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Orbital symmetry and Pericyclic reaction
1. Dr. V. Sivamurugan
Professor in Chemistry
Pachaiyappaâs College
Chennai â 600 030
E mail: sivamu1177@gmail.com
18 hours â 6 lectures
Vitamin D
2. ï Pericyclic reactions-classification, electrocyclic, cycloaddition
reactions. Woodword Hoffman rules, FMO-Analysis of
electrocyclic, cycloaddition and sigmatropic reactions-
correlation diagram for cycloaddition reaction (Ï2s + Ï2s) and
(Ï4s + Ï2s) â butadiene â 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.
7. ï A pericyclic reactions : result of reorganizing the
electrons in the reactant(s).
ï Electrons move round a circle and there are no
positive or negative charges on any intermediatesâ
indeed, there are no intermediates at all. This type of
reaction is called pericyclic reactions
ï The common feature is a concerted mechanism
involving a cyclic TS with continuous electronic
reorganization.
âą electrocyclic reactions
âą cycloaddition reactions
âą sigmatropic rearrangements
11. The formation of stable â benzene like structure in the
intermediate.
12. ï The overlap of p atomic orbitals to form p-molecular orbitals can be
described mathematically using quantum mechanics.
ï Two in-phase 2p atomic orbitals interact, a covalent bond is formed
ï Two out-of-phase atomic orbitals interact, a node is created between
the two nuclei
ï An electron goes into the available molecular orbital with the lowest
energy, and only two electrons can occupy a particular molecular
orbital
ï Orbitals are conservedâtwo atomic orbitals combine to produce two
molecular orbitals, four atomic orbitals
ï combine to produce four molecular orbitals, six atomic orbitals
combine to produce six molecular orbitals (LCAO)
15. ï In a thermal reaction the reactant is in its ground
state (HOMO)
ï In a photochemical reaction the reactant is in an
excited state (LUMO).
ï The ground-state HOMO and the excited-state
HOMO always have opposite symmetriesâone
is symmetric and the other is asymmetric.
16. HOMO for thermal
reactions â Ground
state HOMO
HOMO for
photochemical
reactions â excited
state HOMO
17. ïIt is an intramolecular reaction in which a
new (sigma) bond is formed between the
ends of a conjugated (pi) system.
31. ïThe ground-state HOMO of a compound
with an even number of conjugated double
bonds is asymmetric (C2 â symmetry)
ïwhereas the ground-state HOMO of a
compound with an odd number of
conjugated double bonds is symmetric
(mirror symmetry).
32.
33.
34. ïIn a cycloaddition reaction, two different Ï-
bondâcontaining molecules react to form a
cyclic molecule by rearranging the
electrons and forming two new Ï-bonds.
39. ï The orbitals of one molecule must overlap with the
orbitals of the second molecule.
ï 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.
ï Bond formation is suprafacial if both bonds form on the
same side of the p system.
ï Bond formation is antarafacial if the two bonds form on
opposite sides of the p system
44. âą Addition to opposite faces of the system - called
antarafacial (a)
âą The face-to-face addition is called suprafacial (s).
45.
46.
47.
48. ïReorganization 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. ïIf the migrating group remains associated
with the same face of the conjugated
system throughout the reaction, the
migration is termed suprafacial mode.
ïIf the migrating group moves to the
opposite face of the system during the
course of the migration called antarafacial
mode.
50. âą possess a six-membered pericyclic transition state
57. â Cycloadditions or cycloreversions in which the two bonds are made or
broken to the same atom.
58. âą The ene reaction involves an alkene fragment (the ene) that removes a
hydrogen from an allylic fragment with formation of a new carbon bond
ÎČ-pinene
59. âą 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
60.
61. âą cis-divinylcyclopropane to 1,4-cycloheptadiene, a
reaction that occurs readily at temperatures below â40
oC.
âą Owing to unfavorable molecular geometry, trans-
divinylcyclopropane to cycloheptatriene cannot be
concerted and requires temperatures on the order of
190 C.
62. ï 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.
ï The sets of protons that coalesce undergo sufficiently
rapid interchange with one another to result in an
averaged signal
63. ï It is converted into itself with a first-order rate constant of
3.4Ă10-3 sâ1 at 25 oC.
ï At 10 oC, the 1H-NMR spectrum of bullvalene exhibits a
single peak at 4.22 ppm, which indicates the âfluxionalâ
nature of the molecule.
ï Owing to the threefold axis of symmetry present in
bullvalene, the degenerate rearrangement results in all of
the carbons having an identical averaged environment.