or otherwise of pericyclic reactions
under thermal and photo- chemical conditions using the above methods. Therefore a basic
understanding of molecular orbitals of conjugated polyene systems and their symmetry
properties is essential to apply the above methods. Engineering Chemistry III Prof. S. Sankararaman
Indian Institute of Technology Madras
2
Constructing MO diagram of polyene systems:
1. Although there are C-C and C-H sigma bonds present in the molecule, the π MOs
can be constructed independently of them. Although there may be a change in the
hybridization of carbon atoms during the course of a pericyclic reaction, the MO
levels of the sigma framework are relatively unaffected.
2. For a conjugated polyene system containing n (n = even) π electrons, there will be
n/2 π bonding molecular orbitals that are filled MOs and n/2 antibonding MOs that
are empty in the ground state electronic configuration of the molecule.
3. The lowest energy MO has zero nodes, the next higher one has one node and the
second higher has two nodes and so on.
The nth MO will have (n-1) nodes.
4. The nodal points are found at the most symmetric points in a MO.








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DESH DEEPAK A P S CHAUHAN

M.Sc. CHEMISTRY!

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