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. Benzene C 6 H 6 –Rapid Equilibrium between 2 structures –Proposed by Fredrich Kekule (1865 German chemist) Kekule Structure Rapid Equilibrium

4. * Kekule’s Dream

5. 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! Benzene Structure

6. Each C is sp 2 hybridized Each C has an unhybridized p orbital perpendicular to the plane of the ring The 6 p orbitals overlap to form a  cloud Bonding in Benzene

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

8. Benzene –contributing resonance structures Resonance Hybrid

9. 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 - ) Resonance Hybrids

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

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

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

13. Drawing resonance hybrids Resonance Hybrids

15. Practice Drawing Resonance 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 B is less stable than A Equal Stability

18. 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. Resonance Energy

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

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

25. * Stability of Cations

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

27. Carboxylic acids are stronger acids than alcohols due to resonance effects Effects on pK a

28. Why is this alcohol as acidic as it is? –-OH is on an sp 2 hybridized C –In phenol, some resonance contributors have a + charge on O –3 of phenol’s structures have separated charges Phenol

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

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! Kinetic product – low temp Thermodynamic product – high temp

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

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

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

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

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

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

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 3 4 5 6 7

40. Bicyclics - Nomenclature Examples

41. Bicyclics - Nomenclature Examples