Organic Chemistry 6 th Edition Paula Yurkanis Bruice

1. Organic Chemistry 6th Edition Paula Yurkanis Bruice Chapter 7 Delocalized Electrons and Their Effect on Stability, Reactivity, and pKa More About Molecular Orbital Theory

2. Localized Versus Delocalized Electrons

3. Benzene Has Delocalized Electrons • Each p electron is shared by all six carbons • The p electrons are delocalized • A planar molecule • Has six identical carbon–carbon bonds

4. Resonance Contributors and the Resonance Hybrid Resonance contributors do not depict any real electron distribution

5. electrons cannot delocalize in nonplanar molecules

6. Rules for Drawing Resonance Contributors 1. Only electrons move. 2. Only p electrons and lone-pair electrons move. 3. The total number of electrons in the molecule does not change. 4. An sp3 carbon cannot accept electrons; it already has an octet. 5. Do not break sp3() bonds.

7. Only  or lone pair electrons move! No  bonds broken! Many Resonance Structures Consist of Two-Center and/or Three-Center Systems Two-Center Systems: Three-Center Systems:

8. Allylic Resonance Move p electrons toward an sp2 carbon:

9. Allylic Allylic Allylic Allylic Allylic Multi-Center Allylic Resonance Composed of allylic centers: Benzylic cation:

10. Three-Center Bonding: Nitro Group Resonance

11. Electronegative oxygen Electropositive nitrogen Electronic push-pull Three-Center Bonding: Amide Resonance Move lone-pair electrons toward the sp2 oxygen: Amide resonance responsible for the strength of amide bonds found in hair, skin, muscle, Kevlar vests, etc.

12. (R) (R) derivative Three-Center Bonding: Carboxylic Acid Derivatives B is less stable than A, no electronic push-pull. Ester bonds not as robust as amide bonds. C and D are equally stable: carboxylate resonance responsible for the acidity of carboxylic acids.

13. Carbonyl center Allyl cation ,-Unsaturated carbonyl compounds are toxic because they trap YOUR nucleophiles. ,-Unsaturated Carbonyl Compounds A four-center system composed of two and three center systems: Predicted reactivity:

14. Summary of Electron Delocalization Examples 1. Move p electrons toward an sp2 carbon 2. Move lone-pair electrons toward an sp2 carbon

15. 3. Move lone-pair or  electrons towards an sp carbon: 4. Do not move electrons to an sp3 carbon:

16. Alkene cross conjugated with carboxylate No + charge here, cross conjugated 5. Electrons always move toward the more electronegative atom: 6. Do not break sp3() bonds :

17. Isoelectronic with allyl anion Substituent Effects Resonance release of lone-pair electrons (competes with inductive withdrawal by the electronegative oxygen): The methoxy group makes the benzene ring electron-rich: Electron-releasing groups have a lone pair at the point of attachment.

18. Substituent Effects The nitro group makes the benzene ring electron-deficient by resonance withdrawal: Electron-withdrawing groups possess an electron-deficient center at the point of attachment:

19. More stable More stable Features that decrease the predicted stability of a contributing resonance structure: 1. An atom with an incomplete octet: 2. A negative charge that is not on the most electronegative atom:

20. Not a valid structure More stable charge-separated structure More stable 3. A positive charge that is not on the most electropositive atom: 4. Charge separation:

21. Delocalization Energy The extra stability a compound gains from having delocalized electrons is called the delocalization energy Electron delocalization is also called resonance Delocalization energy is also called resonance energy A resonance hybrid is more stable than any of its resonance contributors is predicted to be

22. Summary • The greater the predicted stability of a resonance • contributor, the more it contributes to the resonance • hybrid • The greater the number of relatively stable resonance • contributors, the greater is the resonance energy • The more nearly equivalent the resonance contributors, • the greater is the resonance energy

23. The amino acid arginine, guanidium pKa = 12.5, cationic at physiological pH= 7.4 Using Resonance to Predict Stability Carbonate is stable and ubiquitous in nature: Cement, shells, limestone… The guanidium ion is stabilized by resonance and deprotonated only in strong base: Important physiologically:

24. Relative Stabilities of Carbocations and Radicals Carbocations: hyperconjugation more important than resonance. Radicals: resonance more important than hyperconjugation.

