1) Pericyclic reactions are single-step, concerted reactions that proceed through a cyclic transition state without intermediates. 2) They are classified as electrocyclic reactions, cycloaddition reactions, cheletropic reactions, sigmatropic rearrangements, or group transfer reactions depending on the type of bonding changes that occur. 3) Examples of pericyclic reactions include the Diels-Alder reaction, electrocyclic ring openings and closings, and the Claisen rearrangement.

1. Pericyclic reactions
(Part-I)
Dr Mishu Singh
Department of Chemistry
Maharana Pratap Govt. P.G. College, Hardoi
www.mpgpdcollegehardoi.in
www.mishchemworld/wix.com
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2. NO MEACHANISHM REACTION
1965 WOODWARD & FUKUI – NOBEL PRIZE
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3. Content in Part-I
Classification of Organic Reactions
Classification of Pericyclic Reactions
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4. Broad Classification: Organic Reactions
➢ Polar Mechanism
+
➢ Radical Mechanism
➢ Pericyclic Reactions
A reaction that occurs by a concerted process through a cyclic
transition state
*Concerted - All the bonding changes occur at the same time
and in a single step – No intermediates involved !
E
+
N
u_ E
N
u
A
. +
B
. A B
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➢Polar Reactions:
Involves carbanions or carbocations as reactive intermediates
e.g. Aldol condensation, Pinacol-Pinacolone rearrangement
Mechanism:
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➢Non polar /Radical Reactions:
Involves radicals as reactive intermediates
e.g. Barton’s decarboxylation, Pinacol formation
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➢Pericyclic Reactions:
Involves no reactive intermediates – single step reactions with only
Transition State a.k.a. “No mechanism reactions!”
e.g. Diels-Alder Reaction,
Electrocyclic ring opening/closing,
Claisen rearrangement
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Pericyclic Reactions
➢ A pericyclic reaction is characterized as a change in bonding relationships
that takes place as a continuous, concerted reorganization of electrons.
➢ The term "concerted" specifies that there is one single transition state
and therefore no intermediates are involved in the process. To maintain
continuous electron flow, pericyclic reactions occur through cyclic
transition states.
➢ More precisely: The cyclic transition state must correspond to an
arrangement of the participating orbitals which has to maintain a
bonding interaction between the reaction components throughout the
course of the reaction
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11. To summarise
➢ Pericyclic reactions do not involve heterolytic or
homolytic fission
➢ They do not involve formation of an intermediate
➢ They are single step concerted reactions involving
formation of a cyclic transition state
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12. Pericyclic Reactions-Classification
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13. Types of Pericyclic Reaction
1.Electrocyclic reactions (Ring opening and ring closing)
2. Cycloaddition reactions (cycloreversion)
3. Cheletropic reactions
4. Sigmatropic rearrangement
5. Group transfer reactions
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14. 1) Electrocyclic reactions (Ring opening and ring closing)
2) Cycloaddition reactions (cycloreversion)
3) Cheletropic reactions
4) Sigmatropic rearrangement
5) Group transfer reactions
Reagents -
Heat () or Light (h)
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15. Electrocyclic Reaction
➢ Cyclization of a conjugated polyene (ring closing)
➢ Outermost π bonds converted to σ bond and π bonds reorganized
➢ Classified based on the number of π electrons
Conjugated triene
Conjugated diene
cyclohexadiene
cyclobutene
6 e⁻ system
4 e⁻ system
Reverse process (ring opening)
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16. Cycloaddition Reaction
[4π + 2π][2π + 2π]
[4π + 4π]
[4π + 6π]
➢ Condensation of two (or more) small π systems to form a ring
➢ σ bonds formed at the expense of π bonds
➢ Classified by the number of π electrons interacting
Cycloreversion is the reverse of cyclo addition
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17. Cheletropic Reaction
➢ A special type of cycloaddition/cyclo-reversion reactions.
➢ Two bonds are formed or broken at a single atom.
➢ Nomenclature same as for cycloadditions.
[4+1]
[2+1]
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18. ➢ Involves the breaking and migration of a sigma bond over p
electron systems
➢ Formation of a new sigma bond occurs with reorganization of
the p systems
X = C; Cope Rearrangement
X = O; Claisen Rearrangement
Classified as [i,j] shift, because
the rearrangement occurred
when the σ-bond migrates across
i atoms of one system and j of
another
Sigmatropic Rearrangement
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19. Group Transfer Reactions
➢ One or more group of atoms get transferred to a second reaction
partner
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