1. Case Western Reserve University
Chapter 29
Pericyclic
Reactions
Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall

2. Reactions of Organic Compounds
A pericyclic reaction, a reaction that occurs as a result of
reorganizing the electrons in the reactant(s)

3. Pericyclic Reaction I
An intramolecular reaction in which a new s bond is
formed between the ends of a conjugated p system

4. Electrocyclic Reactions Are Reversible

5. Pericyclic Reaction II
Two different p bond-containing molecules react to form
a cyclic compound

6. Pericyclic Reaction III
A s bond is broken in the reactant, a new s bond is
formed in the product, and the p bonds rearrange

8. Note The electrocyclic reactions and sigmatropic
rearrangements are intramolecular reactions
The cycloaddition reactions are usually intermolecular
reactions
Common features among the three pericyclic reactions
are concerted reactions
are highly stereoselective
are not affected by catalysts

9. The configuration of the product formed depends on:
the configuration of the reactant
the number of conjugated double bonds or pairs of
electrons in the reacting system
whether the reaction is a thermal or a photochemical
reaction
A photochemical reaction takes place when a reactant absorbs light
A thermal reaction takes place without the absorption of light

10. Conservation of Orbital Symmetry Theory
Explains the relationship among the structure and
configuration of the reactant, the conditions (thermal or
photochemical) under which the reaction takes place,
and the configuration of the products
States that in-phase orbitals overlap during the course of
a pericyclic reaction

11. A Molecular Orbital Description of Ethane

12. Four p atomic orbitals interact to give the four p MOs of
1,3-butadiene
(LUMO)
(LUMO)
(HOMO)
(HOMO)

13. From the MO diagram of 1,3-butadiene …
y1 and y3 are symmetric MOs
y2 and y4 are asymmetric MOs
The ground state HOMO and the excited HOMO have
opposite symmetry

14. Note A MO is bonding if the number of bonding interactions is
greater than the number of nodes
A MO is antibonding if the number of bonding
interactions is fewer than the number of nodes
The normal electronic state of a molecule is known as
its ground state
The ground state electron can be promoted from its
HOMO to its LUMO by absorption of light (excited state)
In a thermal reaction the reactant is in its ground state;
in a photochemical reaction, the reactant is in its excited
state

15. A Molecular Orbital Description of 1,3,5-hexatriene

16. An electrocyclic reaction is completely stereoselective

20. To form the new s bond in the electrocyclic reaction, the p orbitals
at the end of the conjugated system must overlap head-to-head

21. Only the symmetry of the HOMO is important in determining the
course of the reaction

22. A symmetry-allowed pathway is one in which in-phase
orbitals overlap
If a reaction is symmetry-forbidden, it cannot take place
by a concerted pathway
The symmetry of the HOMO of the compound undergoing
ring closure controls the stereochemical outcome of an
electrocyclic reaction

25. Determining the stereochemistry of the product of an
electrocyclic reaction
disrotatory
ring closure
conrotatory
ring closure
disrotatory
ring closure

26. Table 29.2

27. Cycloadditions are classified according to the number of
p electrons that interact in the reaction

28. The frontier molecular orbitals of both reactants must be
considered: the HOMO and LUMO

29. Frontier Orbital Analysis of a [4 + 2] Cycloaddition Reaction

30. A [2 + 2] Cycloaddition Reaction

31. Frontier MO Analysis of the [2 + 2] Cycloaddition Reaction

32. Table 29.3

33. Sigmatropic Rearrangements

35. Consider the transition state of the reaction,
Sigmatropic rearrangements have cyclic transition
states
Rearrangement must be suprafacial if the transition
state has six or fewer atoms in the ring

36. Table 29.4

45. Pericyclic Reactions in Biological Systems