22-05-2012, 12:53 PM
PERICYCLIC REACTIONS
[attachment=22695]
Common features of pericyclic reactions
They are all concerted reactions
They are not affected by use of catalyst or change of solvents
These are highly stereoselective
In fact stereochemistry of product formed depend on the following condition
1.Stereochemistry of reactant 2.Number of double bonds in the reactant 3.Whether the reaction is thermal or photo chemical reaction
Frontier orbital approach
Kenichi Fukui proposed a theory known as Frontier orbital theory
Two most important orbital involved in pericyclic reaction
1. Highest Occupied Molecular Orbital [HOMO]
2.Lowest Unoccupied Molecular Orbital[LUMO]
When two in phase atomic orbitals overlap, a covalent bond formed
There is increased electron density between the nuclei
In these region each of the electron is electrostatically attracted by both nuclei
Orbital formed by this process is called Bonding Molecular orbital
Molecular orbital description of 2 pi electrons
In phase overlap of the two pi atomic orbitals of ethane gives bonding molecules orbitals by pi-1. This than the isolated p- atomic orbital
Out of phase overlap gives antibonding molecular orbital designated as pi-2 which has higher energy than isolated p-atomic orbitals
Under thermal condition - HOMO and LUMO
Under photochemical condition – LUMO and HOMO
CYLCOADDITION REACTION
In these are inter molecular reactions
his reactions two different pi bonds containing molecule react
The resulting cyclic product has two new sigma bonds
Eg: Diels-Alder reactions
[attachment=22695]
Common features of pericyclic reactions
They are all concerted reactions
They are not affected by use of catalyst or change of solvents
These are highly stereoselective
In fact stereochemistry of product formed depend on the following condition
1.Stereochemistry of reactant 2.Number of double bonds in the reactant 3.Whether the reaction is thermal or photo chemical reaction
Frontier orbital approach
Kenichi Fukui proposed a theory known as Frontier orbital theory
Two most important orbital involved in pericyclic reaction
1. Highest Occupied Molecular Orbital [HOMO]
2.Lowest Unoccupied Molecular Orbital[LUMO]
When two in phase atomic orbitals overlap, a covalent bond formed
There is increased electron density between the nuclei
In these region each of the electron is electrostatically attracted by both nuclei
Orbital formed by this process is called Bonding Molecular orbital
Molecular orbital description of 2 pi electrons
In phase overlap of the two pi atomic orbitals of ethane gives bonding molecules orbitals by pi-1. This than the isolated p- atomic orbital
Out of phase overlap gives antibonding molecular orbital designated as pi-2 which has higher energy than isolated p-atomic orbitals
Under thermal condition - HOMO and LUMO
Under photochemical condition – LUMO and HOMO
CYLCOADDITION REACTION
In these are inter molecular reactions
his reactions two different pi bonds containing molecule react
The resulting cyclic product has two new sigma bonds
Eg: Diels-Alder reactions