This document discusses pericyclic reactions, including their types, mechanisms, and synthetic importance, particularly in relation to electron reorganization. It covers electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements, highlighting examples and the quantum mechanical principles behind these processes. The document also touches on specific reactions like the Diels-Alder and ene reactions, as well as the fluxional nature of bulvalene.

1. Dr. V. Sivamurugan
Professor in Chemistry
Pachaiyappa’s College
Chennai – 600 030
E mail: sivamu1177@gmail.com
18 hours – 6 lectures
Vitamin D

2.  Pericyclic reactions-classification, electrocyclic, cycloaddition
reactions. Woodword Hoffman rules, FMO-Analysis of
electrocyclic, cycloaddition and sigmatropic reactions-
correlation diagram for cycloaddition reaction (π2s + π2s) and
(π4s + π2s) – butadiene – cyclobutene system and Inter
conversion of hexatriene to cyclohexadiene. Structure of
bulvalene, a fluxional molecule- MO treatment on Cope,
Claisen rearrangements, Diels-Alder and Ene reaction.

3. Acknowledgements
Chapter 34 Chapter 29

4. Acknowledgements
Chapter 10: concerted pericyclic
reactions

5. Prof. Robert B. Woodward
(1917–1979)
Prof. Roald Hoffmann
(1939-

6. Organic
reactions
Polar reactions
Pericyclic
reactions
Free-radical
reactions
?

7.  A pericyclic reactions : result of reorganizing the
electrons in the reactant(s).
 Electrons move round a circle and there are no
positive or negative charges on any intermediates—
indeed, there are no intermediates at all. This type of
reaction is called pericyclic reactions
 The common feature is a concerted mechanism
involving a cyclic TS with continuous electronic
reorganization.
• electrocyclic reactions
• cycloaddition reactions
• sigmatropic rearrangements

8. Synthetic importance: agricultural fungicide Captan.

9. Complex 4 ring
fused system
assembled in single
step

10. Effective overlap of 2pz orbitals lead to form new bonds

11. The formation of stable – benzene like structure in the
intermediate.

12.  The overlap of p atomic orbitals to form p-molecular orbitals can be
described mathematically using quantum mechanics.
 Two in-phase 2p atomic orbitals interact, a covalent bond is formed
 Two out-of-phase atomic orbitals interact, a node is created between
the two nuclei
 An electron goes into the available molecular orbital with the lowest
energy, and only two electrons can occupy a particular molecular
orbital
 Orbitals are conserved—two atomic orbitals combine to produce two
molecular orbitals, four atomic orbitals
 combine to produce four molecular orbitals, six atomic orbitals
combine to produce six molecular orbitals (LCAO)

13. Sym = s2
Sym = C2

14. HOMO
LUMO
n = 0
n = 1
n = 2
n = 3
Sym = C2
Sym = s2

15.  In a thermal reaction the reactant is in its ground
state (HOMO)
 In a photochemical reaction the reactant is in an
excited state (LUMO).
 The ground-state HOMO and the excited-state
HOMO always have opposite symmetries—one
is symmetric and the other is asymmetric.

16. HOMO for thermal
reactions – Ground
state HOMO
HOMO for
photochemical
reactions – excited
state HOMO

17. It is an intramolecular reaction in which a
new (sigma) bond is formed between the
ends of a conjugated (pi) system.

19. 2E, 4E-isomer

21. 2E, 4E- isomer

23. n = 0
n = 1
n = 2
n = 3
n = 4
n = 5
HOMO
Thermo
chemical
Excited
HOMO
Photo
chemical

28. HOMO
Thermal
Excited HOMO
Photochemical
C2 symm
C2 symm
mirror symm
mirror symm

29. HOMO
Thermal
Excited HOMO
Photochemical
C2 symm
mirror symm

30. 2p, 4p, 6p…..
1p, 3p, 6p…..

31. The ground-state HOMO of a compound
with an even number of conjugated double
bonds is asymmetric (C2 – symmetry)
whereas the ground-state HOMO of a
compound with an odd number of
conjugated double bonds is symmetric
(mirror symmetry).

34. In a cycloaddition reaction, two different π-
bond–containing molecules react to form a
cyclic molecule by rearranging the
electrons and forming two new σ-bonds.

36. 4p 2p

37. 2p 2p

38. 8p
2p

39.  The orbitals of one molecule must overlap with the
orbitals of the second molecule.
 HOMO of one of the molecules (p-donor) and the LUMO
of the other molecule (p-acceptor) because only an
empty orbital can accept electrons.
 Bond formation is suprafacial if both bonds form on the
same side of the p system.
 Bond formation is antarafacial if the two bonds form on
opposite sides of the p system

41. suprafacial

43. suprafacialantrafacial

44. • Addition to opposite faces of the system - called
antarafacial (a)
• The face-to-face addition is called suprafacial (s).

48. Reorganization of electrons attached by a
s-bond migrates to the other terminus of a
conjugated p-electron system, with a
simultaneous shift of the p-electrons.

49. If the migrating group remains associated
with the same face of the conjugated
system throughout the reaction, the
migration is termed suprafacial mode.
If the migrating group moves to the
opposite face of the system during the
course of the migration called antarafacial
mode.

50. • possess a six-membered pericyclic transition state

51. • Homoallylic alkoxide, which is hydrolyzed to the corresponding alcohol

52.  Aryl allyl ether was heated without solvent to
produce ortho-allyl phenol.

55. 1. Thermolysis of 1,5-dienes leads to a six π -electron reorganization
anionic oxy-Cope reaction

56. • Transition state for the Cope rearrangements is a chair
like

57. ① Cycloadditions or cycloreversions in which the two bonds are made or
broken to the same atom.

58. • The ene reaction involves an alkene fragment (the ene) that removes a
hydrogen from an allylic fragment with formation of a new carbon bond
β-pinene

59. • Carbonyl compounds are enophiles in the ene reaction
• Cyclization of octa-1,6-diene (a cis-trans mixture) at 475° C
gave exclusively cis- isomer

61. • cis-divinylcyclopropane to 1,4-cycloheptadiene, a
reaction that occurs readily at temperatures below −40
oC.
• Owing to unfavorable molecular geometry, trans-
divinylcyclopropane to cycloheptatriene cannot be
concerted and requires temperatures on the order of
190 C.

62.  It is a reaction process in which no overall change in
structure occurs, and the product of rearrangement is
structurally identical to the starting material.
 The sets of protons that coalesce undergo sufficiently
rapid interchange with one another to result in an
averaged signal

63.  It is converted into itself with a first-order rate constant of
3.4×10-3 s−1 at 25 oC.
 At 10 oC, the 1H-NMR spectrum of bullvalene exhibits a
single peak at 4.22 ppm, which indicates the “fluxional”
nature of the molecule.
 Owing to the threefold axis of symmetry present in
bullvalene, the degenerate rearrangement results in all of
the carbons having an identical averaged environment.