Organic Pedagogical Electronic Network Molecular Orbitals Mariana Neubarth Coelho Edited by Margaret Hilton Honors Organic Chemistry University of Utah

Molecular Orbitals Lennard-Jones, J.E. (1929) Trans.Faraday Soc. 25, 668. LinkLink John Lennard-Jones describes “Atomic states” and “Molecular states” Molecular Orbital Theory Representation of Molecular Orbials σ bond Bonding : in phase (matching colors) Antibonding : out of phase (non matching colors) π bond Bonding : in phase (matching colors) Antibonding : out of phase (non matching colors)

Building Energy Diagrams Lewis, D. Journal of Chemical Education, 1999, 76, 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 CH 3 Br 2 atomic orbitals (AO)  2 molecular orbitals (MO) 1 bonding and 1 antibonding bonding MO energy sp 3 C (AO) Br (AO) antibonding MO

Frontier Molecular Orbitals Lewis, D. Journal of Chemical Education, 1999, 76, HOMO: highest-energy occupied molecular orbital – acts as a nucleophile LUMO: 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 S N 2 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. energy sp 3 C (AO) Br (AO) HOMO LUMO H 3 C-Br (σ bond) Frontier Orbitals of CH 3 Br

Why are antibonding orbitals important? Lewis, D. Journal of Chemical Education, 1999, 76, 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). S N 2 reactions This reaction does not occur because the antibonding orbital is too hindered. Primary or secondary alkyl halides Tertiary alkyl halides

Why are antibonding orbitals important? Lewis, D. Journal of Chemical Education, 1999, 76, 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). E 2 reactions 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 Occupied orbitals can be occupied non-bonding orbitals (lone pairs) bonding orbital of a σ bond bonding orbital of a π bond Unoccupied orbitals can be unoccupied non-bonding orbitals (cations) antibonding orbital of a σ bond antibonding orbital of a π bond Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

Molecular Orbitals: π Systems 1,3 diene 4 atomic orbitals4 moleclar orbitals bonding antibonding  Each carbon is sp 2 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 energy Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

Molecular Orbitals: π Systems Bonding Non-bonding Antibonding Each carbon is sp 2 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 orbitals3 molecular orbitals  0 nodes 1 node 2 nodes energy Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

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+H3O+– OH The low LUMO on H 3 O + 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 LUMO AE vs. AE energy As the formal charge on a species becomes more positive, the energies of its frontier orbitals decrease. AE: Activation energy Example: acid – base chemistry Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

Applications: Diels-Alders Reaction Diels-Alders Reaction HOMO LUMO HOMO energy 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 of the diene Lewis, D. Journal of Chemical Education, 1999, 76, 1718.

Reverse Diels-Alders HOMO LUMO HOMO energy 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 of the dienophile LUMO of the diene As a consequence, the HOMO on the dienophile interacts with the LUMO on the diene. Lewis, D. Journal of Chemical Education, 1999, 76, Applications: Diels-Alders Reaction

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