Organic Pedagogical Electronic Network Molecular Orbitals Mariana Neubarth Coelho Edited by Margaret...

Organic Pedagogical Electronic Network

Molecular Orbitals

Mariana Neubarth Coelho

Edited by Margaret Hilton

Honors Organic Chemistry

University of Utah

CC

H

HH

H

H

H

CC

H

HH

H

H

H

Molecular Orbitals

Lennard-Jones, J.E. (1929) Trans.Faraday Soc. 25, 668. Link

1929 - John Lennard-Jones describes “Atomic states” and “Molecular states”

Molecular Orbital Theory

Representation of Molecular Orbials

CC

H

HH

H

H

H

CC

H

HH

H

H

H

σ bond

Bonding:in phase

(matching colors)

Antibonding:out of phase

(non matching colors)

π bond

H

H H

H H

H H

H

Bonding:in phase

(matching colors)

Antibonding:out of phase

(non matching colors)

http://pubs.rsc.org/en/content/articlepdf/1929/tf/tf9292500668

Building Energy Diagrams

Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

• Molecular orbitals result of the combination of atomic orbitals.

• Orbitals cannot be created nor destroyed. The number of molecular orbitals equals the number of atomic orbitals involved.

• Orbitals need to be in phase in order to overlap. Thus, bonding orbitals are in phase and antibonding orbitals are out of phase.

• More electronegative atoms have lower-energy atomic orbitals.

• Lower-energy molecular orbials are filled first (Aufbau principle).

Example Energy Diagram of CH3Br

2 atomic orbitals (AO)

2 molecular orbitals (MO) 1 bonding and 1 antibonding

bondingMO

ener

gy

sp3 C(AO) Br

(AO)

antibondingMO

C Br

H

HH

C Br

H

HH

http://pubs.acs.org/doi/abs/10.1021/ed076p1718?journalCode=jceda8&quickLinkVolume=76&quickLinkPage=1718&selectedTab=citation&volume=76







Frontier Molecular Orbitals


HOMO: highest-energy occupied molecular orbital – acts as a nucleophileLUMO: lowest-energy unoccupied molecular orbital – acts as an electrophile

• Because bromine is more electronegative than carbon, the σ-bond is polarized with bromine bearing more of the electron density.

• The HOMO contains the electrons of this bond. It has more contribution from bromine because they are closer in energy.

• The LUMO is empty and has more contribution from carbon.

• If this molecule was attacked by a nucleophile in an SN2 reaction, the antibonding orbital would receive electrons, forming a new bond between carbon and the nucleophile. Bromine, the leaving group, would take the electrons from the C-Br bond.

Frontier orbitals are where reactions take place.

ener

gy

sp3 C(AO) Br

(AO)

HOMO

LUMO

H3C-Br (σ bond)

Frontier Orbitals of CH3Br








Why are antibonding orbitals important?


• The transfer of electrons from one reactant to another requires the overlap of a filled orbital (HOMO) and an empty orbital (LUMO).

• Orbitals need to be in phase in order to overlap.

• If the LUMO is inaccessible, the reaction may not occur.

• Filled orbitals cannot receive electrons (Pauli exclusion principle).

SN2 reactions

This reaction does not occur because the antibonding orbital is too hindered.

OH C Br

H

HH

OH C

H

HH

+ Br

OH C Br

H3C

H3CCH3

X

X

Primary or secondary

alkyl halides

Tertiary alkyl

halides








Why are antibonding orbitals important?


• The transfer of electrons from one reactant to another requires the overlap of a filled orbital (HOMO) and an empty orbital (LUMO).

• Orbitals need to be in phase in order to overlap.

• If the LUMO is inaccessible, the reaction may not occur.

• Filled orbitals cannot receive electrons (Pauli exclusion principle).

H

BrHH

HH

OH

H

H H

H

+ Br

E2 reactions

Br

H

HH

H

HO

H

xX No Reaction

Antiperiplanar: the bonding orbital of the C-H bond and the antibonding orbital of the C-Br bond must rehybridize in order to form the π bond (note: they are in phase).

