2012 Orbital Hybrization, Sigma and Pi Bonds


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2012 Orbital Hybrization, Sigma and Pi Bonds

  1. 1. IB Chemistry Power Points Topic 14 (AHL) Bondingwww.pedagogics.ca LESSON Hybridization (video tutorial links included)
  2. 2. Recall : methane, CH4 – tetrahedralshape (4 equal covalent bonds) Carbon has an electron arrangement 1s22s22p2 There is a contradiction here . . . . . .
  3. 3. Recall: the s orbital shape ONE s orbital in each energy level
  4. 4. Recall: the p orbital shape THREE p orbitals in each energy level (beginning n = 2)
  5. 5. Carbon’s valence shell - 2nd energy level orbitals The 4 orbitals are oriented like so:Problem #1This is not a tetrahedralshape. The 3 p orbitals are at900 to each other and the sorbital is “non-directional”
  6. 6. Carbon ground state electron configuration What is the expected orbital notation of carbon in its ground state?Problem #2? Only 2 electrons appear to be available for bonding
  7. 7. Solution – require 4 unpaired electronsPerhaps carbon “promotes”one of its 2s electrons… …to the empty 2p orbital.
  8. 8. BUT. . . this would mean that three of the bonds in a methanemolecule would be identical,But what about the fourthbond…? Must have a lower bond energy This was not observed by chemists
  9. 9. “hybridization” theory - What if the s orbital is combined with three p orbitals to create four equal hybrid orbitals? “hybridized”These new orbitals have slightly MORE energy thanthe 2s orbital… and slightly LESS energy than the 2p orbitals. … But all 4 hybrid orbitals have equal energy.
  10. 10. Since the 4 hybrid orbitals result from a combination of 1 sand 3 p orbitals, these hybrids are termed sp3 orbitals.
  11. 11. 3 3 3 31s 2s 2px 2py 2pz sp sp sp sp y Hybridize x o 109.5 z Summary of sp3 hybridization
  12. 12. sp3 hybridizationin methaneCH4
  13. 13. Summary of sp3hybridization
  14. 14. Sigma (σ) bonds The bonds formed in methane between carbon and hydrogen atoms are called sigma bonds.As shown in the diagram, a sigma bond is formed by the headon overlap of two orbitals. THE OVERLAP REGION LIESDIRECTLY BETWEEN THE TWO NUCLEI. Nuclei can rotateabout a sigma bond. Video tutorial link to ethane
  15. 15. sp2 hybridizationConsider the electron configuration of boron:When boron bonds with hydrogen, VSEPR predicts atrigonal planar geometry.One s orbital hybridizes with 2p orbitals to form 3 sp2 orbitals
  16. 16. sp2 hybridization also occurs in molecules that contain doublecovalent bonds. For example, ethene C2H4
  17. 17. Each carbon : 3 hybridized sp2 orbitals and 1 p orbitalSigma bond – single bond C-HSigma bond – sp2 orbitals: one half of double bond C=CPi bond – p orbitals: other half of double bond C=C
  18. 18. Pi bondsA pi bond consists of two p orbitals that overlap above andbelow a sigma bond. A double bond always consists of ONEsigma bond and ONE pi bond.The molecule cannot be twisted without breaking the pi bond. Video tutorial ethene
  19. 19. PracticeDraw a diagram to show the orbital overlap in the sigma and pibonds in a molecule of formaldehyde CH2O. State the shape ofthe molecule and the arrangement of negative charge centersaround the central atom.
  20. 20. Formaldehyde 2 Lone Pairs  bond Sigma bondtrigonal planar (3 regions of electron density)
  21. 21. sp hybridizationConsider the electron configuration of beryllium:When beryllium bonds with chlorine, VSEPR predicts alinear geometry.One s orbital hybridizes with 1p orbitals to form 2 sp orbitals
  22. 22. sp hybridizationsp hybridization also occurs in molecules that containtriple covalent bonds. For example, acetylene C2H2
  23. 23. In ethyne C2H2 each carbon has 2 sp orbitals and 2 un-hybridizedp orbitals.The single bond between carbon and hydrogen is a sigma bond.The triple bond between the two carbons consists of a sigma bondbetween two sp2 orbitals and two pi bonds between the two sets ofp orbitals.
  24. 24. The two pi bonds are in different planes 900 to each other . Atriple bond always consists of ONE sigma bond and TWO pibonds. The molecule cannot be twisted without breaking thepi bonds.Show video tutorial acetylene