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Organic Chemistry

Organic Chemistry. Chapter 7 - Resonance. Electron Delocalization and Resonance. Localized electrons = restricted to a particular region Delocalized electrons do not belong to a single atom or exclusively to a bond between 2 atoms. Kekule Structure. Benzene C 6 H 6

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Organic Chemistry

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  1. Organic Chemistry Chapter 7 - Resonance

  2. Electron Delocalization and Resonance • Localized electrons = restricted to a particular region • Delocalized electrons do not belong to a single atom or exclusively to a bond between 2 atoms

  3. Kekule Structure • Benzene C6H6 • Rapid Equilibrium between 2 structures • Proposed by Fredrich Kekule(1865 German chemist) Rapid Equilibrium

  4. Kekule’s Dream • *

  5. Benzene Structure • Kekule Structures of Benzene were accepted in the 1930’s when X-ray studies showed ALL SIX C-H bonds equal and ALL SIX C-C bonds equal!

  6. Bonding in Benzene • Each C is sp2 hybridized • Each C has an unhybridized p orbital perpendicular to the plane of the ring • The 6 p orbitals overlap to form a  cloud

  7. Resonance Hybrid • A compound with delocalized e- is said to have resonance • resonance contributor • resonance structure • contributing resonance structure

  8. Resonance Hybrid • Benzene • contributing resonance structures

  9. Resonance Hybrids Drawing resonance hybrids • 1) Only e- move(not atoms) • 2) Only  and non-bonding e- move • 3) Total # e- stays same(as does unpaired e-)

  10. Resonance Hybrids e- can be moved only by…  e- move toward + or toward  bond

  11. Resonance Hybrids e- can be moved only by… Nonbonding pair e- toward a  bond

  12. Resonance Hybrids e- can be moved only by… Nonbonding single e- toward a  bond

  13. Resonance Hybrids • Drawing resonance hybrids

  14. Resonance Hybrids

  15. Practice Drawing Resonance Practice Practice Practice

  16. What makes a Resonance Structure Have Decreased Stability? • 1) an atom with an incomplete octet • 2) a negative charge that is not on the most electronegative atom • 3) a positive charge not on the most electropositive atom • 4) charge separation

  17. Examples To Examine Equal Stability B is less stable than A

  18. Resonance Energy • 1) The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid. • 2) The greater the number of relatively stable resonance contributors, the greater the resonance energy. • 3) The more nearly equivalent the resonance contributors, the greater the resonance energy.

  19. Resonance Energy The more nearly equivalent the resonance contributors, the greater the resonance energy

  20. Stability of Dienes

  21. Stability of butadiene

  22. Butadiene – Example

  23. Allylic Cations • Allylic cations have delocalized e- • They are more stable due to resonance contributors:

  24. Benzylic Cations • Benzylic cations have delocalized e- • They are more stable due to resonance contributors:

  25. Stability of Cations • *

  26. Consequences • Because of the allyl and benzyl cations and radicals are more stable, some products can be formed easier. Mechanisms Mechanisms Mechanisms Look at how conjugation affects reactions

  27. Effects on pKa • Carboxylic acids are stronger acids than alcohols due to resonance effects

  28. Phenol • Why is this alcohol as acidic as it is? • -OH is on an sp2 hybridized C • In phenol, some resonance contributors have a + charge on O • 3 of phenol’s structures have separated charges

  29. Molecular Orbital Theory • Molecular Orbital Theory can also help explain increased stability • READ THROUGH THIS SECTION OF THE BOOK

  30. Thermodynamic vs. Kinetic Control • If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions. • If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored. • If carried out at sufficiently vigorous (hi-temp) conditions, the reaction will be reversible and the thermodynamic product will be favored.

  31. Thermodynamic vs. Kinetic Control • Kinetic product and thermodynamic product: Formed fastest Most stable

  32. Thermodynamic vs. Kinetic Control • Reaction conditions control products! Kineticproduct – low temp Thermodynamic product – high temp

  33. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Reaction • This reaction makes new carbon carbon bonds • Uses a conjugated diene and a dienophile

  34. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Reaction • Electron withdrawing groups (O or N) increases the reactivity of the dienophile and increase the reactivity acts as an electrophile

  35. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Reaction • 1,4 addition to a conjugated diene

  36. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Stereochemistry • If a chiral center is created, equal amounts of R, S produced (racemic)

  37. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Stereochemistry • Stereospecific: cis dienophile remains cis • trans dienophile remains trans

  38. Kurt Alder Cologne University Cologne Otto Diels Kiel University Kiel Diels-Alder Examples • Dienophiles can also be C triple bond C

  39. Bicycloalkanes • Nomenclature • parent is the alkane of the same number of carbons as are in two rings • number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc. • show the lengths of bridges in brackets, from longest to shortest 1 2 6 7 3 5 4

  40. Bicyclics - Nomenclature • Examples

  41. Bicyclics - Nomenclature • Examples

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