Synperiplanar: the bonding orbital of the C-H bond and the antibonding orbital of the C-Br bond are out of phase, therefore they cannot overlap to form the π bond.








Occupied and Unoccupied Orbitals

Occupiedorbitalscan be

occupied non-bonding orbitals (lone pairs)bonding orbital of a σ bondbonding orbital of a π bond

Unoccupiedorbitals

can be

unoccupied non-bonding orbitals (cations)antibonding orbital of a σ bondantibonding orbital of a π bond









Molecular Orbitals: π Systems

1,3 diene

4 atomic orbitals 4 moleclar orbitals

bonding

antibonding

• Each carbon is sp2 hybridized.

• There are 4 electrons in the conjugated π system of this molecule.

• The nodes (dashed lines) represent a switch in phase, meaning that orbitals can not overlap. No electrons can be found where there is a node.

• One node is added (based on symmetry) for each increase in energy level.

HOMO

LUMO

Conjugation: p orbitals on adjacent carbons can overlap allowing the π electrons to delocalize, thus lowering the energy of the molecule. In order for there to be overlap, orbitals need to be in phase.

0 nodes

1 node

2 nodes

3 nodes

ener

gy









Molecular Orbitals: π Systems

Bonding

Non-bonding

Antibonding• Each carbon is sp2 hybridized.

• Conjugation still exists even if there are only 2 electrons. The positive charge and the π electrons are delocalized over the 3 carbons.

• One node is added (based on symmetry) for each increase in energy level.

3 atomic orbitals 3 molecular orbitals

0 nodes

1 node

2 nodes

ener

gy


allylic carbocation








Molecular Orbitals: Predicting Interactions

• The HOMO on one reactant donates electrons to the LUMO on the other. Both reactants have a HOMO and LUMO, but the best HOMO-LUMO combination will be have the best orbital overlap, or in other words, the best match in energy.

H3O+–OH

• The low LUMO on H3O+ makes it a good acid, but also prevents it from acting as a base under normal circumstances, despite the presence of a lone pair on the oxygen. Similarly, the high HOMO on –OH makes it a good base, but also prevents it from acting as an acid despite the potentially acidic O-H bond.

HOMO

HOMO

LUMO

LUMO

AE vs. AE

ener

gy

• As the formal charge on a species becomes more positive, the energies of its frontier orbitals decrease.

AE: Activation energy

Example: acid – base chemistry









Applications: Diels-Alders Reaction

Diels-Alders Reaction

HOMO

LUMO

LUMO

HOMO

ener

gy

AE

The HOMO on the diene overlaps with the LUMO on the dienophile. This is the best match in energy.

LUMO of the dienophile

HOMO ofthe diene


diene dienophile

Reverse Diels-Alders

HOMO

LUMOLUMO

HOMO

ener

gy

AE

• The presence of an electron withdrawing group (EWG) on the dienophile decreases its energy.

• The presence of an electron donating group (EDG) on the diene increases its energy.

HOMO ofthe dienophile

LUMO ofthe diene

As a consequence, the HOMO on the

dienophile interacts with the LUMO on the

diene.


EWG

EDG

EWG

EDG

diene dienophile

Applications: Diels-Alders Reaction








Problems

1. Draw the molecular orbitals and label the HOMO and LUMO for the pi electrons in hexatriene:

3. Using molecular orbitals, explain why differentalkenes are produced in each case.

4. Predict the product of the reaction below and draw the correct molecular orbitals from each reagentwhich react.

ClNaOH NSO2Ph

+OEt

ClNaOH

(Hint: draw the chair conformations)

2. Rationalize which is more stable, based on the relative energies of their HOMO/LUMOs.

5. Is the following transformation likely to occur? UseFMO analysis to explain your answer.

+

Organic Pedagogical Electronic Network Molecular Orbitals Mariana Neubarth Coelho Edited by Margaret...

Documents

Transcript of Organic Pedagogical Electronic Network Molecular Orbitals Mariana Neubarth Coelho Edited by Margaret...