Dr. Sheppard CHEM 4201 CONJUGATED SYSTEMS (CONTINUED)

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

 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 III. MOLECULAR ORBITAL THEORY

 BONDING MO Formation of a  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 (  bond)

 ANTIBONDING MO Formation of a  * 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

  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  bonding MO is formed  When + and - lobes overlap (destructive), waves cancel out and a node forms; this results in an  antibonding MO  Electron density is centered above and below the  bond PI BONDING

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

 1 MO FOR 1,3-BUTADIENE  Lowest energy  All bonding interactions  Electrons are delocalized over four nuclei  Contains first pair of  electrons

 2 MO FOR 1,3-BUTADIENE  Two bonding interactions  One antibonding interaction  One node  A bonding MO  Higher energy than  1 MO and not as strongly bonding  Contains second pair of  electrons

 3 * MO FOR 1,3-BUTADIENE  Antibonding MO  Two nodes  Unoccupied in the ground state

 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

 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 PERICYCLIC REACTIONS AND MOs

 Conrotatory vs. disrotatory  Thermal vs. photochemical ELECTROCYCLIC REACTIONS

 Motivation for conrotatory or disrotatory has to do with overlap of outermost  lobes of MO  Orbitals that overlap when  bond formed  Two possibilities:  These lobes must rotate so like signs overlap 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) ELECTROCYCLIC REACTIONS Disrotatory (thermal) Conrotatory (photochemical)

ELECTROCYCLIC REACTIONS

 MOs of 1,3-butadiene (even # electron pairs) ELECTROCYCLIC REACTIONS Disrotatory (photochemical) Conrotatory (thermal)

ELECTROCYCLIC REACTIONS

 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 DIELS-ALDER REACTION

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

 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 [2+2] CYCLOADDITIONS AND SKIN CANCER

 These reactions also have suprafacial and antarafacial stereochemistry  Suprafacial = migration across same face of  system  Antarafacial = migration across opposite face of  system  Both are allowed, but suprafacial are easier SIGMATROPIC REARRANGEMENT

SUPRAFACIAL AND ANTARAFACIAL  Rules are the same as for Diels-Alder reactions:

 The electrons circle around  Thermal reactions with an  Even number of electron pairs are  Conrotatory or  Antarafacial SUMMARY OF PERICYCLIC REACTIONS AND MOs