What is Pericyclic Reaction?
Simply put, any concerted reactions featuring a cyclic flow of electrons via a single transition state are referred to as “pericyclic.”
There are no ions, free radicals, or catalysts in these reactions. The reactions take place under extreme temperature and photochemical conditions, making molecular orbital symmetry a major consideration. Pericyclic reactions are stereospecific to a large extent.
In 1965, the scientists Hoffman and Woodward were the first to explain pericyclic reactions (pericyclic meaning transferring electrons around a circle).
Table of Contents
- Classification of Pericyclic Reaction
- Electrocyclic Reaction
- Cycloadditions Reaction
- Sigmatropic Reaction
- Group Transfer Reaction
- Frequently Asked Questions – FAQs
Classification of Pericyclic Reaction
The bond-forming or -breaking processes take place in a concerted way, without the formation of any intermediate through a cyclic transition state.
There are four types of reactions that are considered as pericyclic reactions.
- Electrocyclic Reaction
- Cycloadditions Reaction
- Sigmatropic Reaction
- Group Transfer Reaction
Concerted reactions are pericyclic reactions. They are distinguished by the formation or dissolution of bonds in a single coordinated step via a cyclic transition state involving x or an electron. Heat is used to activate pericyclic processes in a thermally induced reaction, and ultraviolet light is used in a photo induced reaction.
Because pericyclic reactions are extremely stereospecific, thermal and photochemical activities frequently result in molecules with diverse but distinct stereochemistry. Solvents are not used in pericyclic reactions since there are no ionic or free radical intermediates. Reagents that are nucleophilic or electrophilic have no effect on the outcome of these reactions.
Electrocyclic Reaction
Electrocyclic reactions are reversible cyclisation processes involving a molecule having conjugated double bonds. Two pi electrons are used to generate a sigma bond in this method. An electrocyclic reaction is a reversible reaction in which a conjugated polyene’s ring closes to form a cycloalkene, or a cycloalkene’s ring opens to form a conjugated polyene. Formation of a (sigma) bond across the conjugated polyene.
Note: ring formation from an open-chain conjugated system.
Cycloaddition Reaction
Two molecules join to form a ring in a cycloaddition reaction, with two pi bonds being changed to two sigma bonds in the process. The Diels-Alder reaction, shown below, is the most well-known cycloaddition reaction.
Two components make a ring when they come together and form two new bonds, one at each end of both components. This is the most widespread type of pericyclic reaction. It is defined by two fragments coming together in a ring to produce two new sigma-bonds.
Sigmatropic Reaction
Sigmatropic rearrangements are intramolecular rearrangements in which an atom or a group of atoms shift from one place to another in a coordinated manner. In organic chemistry, a sigmatropic reaction is a pericyclic reaction via which one -bond is transformed into another -bond in an uncatalyzed intramolecular reaction.
Group Transfer Reaction
A group transfer reaction is a pericyclic process in organic chemistry in which one or more groups of atoms are transferred from one molecule to another. The ene reaction, in which allylic hydrogen is transferred to an alkene, is the most well-known group transfer reaction.
Group transfer reactions, unlike other pericyclic reaction types, do not have a particular conversion of pi bonds and sigma bonds or vice versa, and are hence less common. They must follow the Woodward–Hoffmann laws, just as all pericyclic reactions.
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Frequently Asked Questions on Pericyclic Reactions
What are pericyclic reactions and their types?
Reactions of this kind often proceed by nearly simultaneous reorganization of bonding electron pairs by way of cyclic transition states; they have been termed pericyclic reactions.
What happens in an electrocyclic reaction?
An electrocyclic reaction is the concerted cyclization of a conjugated π-electron system by converting one π-bond to a ring-forming σ-bond. Once again, the number of curved arrows that describe the bond reorganization is half the total number of electrons involved in the process.
What is the intermolecular cycloaddition reaction?
A cycloaddition is a chemical reaction in which “two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity.” The resulting reaction is a cyclization reaction.
What is the difference between Suprafacial and Antarafacial?
If all the bonding events take place on the same face of the π-system the configuration of the reaction is termed suprafacial. If the bonding events occur on opposite sides or faces of the π-system, the reaction is termed antarafacial.
Is Claisen rearrangement a Sigmatropic reaction?
Discovered in 1912 by Rainer Ludwig Claisen, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement. This rearrangement is a useful carbon-carbon bond-forming reaction.
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