Pericyclic reactions are defined as reactions that occur by concerted cyclic shift of electrons. In concerted reaction, reactant bonds are broken and product bonds are formed at the same time.

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Pericyclic Reactions
Mohd Kamil Hussain, Phd, MRSC
Department of Chemistry
Govt Raza PG College Rampur
Pericyclic Reactions:
Pericyclic reactions are defined as reactions that occur by concerted cyclic shift of
electrons. In concerted reaction, reactant bonds are broken and product bonds are formed
at the same time. These reactions are classified as.
1. Electrocyclic reactions.
2. Cycloaddition reactions
3. Sigmatropic rearrangements
4. Group transfer reactions
Electrocyclic Reactions
An intramolecular reaction in which a new sigma bond is formed between the ends of a
conjugated π-system to generate a ring that has an extra sigma bond and one less π bond
compared to the reactant are known as electrocyclic reaction.
In above elctrocyclic reaction forward process is electrocyclic ring closing and backward
process is electrocyclic ring opening.

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Woodward-Hoffmann selection rule for electro cyclic reaction: Frontier Molecular
orbital (FMO) method:
The FMO method was introduced by fukui, Woodward and Hoffmann. According to this
method, an electrocyclic reaction will be allowed when the termini orbitals of the HOMO
of a conjugated π-system overlaps between them in such a way that positive lobe overlap
with another positive lobe or negative lobe with another negative lobe.
 Frontier molecular orbital = HOMO and LUMO
 Derivation of selection rule needs consideration of only HOMO of acyclic conjugated
system

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Summary
Woodward-Hoffmann
selection rule for electro
cyclic reaction
Acyclic conjugated
system
Condition Rotation allowed
4nπ system Thermal Conrotation
Photochemical Disrotation
(4n +2)π system Thermal Disrotation
Photochemical Conrotation
Stereochemistry of Electrocyclic reaction
Rotation of terminal lobes in the same direction to form sigma bond is known as
conrotation an rotation of terminal lobes of orbital in opposite directions to form sigma
bond is known as disrotation.
 Substituents on the two termini in the open chain reactant pointing in the same will be
cis after conrotation.
 Substituents on the two termini in the open chain reactant pointing in the same will be
trans
after disrotation.
Electrocyclization of (2E,4E)-hexa-2,4-diene:
Thermal cyclization: This is 4nπ system, under thermal condition HOMO of
(2E,4E)-hexa 2,4-diene is ψ2 which has C2 symmetry. Hence for formation of a
sigma bond cyclization will take place through conrotation. Stereochemistry of
cyclised product will be Trans.
Photochemical cyclization: After absorbing a photon electron will excite to ψ3
which will be the HOMO of (2E,4E)-hexa-2,4-diene. HOMO of this butadiene will
have m-symmetry hence cyclization will take place through disrotation.

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Other examples
Electrocyclic ring opening
In ring opening thermal reaction stereochemistry of the reaction is determined by the
symmetry of HOMO of open chain partner. In thermal reaction HOMO of butadiene (4nπ)

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has C2- symmetry and HOMO of hexatriene (4n+2)π has m-symmetry. In photochemical
reaction HOMO of 4nπ system has m-symmetry while (4n+2)π has C2- symmetry.
Torque selectivity in electrocyclic ring opening