Dr. SheppardCHEM 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

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

H2: 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

p molecular orbitals are the sideways overlap of p orbitalsp orbitals have two lobes

Plus (+) and minus (-) indicate the opposite phases of the wave

function, not electrical chargesWhen lobes overlap constructively (+ and +, or - and -), a p

bonding MO is formedWhen + and - lobes overlap (destructive), waves cancel out

and a node forms; this results in an * p antibonding MOElectron density is centered above and below the s bond

PI BONDING

ETHYLENE PI MOs

The combination of two p orbitals gives two molecular orbitals

Constructive overlap is a bonding MODestructive 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 lineLarger and smaller orbitals are used to show which

atoms bear more of the electron density in a particular MO

1 432

1 MO FOR 1,3-BUTADIENE

Lowest energyAll bonding

interactionsElectrons are

delocalized over four nuclei

Contains first pair of p electrons

2 MO FOR 1,3-BUTADIENE

Two bonding interactions

One antibonding interaction

One nodeA bonding MOHigher energy than1 MO and not as strongly bonding

Contains second pair of p electrons

3* MO FOR 1,3-BUTADIENE

Antibonding MOTwo nodesUnoccupied in the

ground state

4* MO FOR 1,3-BUTADIENE

Strongly antibondingVery high energyUnoccupied 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. disrotatoryThermal vs. photochemical

ELECTROCYCLIC REACTIONS

Motivation for conrotatory or disrotatory has to do with overlap of outermost p lobes of MOOrbitals 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)

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 photochemicallyEven # electron pairs (e.g. [2+2]) = photochemicalOdd # electron pairs (e.g. [4+2]) = photochemical

Reactions are either symmetry allowed or forbiddenAgain, 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 HOMODienophile accepts electrons into its LUMOButadiene 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 symmetrySuprafacial

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 difficultException: [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

http://chm234.asu.edu/reallife/332thymine/thymine.html

These reactions also have suprafacial and antarafacial stereochemistrySuprafacial = migration across same face of p systemAntarafacial = migration across opposite face of p

systemBoth 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 anEven number of electron pairs areConrotatory orAntarafacial

SUMMARY OF PERICYCLIC REACTIONS AND MOs