This document discusses pericyclic reactions, which are concerted reactions where the transition state involves electrons moving in a cyclic pattern. It describes the key properties of pericyclic reactions, including that they are stereospecific and often light- or heat-activated. It then covers various classes of pericyclic reactions like electrocyclic reactions, cycloadditions like the Diels-Alder reaction, sigmatropic rearrangements including the Cope rearrangement, and group transfer reactions. Examples are provided to illustrate each reaction type.

1. PERICYCLIC REACTION
Prepared by-
Ranadeep Borgohain
M.Pharm 2nd semester
Tripura University

2. CONTENT
• A) INTRODUCTION
• B) PROPERTIES OF PERICYCLIC
REACTION
• C) CLASSIFICATION
• D) EXAMPLES OF VARIOUS PERICYCLIC
REACTIONS
• E) REFERENCE

3. INTRODUCTION
• Pericyclic reactions are “Any concerted
reactions in which bonds are formed or broken
in a cyclic transition state”.(Electrons move
around in a circle)
i.e there is a single transition state from start to
finish in a contrast to a step wise reaction.

5. Pericyclic activity can be understood in terms
of frontier moleculer orbital theory(FMO
theory) and the outcome reaction can be
predicted using the Woodward-Hoffman rules.
Pericyclic reactions require light or heat
and are completely stereospecific; i.e ,a single
stereo isomer of the reactant forms a single
stereoisomer of the product

6. What are pericyclic reaction?
• The word 'pericyclic' comes from how the
electrons flow round the outside of the ring.
• It is a type of organic reaction where the
transition state has a cyclic geometry.
X= CR2, NR, O
X
X

7. Properties of a pericyclic reaction
• They are highly stereo specific in nature. That
is a single stereoisomer of the reactant forms a
single stereoisomer of the product.
• No electrophiles or nucleophiles are involved.
• Not generally catalysed by lewis acid.
• Often photochemically promoted.

8. CLASSIFICATION OF
PERICYCLIC REACTION
• ELECTROCYCLIC REACTION
• CYCLO ADDITION RECTION
• SIGMATROPIC REARRANGEMENTS
• GROUP TRANSFER REACTION

9. ELECTROCYCLIC REACTION
• Under the influence of heat and light, a
conjugate polyene can undergo isomerization
to form a cyclic compound with a single bond
between the terminal carbons of the original
conjugate system.
example- CH2
CH2
(3Z)-hexa-1,3,5-triene cyclohexa-1,3-diene

10. • The reverse process can also occur ; a single bond
is broken and a cyclic compounds yields an open-
chain polyene.
Example- CH2
CH2
buta-1,3-dienecyclobutene

11. CYCLOADDITION REACTIONS
• A cycloaddition is a pericyclic 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.

12. TYPES OF CYCLOADDITION
REACTIONS.
• 1) Diels-Alder reactions
CH2
CH2
CH2
CH2
diene dienophile

13. • 2) HUISGEN CYCLOADDITIONS
The Huisgen cycloaddition reaction is (2+3) cycloaddition.
O
CH
+ N N
+
N
-
O
N
N
N

14. SIGMATROPIC REACTIONS
• Molecular rearrangements in which a ∂-
bonded atom or group, flanked by one or more
∏ electron systems, shifts to a new location
with a corresponding reorganization of the ∏-
bonds are called sigmatropic reactions.

15. EXAMPLE OF SIGMATROPIC
REACTION
Few examples of sigmatropic rearrangements
are:
These rearrangements are described by two
numbers set in brackets [I,j] that refers to the
relative position of the atoms involved in
migration. Few examples of sigmatropic
rearrangements involving shift of hidrogen
atoms are [1,3],[1,5] and [1,7] rearrangements
are described below:

16. 1
2 3
H
4
H
5
H
6
H
7
H
8
H
9
1 2
3
H
4
H
5
H
6
H
7
H
8
H
9[1,3]
[1,3] Shift of H in an allyl system
1 2 3
4 5
H
6
H
7
H
8
H
9
H
10
H
11
H
12
H
13
1 2 3
4 5
H
6
H
7
H
8
H
9
H
10
H
11
H
12
H
13[1,5]
[1,5] Shift of H in a 1,3-pentadiene system
CH3 2 3
4
5
6 7
H
8
H
9
H
10
H
11
H
12
H
13
H
14
1 2
3
4
5 6 7
H
8
H
9
H
10
H
11
H
12
H
13
H
14
H
15
H
16
H
17[1,7]
[1,7] Shift of H in a 1,3,5-heptatriene system

17. COPE REARRANGEMENT
The cope rearrangement is an example of [3,3] sigmatropic
rearrangements. In this case the first atom of the migrating group is
not bonded to alkenyl chain.
CH2
CH3
CH2
CH2
CH3[3,3]
(5E)-hepta-1,5-diene 3-methylhexa-1,5-diene
4
3
2
1
1
2 3
H
H
H
H
H
HH
H
H
H H
H 3
21
H
H
HH
H
3
2
1
4
H
H
HH
H
H
H

18. It is atom [i=3] of the migrating group which
become bonded to atom 3 (j=3) of the alkenyl
chain. Thus, this is an example of [3,3]
sigmatropic rearrangement.

19. GROUP TRANSFER REACTION
• A group transfer reaction is a pericyclic process
where one or more groups of atoms is transferred
from one molecule to another.
• They can be sometime difficult to identify when
separate reactant molecules combine into a single
product molecule.
• Unlike other pericyclic reaction classes, group
transfer reactions do not have a specific
conversion of pi bonds into sigma bonds or vice
versa, and tend to be less frequently encountered.

20. • Like all pericyclic reactions they must obey
Woodward- Hoffmann rules.
• Example-
H
H
H
H H
H
CH2
CH2
H
H
H
H
H H
H
H
H
H

21. REFERENCE
• Organic reaction mechanism by V.K Ahluwalia
• Wikipedia
• Google