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Advanced Theories of Chemical Bonding Chapter 9

Advanced Theories of Chemical Bonding Chapter 9. Atomic Orbitals. Molecules. Two Theories of Bonding. MOLECULAR ORBITAL THEORY — Robert Mullikan (1896-1986) valence electrons are delocalized valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

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Advanced Theories of Chemical Bonding Chapter 9

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  1. Advanced Theories of Chemical BondingChapter 9 Atomic Orbitals Molecules

  2. Two Theories of Bonding • MOLECULAR ORBITAL THEORY — Robert Mullikan (1896-1986) • valence electrons are delocalized • valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.

  3. Two Theories of Bonding • VALENCE BOND THEORY — Linus Pauling • valence electrons are localized between atoms (or are lone pairs). • half-filled atomic orbitals overlap to form bonds. Linus Pauling, 1901-1994

  4. Sigma Bond Formation by Orbital Overlap Two s orbitals overlap

  5. Sigma Bond Formation Two s orbitals overlap Two p orbitals overlap

  6. Using VB Theory Bonding in BF3 planar triangle angle = 120o

  7. Bonding in BF3 • How to account for 3 bonds 120o apart using a spherical s orbital and p orbitals that are 90o apart? • Pauling said to modify VB approach with ORBITAL HYBRIDIZATION • — mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.

  8. Why Hybridize? Just looking at valence electrons: Be should form no covalent bonds B should form one covalent bond C should form 2 covalent bonds But… BeF2, BF3 and CF4 Exist! HOW?

  9. Hybrid Orbitals: Why? • To explain the bonding in molecules like BeF2, BF3 and CF4, Linus Pauling proposed that orbitals become ‘hybridized’ • Hybrid orbitals are orbitals created by mixing the s, p or d orbitals of an atom.

  10. Hybrid Orbitals: The Rules • The number or hybrid orbitals is ALWAYS equal to the number of atomic orbitals that are combined to make the hybrid set • Hybrid orbital sets are always built by combining an s orbital with as many p or d orbitals necessary to accommodate the bonding and lone pairs on the central atom (Remember Electron Pair Geometry?) • The Hybrid Orbitals are directed TOWARDS the terminal atoms • This results in a better orbital overlap AND stronger bonds between the central and terminal atoms

  11. sp Hybrid Orbitals Mix an s orbital with a p orbital to create two sp orbitals

  12. sp2 Hybrid Orbitals Mix an s orbital with 2 p orbitals to create three sp2 orbitals

  13. sp3 Hybrid Orbitals Mix an s orbital with 3 p orbitals to create four sp3 orbitals

  14. sp3 Hybrid Orbitals: Examples

  15. 2p 2s hydridize orbs. rearrange electrons 2 unused p three sp orbital hybrid orbitals Bonding in BF3

  16. Bonding in BF3 • The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp2 hybrids. • Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.

  17. Bonding in BF3 An orbital from each F overlaps one of the sp2 hybrids to form a B-F  bond.

  18. BF3, Planar Trigonal

  19. Bonding in CH4 How do we account for 4 C—H sigma bonds 109o apart? Need to use 4 atomic orbitals — s, px, py, and pz — to form 4 new hybrid orbitals pointing in the correct direction.

  20. Bonding in a Tetrahedron Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.

  21. Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals.

  22. Bonding in CH4

  23. Bonding in Glycine

  24. Bonding in Glycine

  25. Bonding in Glycine

  26. Bonding in Glycine

  27. Bonding in Glycine

  28. 2 e- clouds 3 e- clouds 4 e- clouds 5 e- clouds 6 e- clouds Orbital Hybridization

  29. H H 2 sp 120 ° C C H H Multiple Bonds Consider ethylene, C2H4

  30. H H 2 sp 120 ° C C H H Sigma Bonds in C2H4

  31. π Bonding in C2H4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond.

  32. π Bonding in C2H4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond.

  33. Multiple Bondingin C2H4

  34. sand π Bonding inC2H4

  35. sand π Bonding inCH2O

  36. sand π Bonding inC2H2

  37. sand π Bonding inC2H2

  38. Consequences of Multiple Bonding There is restricted rotation around C=C bond.

  39. Consequences of Multiple Bonding Restricted rotation around C=C bond.

  40. Consequences of Multiple Bonding Formation of Isomers One isomer may have biological activity while the other may not

  41. Double Bonds and Vision

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