25. Cation Stabilization by Resonance and Hyperconjugation

26. Example: Delocalized Electrons Can Affect the Product of a Reaction

27. Delocalized Electrons Affect pKa Carboxylic acid is a stronger acid because… Electron withdrawal and electron delocalization stabilize the conjugate base:

29. Phenol is a stronger acid than cyclohexanol because of phenolate ion delocalization: Localized Anion pKa = 16 pKa = 10 Delocalized Anion

30. Protonated aniline is a stronger acid than protonated cyclohexylamine because the aniline lone pair is delocalized: Localized lone pair Delocalized lone pair pKa = 4.62 pKa = 11.2

31. Connecting Delocalization, Substituent Effects, and pKa Values Important in understanding drug design and reaction mechanisms: Which phenol is the stronger acid? First, show the resonance delocalization of the phenolate anion:

32. Anion delocalizes into the nitro group Anion adjacent to electron-withdrawing nitro group Stronger acid, pKa = 7.2 Weaker acid, pKa = 8.4. Nitro only exerts inductive withdrawal Second, pick the structure in which the anion interacts with the nitro group. This structure represents the more stable anion: Third, the conjugate acid of the more stable anion is the strongest acid:

33. Stability of Dienes

34. Closer in energy to pentane The most stable diene has the lower -DHo value: Why: The hybridization of the orbitals forming the carbon-carbon single bonds also causes a conjugated diene to be more stable than an isolated diene:

36. A Molecular Orbital Description of Stability • Bonding MO: constructive (in-phase) overlap • Antibonding MO: destructive (out-of-phase) overlap

37. MOs for the Allyl System

38. The Molecular Orbitals of 1,3-Butadiene

39. Symmetry in Molecular Orbitals y1 and y3 in 1,3-butadiene are symmetrical molecular orbitals y2 and y4 in 1,3-butadiene are fully asymmetrical orbitals

40. The highest-energy molecular orbital of 1,3-butadiene • that contains electrons is y2 (HOMO) • The lowest-energy molecular orbital of 1,3-butadiene • that does not contain electrons is y3 (LUMO) • HOMO = the highest occupied molecular orbital • LUMO = the lowest unoccupied molecular orbital

41. Consider the p molecular orbitals of 1,4-pentadiene: This compound has four p electrons that are completely separated from one another

42. The Molecular Orbitals of 1,3,5-Hexatriene

43. Benzene has six p molecular orbitals

44. As the energy of the p orbitals increase, the net number of bonding interactions decreases

45. Benzene is unusually stable because of large delocalization energies:

46. Needs 4N+2 (N = 0, 1, 2, 3…) electrons to fill orbitals Needs an even number of electrons to fill orbitals Why isn’t cyclooctatetrene flat? Pattern of bonding orbitals of planar cyclic  systems (e.g., benzene): Pattern of bonding orbitals of non-planar or non-cyclic  systems (e.g., 1,3-butadiene): Answer: Cyclooctatetrene has 8electrons, which cannot fill the degenerate orbitals of a planar  system. Therefore it has a tub shape to remove these degenerate orbitals.

47. Reactions of Isolated Dienes In the presence of limiting electrophilic reagent, only the more reactive double bond reacts

48. Reactions of Conjugated Dienes An isolated diene undergoes only 1,2-addition A conjugated diene undergoes both 1,2- and 1,4-addition

49. More stable alkene More stable cation Kinetic product Thermodynamic product Mechanism of HBr Addition to a Conjugated Diene