MO Theory

Report
CONJUGATED SYSTEMS
(CONTINUED)
Dr. Sheppard
CHEM 4201
OUTLINE
I.
II.
III.
IV.
Structure
Reactions
MO Theory
UV Spectroscopy
III. MOLECULAR ORBITAL THEORY
 Sigma bonding
 Electron density lies between the nuclei
 Formed from overlap of hybrid orbitals
 Hybrid orbitals formed from the combination of atomic orbitals
 Another approach…
 Molecular orbitals (MOs)
 Produced when atomic orbitals on different atoms interact
 The bonding molecular orbital is lower in energy than the
original atomic orbitals.
 The antibonding MO is higher in energy than the atomic
orbitals
s BONDING MO
• Formation of a s
bonding MO
• When the 1s orbitals of
two hydrogen atoms
overlap in phase with
each other, they interact
constructively to form a
bonding MO
• The result is a
cylindrically symmetrical
bond (s bond)
s* ANTIBONDING MO
• Formation of a s*
antibonding MO
• When two 1s orbitals
overlap out of phase, they
interact destructively to
form an antibonding (*)
MO
• Result in node separating
the two atoms
H 2 : s—s OVERLAP
• Bonding MOs are
lower in energy than
the atomic orbitals
• Antibonding MOs
are higher in energy
than the atomic
orbitals
• In stable molecules,
bonding orbitals are
usually filled and
antibonding orbitals
are usually empty
PI BONDING
 p molecular orbitals are the sideways overlap of p orbitals
 p orbitals have two lobes
 Plus (+) and minus (-) indicate the opposite phases of the wave
function, not electrical charges
 When lobes overlap constructively (+ and +, or - and -), a p
bonding MO is formed
 When + and - lobes overlap (destructive), waves cancel out
and a node forms; this results in an p* antibonding MO
 Electron density is centered above and below the s bond
ETHYLENE PI MOs
 The combination of two p orbitals gives two molecular
orbitals
 Constructive overlap is a bonding MO
 Destructive overlap is an antibonding MO
MOS OF 1,3-BUTADIENE
 p orbitals on C1 through C4
 Four MOs (2 bonding, 2 antibonding)
 Represent by 4 p orbitals in a line
 Larger and smaller orbitals are used to show which
atoms bear more of the electron density in a particular
MO
p 1 MO FOR 1,3-BUTADIENE
 Lowest energy
 All bonding
interactions
 Electrons are
delocalized over
four nuclei
 Contains first pair
of p electrons
p 2 MO FOR 1,3-BUTADIENE
 Two bonding
interactions
 One antibonding
interaction
 One node
 A bonding MO
 Higher energy than
p 1 MO and not as
strongly bonding
 Contains second pair of
p electrons
p 3 * MO FOR 1,3-BUTADIENE
 Antibonding MO
 Two nodes
 Unoccupied in the
ground state
p 4 * MO FOR 1,3-BUTADIENE
 Strongly antibonding
 Very high energy
 Unoccupied in ground
state
MO FOR 1,3-BUTADIENE AND ETHYLENE
 The bonding MOs of
both 1,3-butadiene and
ethylene are filled
 The antibonding MOs
are empty
 Butadiene has lower
energy than ethylene
(stabilization of the
conjugated diene)
 Frontier orbitals
 Highest energy occupied
molecular orbital
(HOMO)
 Lowest energy
unoccupied molecular
orbital (LUMO
PERICYCLIC REACTIONS AND MOs
 How can MO Theory explain the products of pericyclic
reactions?
 Theory of conservation of orbital symmetry
 Woodward and Hoffmann (1965)
 Frontier MOs must overlap constructively to stabilize the
transition state
 Drastic changes in symmetry may not occur
ELECTROCYCLIC REACTIONS
 Conrotatory vs. disrotatory
 Thermal vs. photochemical
ELECTROCYCLIC REACTIONS
 Motivation for conrotatory or disrotatory has to do
with overlap of outermost p lobes of MO
 Orbitals that overlap when s bond formed
 Two possibilities:
 These lobes must rotate so like signs overlap
ELECTROCYCLIC REACTIONS
ELECTROCYCLIC REACTIONS
 Which MO do you look at?
 Thermal reactions = Ground state HOMO
 Photochemical reactions = Excited state HOMO* (the
ground state LUMO)
ELECTROCYCLIC REACTIONS
 MOs of 1,3,5-hexatriene (odd # electron pairs)
Conrotatory
(photochemical)
Disrotatory
(thermal)
ELECTROCYCLIC REACTIONS
ELECTROCYCLIC REACTIONS
 MOs of 1,3-butadiene (even # electron pairs)
Disrotatory
(photochemical)
Conrotatory
(thermal)
ELECTROCYCLIC REACTIONS
DIELS-ALDER REACTION
 Reactions are favored thermally or photochemically
 Even # electron pairs (e.g. [2+2]) = photochemical
 Odd # electron pairs (e.g. [4+2]) = photochemical
 Reactions are either symmetry allowed or forbidden
 Again, based on MOs of interacting lobes
 Look at MOs of both reactants
 Suprafacial vs. Antarafacial
SUPRAFACIAL AND ANTARAFACIAL
SYMMETRY-ALLOWED THERMAL [4+2]
CYCLOADDITION
 Diene donates electrons from its HOMO
 Dienophile accepts electrons into its LUMO
 Butadiene HOMO and ethylene LUMO overlap with
symmetry (constructively)
 Suprafacial
“FORBIDDEN” THERMAL [2+2]
CYCLOADDITION
 Thermal [2 + 2] cycloaddition of two ethylenes to
form cyclobutene has antibonding overlap of HOMO
and LUMO
 For reaction to occur, one of the MOs would have to
change its symmetry (orbital symmetry is not
conserved)
 Antarafacial
PHOTOCHEMICAL [2+2] CYCLOADDITION
 Absorption of correct energy photon will promote an
electron to a higher energy level (excited state)
 The ground state LUMO is now the HOMO* (HOMO
of excited molecule)
PHOTOCHEMICAL [2+2] CYCLOADDITION
 LUMO of ground state
ethylene and HOMO* of
excited ethylene have
same symmetry
 Suprafacial
 The [2+2] cycloaddition
can now occur
 The [2+2] cycloaddition
is photochemically
allowed, but thermally
forbidden
DIELS-ALDER REACTION
 Update favored vs. non-favored chart:
 Antarafacial reactions aren’t forbidden, just difficult
 Exception: [2+2] geometry is too strained to twist, so
this thermal antarafacial reaction does not occur
[2+2] CYCLOADDITIONS AND SKIN CANCER
 Dimerization of
thymine in DNA
 Exposure of DNA to
UV light induces the
photochemical
reaction between
adjacent thymine
bases
 Resulting dimer is
linked to development
of cancerous cells
http://chm234.asu.edu/reallife/332thymine/thymine.html
SIGMATROPIC REARRANGEMENT
 These reactions also have suprafacial and
antarafacial stereochemistry
 Suprafacial = migration across same face of p system
 Antarafacial = migration across opposite face of p
system
 Both are allowed, but suprafacial are easier
SUPRAFACIAL AND ANTARAFACIAL
 Rules are the same as for Diels-Alder reactions:
SUMMARY OF PERICYCLIC REACTIONS
AND MOs
 The electrons circle around
 Thermal reactions with an
 Even number of electron pairs are
 Conrotatory or
 Antarafacial

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