A pericyclic reaction is one in which bonds are made or broken in a concerted cyclic transition state. A concerted reaction is one which involves no intermediate. The word pericyclic means around the circle. Pericyclic reactions thus are characterised by a cyclic transition state involving the π bonds.
Characteristics of Pericyclic Reactions:
Pericyclic reactions have certain characteristic properties.
- There is no nucleophilic or electrophilic component.
- No catalyst is needed.
- They show very high stereospecificity.
- They can be promoted by heat or light.
- Can occur in gas phase with no solvent.
- They occur in a single step.
- Bond breaking and making occur simultaneously.
- The configuration of the product depends on the configuration of the reactants and reaction conditions.
- At least one reactant is unsaturated.
- It involves formation or break up of sigma and pi bonds.
Types of Pericyclic Reactions
Based on the identification of certain common features, Woodward and Hoffmann classified pericyclic reactions into five basic types.
i. Electrocyclic reactions –An electrocyclic reaction is the concerted inter conversion of a conjugated polyene and a cycloalkene. Electrocyclic reactions are induced either thermally or photochemically. An electrocyclic reaction is a reversible intramolecular reaction that involves ring opening or ring closure of sigma bonds to pi bonds or vice versa.
Cycloaddition reactions – These are characterized by cyclization between 2 or more molecules to form a ring via transformation of pi bonds or vice versa.
ii. Sigmatropic reactions – These are the reactions in which a bond migrates over a conjugated system.
iii. Cheletropic reactions – They are a special class of cycloaddition/cycloreversion reactions in which 2 bonds are formed or broken at a single atom.
iv. Group transfer reactions – One or more groups are transferred from one component to the other.
Reaction profile for a concerted reaction
A concerted reaction is one in which the conversion of reactants R into the products P occurs directly by way of a single transition state TS. When the conversion of reactants into products proceeds by way of more than one transition state, such that one or more intermediates are formed, the processes are accordingly non-concerted.
The transition state of organic reactions may be placed into one of three broad categories
- Polar (involves separation or dispersal of charge)
- Free radical (involves creation or transfer of unpaired electrons)
- Isopolar (neither polar nor radical in nature) Pericyclic reactions proceed through isopolar transition states.
Background of Molecular Orbitals
Molecular orbital (MO) theory describes bonds as the mathematical combination of atomic orbitals that forms a new set of orbitals called molecular orbitals (MOs). The number of atomic orbitals used equals the number of molecular orbitals formed.
Symmetry in pi molecular orbitals
Pi molecular orbitals contain either the mirror plane or centre of symmetry. Both will not be present together.
Dr. Ashish Kumar
Assistant Professor, Dept. of Chemistry
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