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Electron Delocalization and Resonance Chapter 7

Electron Delocalization and Resonance Chapter 7. Contents of Chapter 7. Benzene Delocalized Electrons and Resonance Stability of Allylic and Benzylic Cations and Radicals Effect of Electron Delocalization on p K a Relative Stabilities of Dienes Reactivity Considerations

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Electron Delocalization and Resonance Chapter 7

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  1. Electron Delocalization and ResonanceChapter 7 Chapter 7

  2. Contents of Chapter 7 • Benzene • Delocalized Electrons and Resonance • Stability of Allylic and Benzylic Cations and Radicals • Effect of Electron Delocalization on pKa • Relative Stabilities of Dienes • Reactivity Considerations • Electrophilic Addition Reactions • Direct (1,2-) and Conjugate (1,4-) Additions Chapter 7

  3. Delocalized Electrons and Resonance Chapter 7

  4. Bonding in Benzene (a) Carbon–carbon and carbon–hydrogen s bonds (b) Carbon p-orbitals overlap with neighbors (c) Cloud of p electrons above and below ring (d) Electrostatic potential map for benzene Chapter 7

  5. Delocalized Pi Systems • Extend continuously through all connected atoms which lack 4 single bonds (except H) • Broken up by saturated atoms with 4 single bonds (C). • Two double bonds on same carbon break up system (cumulated dienes) • Broken by two saturated connected atoms with same Lewis acid/base properties (ie. 2 O atoms or C+ connected to B) Chapter 7

  6. Resonance Tricks Lone-Pair/Double-Bond Interconversion • One atom charge goes down, other goes up. • Atom receiving e’s has charge go more negative. Chapter 7

  7. Resonance Tricks Double Bond Flip • Double bond flips like page in book • Instability (+, -, or •) jumps 1,3 • Electrons move diff. but effect same for +-• Chapter 7

  8. Resonance Tricks Electron Dominoes • Combinations of interconversions and flips • Instability moves thru pi system in 2-atom increments. Chapter 7

  9. Resonance Structures Chapter 7

  10. Resonance Structures Chapter 7

  11. Resonance Structures Chapter 7

  12. Rules for Drawing Resonance Structures Chapter 7

  13. The Resonance Hybrid When there is a choice, electrons tend to move toward the more electronegative atom Chapter 7

  14. Resonance Energy • The resonance energy tells us how much more stable the compound with delocalized electrons is due to that delocalization • The greater the number of relatively stable resonance contributors, the greater the resonance energy Chapter 7

  15. Resonance Energy The more nearly equivalent the resonance structures, the greater the resonance energy Chapter 7

  16. Stability of Allylic and Benzylic Cations Chapter 7

  17. Stability of Allylic and Benzylic Cations Chapter 7

  18. Crude Carbocation Stability Index • Add 1 for each attached carbon. • Add 1 for adjacent double bond or phenyl ring. • Subtract 1 if C+ on double bond. • Alkene C doesn’t count but C attached to either alkene C does. Chapter 7

  19. Relative Stabilities of Carbocations Vinyl cations are one level less stable than alkyl cations Chapter 7

  20. Effect of Delocalization on pKa Carboxylic acids (RCOOH) are much more acidic than and alcohols (ROH) pKa = 4.76 pKa = 15.9 Chapter 7

  21. Effect of Delocalization on pKa Chapter 7

  22. Effect of Delocalization on pKa A protonated aniline is more acidic than a protonated cyclohexylamine pKa = 4.6 pKa = 11.2 Chapter 7

  23. Types of Dienes When double bonds are separated by at least one sp3carbon, isolated diene Chapter 7

  24. Types of Dienes When double bonds are separated by only one single bond (i.e. four sp2 carbons in a row), conjugated diene Chapter 7

  25. Types of Dienes When both sets of double bonds emanate from the same carbon, cumulated diene Chapter 7

  26. Relative Stabilities of Dienes Chapter 7

  27. Relative Stabilities of Dienes • Doubly-bonded carbons in isolated and conjugated dienes all are sp2hybridized • The central carbon in a cumulated diene is sp hybridized Chapter 7

  28. Electrophilic Addition Reactions of Isolated Dienes • Reaction of 1,5-hexadiene with excess HBr adds HBr independently to each double bond • Markovnikov’s Rule is followed Chapter 7

  29. Electrophilic Addition Reactions of Conjugated Dienes Conjugated dienes can give both 1,2- and 1,4- addition products Chapter 7

  30. Mechanism of Addition of HBr to Conjugated Dienes The positive charge on the allylic cation is not localized on C-2, but is shared between C-2 and C-4 Chapter 7

  31. Electrophilic Additions to Conjugated Dienes • Electrophile adds to one end of pi system • Draw all resonance structures for C+ for adding electrophile to each end of pi system • Evaluate which set of structures is more stable (stability index) • Number most stable C+ positions with even numbers, end of pi system #1 • Electrophile (H) adds to position #1 • Add nucleophile to each C+ (even-numbered C atom) in most stable set of resonance structures to get observed products Chapter 7

  32. Electrophilic Addition to Conjugated Dienes Example Chapter 7

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