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Chapter 7

Chapter 7. Delocalized Electrons and Their Effects on Stability, Reactivity and pKa More about MO Theory. Localized and delocalized electrons. Substitution product of benzene. hydrogens on benzene must be identical. two disubstituted products. four disubstituted products.

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Chapter 7

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  1. Chapter 7 Delocalized Electrons and Their Effects on Stability, Reactivity and pKa More about MO Theory

  2. Localized and delocalized electrons

  3. Substitution product of benzene hydrogens on benzene must be identical

  4. two disubstituted products four disubstituted products

  5. Kekulé structure of benzene

  6. 1901 Sabatier 1930’s X-ray and electron diffraction Planar structure C-C bond length 1.39 Å

  7. Structure of Benzene ≡ Dewar benzene Ladenburg benzene : prismane

  8. Bonding in Benzene

  9. Drawing Resonance Contributors

  10. Rule for Drawing Resonance Contributors • Only electrons move. Atoms never move. • Only p-electrons and lone-pair electrons can move; • s-electrons never move. • 3. The total number of electrons in the molecule does not change. Predicted Stabilities of Resonance Contributors • 1. The greater the predicted stability of the resonance contributor, • the more it contributes to the structure of the resonance hybrid. • 2. Resonance contributors with separated charges are relatively unstable, • because energy is required to keep opposite charges separated.

  11. Predicted Stabilities of Resonance Contributors

  12. Effect of delocalization.

  13. carbon-carbon single bonds in conjugated dienes

  14. Cumulated double bonds in Allenes

  15. Stability of Allylic and Benzylic Cations.

  16. Relative stabilities of carbocations.

  17. Relative reactivity with HBr

  18. Molecular Orbital (MO) Ethene CH2=CH2

  19. 1,3-Butadiene LCAO : Linear Combination of Atomic Orbital MO p-orbitals (AO) ψ1 = 0.371 Φ1 + 0.6Φ2 + 0.6Φ3 + 0.371Φ4 ψ2 = 0.6 Φ1 + 0.371 Φ2- 0.371 Φ3- 0.6 Φ4 ψ3 = 0.6 Φ1- 0.371 Φ2- 0.371 Φ3 + 0.6 Φ4 ψ4 = 0.371Φ1- 0.6 Φ2 + 0.6 Φ3- 0.371 Φ4

  20. CH2=CH-CH=CH2

  21. Molecular orbitals of benzene

  22. Effect of Delocalization on pKa Energy diagram ?

  23. Effect of Delocalization on pKa Energy diagram ? charge separation no charge separation

  24. Effect of Delocalization on pKa Energy diagram ?

  25. Addition of hydrogen bromide to 1-phenyl-1-propene

  26. Site of protonation

  27. Reaction of isolated diene with HBr

  28. Reaction of conjugated diene with HBr

  29. Kinetic vs. thermodynamic product

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