1. Chapter 6 Resonance and Electron Delocalization

2. Chapter 6 Topics for Test u Sections 6.1 through 6.13 u I will emphasize drawing resonance structures-- Sections 6.1-6.3 u Be sure to look at the supplement and problems on Hückel’s rule available from my website (Not covered Winter 2007)

3. Problem Assignments u In Text Problems: 1, 2, 3, 4, 5, 6, 710, 11, 12a, c, d u End of Chapter Problems: 17, 18, 19, 20, 21, 22, 23, 24 2830, 31 u Problems on Hückel’s rule

4. Chapter 6 Topics u Section 6.1- -Failure of Lewis Dot Notation a) Get more than one Lewis structure when have non-bonded electrons next to a double bond - - always shift electrons!! b) Individual structures are called resonance structures c) The “real structure” is a hybrid of individual resonance structures-- “summary structure”

5. Chap 6 Topics, Continued u We draw resonance structures because we are interested in figuring out what the “real molecule or ion“ looks like! u When we delocalize electrons (push electrons) this leads to resonance stabilization u resonance = delocalization --> stabilization

6. Resonance u The real molecule cannot be represented by any single Lewis (line-bond) formula. u The real molecule is a hybrid of the contributing Lewis (line-bond) structures.

7. RHINOCEROS DRAGONUNICORN real mythical Wheland’s Analogy

8. .. : _ O C O O.. : : :: _ _ O :: O C O.. : : _ _ Resonance in the Carbonate Ion Resonance in the Carbonate Ion O C O O.. : : :: _ CO 3 2- THREE EQUIVALENT STRUCTURES RESONANCE HYBRID

9. : O C O O.. : :: _ _ Typical bond lengths C=O1.22 A C O 1.43 A o o short long All bond lengths are equal in carbonate ion, at about 1.30 A o but ….. Charge is evenly distributed to the three oxygen atoms The behavior of a real carbonate ion does not match the Lewis diagram. determined by x-ray crystallography of CaCO 3 crystals

10. O O O _ C _ 2/3 _ Summary Structure for Carbonate Ion Summary Structure for Carbonate Ion 2 _

11. Carbonate Ion Ion van der Waal’s (isodensity) surface electrostatic potential mapped on isodensity surface (ab initio 3-21g calculation) the same … showing location of atoms red = neg blue = pos

12. Sect. 6.2 and 6.3: Resonance Structures (electron pushing) a) Anions: move an electron pair away from the negative charge towards double bond! Two “flips” necessary. b) Cations: move electron pair towards positive charge! c) Neutral compound: move towards electronegative atom d) Assign formal charges after “moving e - ”

13. 2007 Chemistry Shirt design. Return your order form with the appropriate amount of money to the main office (Bernadette) by 5 PM Friday, February 23rd. The orders will be placed the following Monday. Check the WWU Chemistry website for more information. WWU Chemistry: Caution.. Herd of Nerds! (left: front; right: back)frontback

14. Short sleave shirt: $15 to 17 Long sleave shirt: $17 to 19 Sweat shirt: $28 to 30

15. Phenolate Ion Resonance Phenolate Ion Resonance - notice the alternate pattern (-) Not all contributing structures are equal. Why? SUMMARY

16. Benzyl Cation + (+)

17. Sect. 6.2 and 6.3 Rules for Evaluating Resonance u The octet rule should be obeyed, if possible. u There should be a minimum of charge separation. u The formal charges may correspond to the relative electronegativities of the atoms, but don’t be surprised if an electronegative atom has a positive charge! u Avoid formal charges of the same sign on adjacent atoms.

18. Rules for Evaluating Resonance Forms - Part 2 u Keep the magnitude of the formal charges as low as possible u Avoid multiple formal charges on any one atom.

19. Section 6.4 Molecular Orbital Picture of Resonance

20. Allyl Anion....  Energy levels nodes 0 1 2

21. Three atomic p orbitals combine to make a pi system of three molecular orbitals. nodes 0 1 2

22. .. + Allyl Cation nodes 0 1 2

23. RESONANCE IS FOUND IN CONJUGATED SYSTEMS Sect. 6.5 Conjugated Systems A conjugated system has alternating double and single bonds.

24. These systems are not conjugated. Unconjugated Systems Unconjugated Systems two single bonds three single bonds Unconjugated

25. 1,3-Butadiene

26. Resonance in Butadiene Resonance provides single bond character between carbons 1 and 2 and also between carbons 3 and 4. These bonds become LONGER 12 34

27. Resonance in Butadiene Resonance provides double bond character between carbons 2 and 3. This bond become SHORTER 123 4

28. Bond Lengths in Butadiene Full double bond 1.330 Full single bond 1.370

29. 3 Sect. 6.6 M.O. Description of Sect. 6.6 M.O. Description of Butadiene Butadiene.... 2 1 0 nodes Four atomic p-orbitals = Four molecular orbitals There are four electrons!

30. four p orbitals combine to make a system of four pi molecular orbitals ( a  system )  node  node  nodes 3 nodes Pictures of Molecular orbitals!

31. Section 6.7 Bond Order Double bonds = 2 Single bonds = 1 Resonance = bond orders between 1 and 2

32. Sect. 6.8-6.13 Benzene Kekule Structures

33. Molecular Orbital Picture of Benzene

34. Friedrich Auguste Kekulé Source: Michigan State University, Department of Chemistry http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml

35. Benzene Pi Molecular Orbitals Benzene Pi Molecular Orbitals six p’s six mo’s top view 0 node 1 node 2 node 3 node

36. Resonance Energy

37. Resonance in other Systems

38. Cyclopentadienyl Anion Cyclopentadienyl Anion 0 node 1 node 2 node

39. HH H H.... H.. Orbitals in Cyclopentadienyl Anion Orbitals in Cyclopentadienyl Anion

40. Cyclopentadienyl Anion Cyclopentadienyl Anion