Dr. Sheppard
CHEM 4201

I.
Structure
II.
Reactions
III.
MO Theory
IV.
UV Spectroscopy
OUTLINE

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 1 s 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 1 s 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
MO and not as
1 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
 T hermal reactions with an
 E ven number of electron pairs are
 C onrotatory or
 A ntarafacial