sections 9 .1 – 9.3 valence bond theory hybrid orbitals sigma and pi bonding
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Sections 9 .1 – 9.3 Valence Bond Theory Hybrid Orbitals Sigma and Pi Bonding. Chemical Bonding Theories. Valence Bond Theory: Uses Lewis Structures Bonds form using shared electrons between overlapping orbitals on adjacent atoms. Orbitals arrange around central atom to avoid each other. - PowerPoint PPT PresentationTRANSCRIPT
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Sections 9.1 – 9.3Valence Bond Theory
Hybrid OrbitalsSigma and Pi Bonding
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Chemical Bonding TheoriesValence Bond Theory: Uses Lewis StructuresBonds form using shared electrons between overlapping orbitals on adjacent atoms.Orbitals arrange around central atom to avoid each other.Two types of bonds: sigma and pi.
Molecular Orbital Theory: Uses MO DiagramsOrbitals on atoms “mix” to make molecular orbtials, which go over 2 or more atoms. Two electrons can be in an orbital.Orbitals are either: bonding, antibonding, or nonbonding.Bonds are either: sigma or pi.
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Orbitals on bonding atoms overlap directly between bonding atoms
Sigma (s) Bonding
Between s orbitals
Between p orbitals
Between s and p orbitals
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Consider VSEPR Shapes and bonding:
Sigma (s) Bonding
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What’s wrong with this picture?
Atoms bond by having their valence orbitals overlap
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Orbitals don’t go in same directions as atomic orbitals.
2s
2pz 2px 2py
Orbitals in CH4Orbitals on C
Conclusion: Atomic orbitals change shape when they make molecules.
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Hybrid Orbitals
Atomic valence orbitals “combine and mix” to form new “Hybrid Orbitals”
Hybrid orbitals go in the VSEPR electron geometry directions.
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Types of Hybrid Orbitals
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4 Pairs: sp3 hybrid orbitals
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sp3 hybrid orbitals: energetics
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sp3 bonding examples
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3 Pairs: sp2 hybrid orbitals
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sp2 bonding example: BF3
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2 Pairs: sp hybrid orbitals
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sp bonding example: BeF2
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More complex example:
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Expanded Valence Cases: 5 structural pairs
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Expanded Valence Cases: 6 structural pairs
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Summary
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Bond angles
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sigma Bonding involves: -- hybrid orbitals -- H 1s orbitals -- terminal atom p orbitals
pi bonding involves unhybridized p orbitals
Sigma vs. Pi Bonding
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sigma bond formation
Orbitals overlap directly between two nuclei:
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pi Bond Formation
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Bonding in Ethene
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Bonding in Acetylene
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Advanced: Allene
Each end carbon is a flat trigonal.
Are they co-planar or perpendicular?
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Bonding in Allene
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Bonding in Benzene
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Summary
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Conformations vs. Isomers
• Isomers: Molecules with same formula but different structure
• Conformers: Different temporary shapes of the same molecule
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Bond Rotations• CAN happen around single bonds• Cannot happen around double bonds
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cis-trans IsomerizationWhen two groups are on a “side” of a molecule.
cis trans
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Can these molecules have cis-trans isomers?