1. Lecture from Monday Ch14C, 4/9/12 Topics covered: -Functional groups -ChemDraw, Chem3D (computer modeling) -Hybridization picture of H 2 C=C=CH 2 (allene) -Drawing resonance structures (Klein handout) -Assessing “good” vs. “bad” resonance structures -Introduction to Conjugation (Part 1 of 2)

5. Made using ChemDraw and Chem3D Programs available on computers in UCLA Science Learning Center (Young 4335 and others) Intro tutorial to how to use ChemDraw: http://www.chem.umass.edu/~samal/267/chemdraw.pdfhttp://www.chem.umass.edu/~samal/267/chemdraw.pdf

6. Vollhardt, Figure 14-4 (p. 618) So, the hydrogens of allene are NOT in the same plane, but are instead in two  planes. This is not something we might have been able to predict just from looking at the Lewis structure IUPAC name: propanediene Common name: allene

7. Which Resonance Contributor Represents Reality? The carbon monoxide case: Neither contributor fully represents CO Resonance hybrid : A weighted average or blend of resonance contributors; the most accurate representation of the electronic structure of a molecule. PeachPlum “True” structure: Nectarine Your friend asks you to describe what a nectarine is because he’s never seen or eaten one… (Klein, Sec 2.1)

8. X Which Resonance Contributor Represents Reality? Once upon a study break... Real creatureFantasy creatures Neither fully represents reality A unicorn-dragon hybrid? The carbon monoxide case: Neither contributor fully represents CO Resonance hybrid : A weighted average or blend of resonance contributors; the most accurate representation of the electronic structure of a molecule. X

9. Drawing the Resonance Hybrid Example: Draw the resonance hybrid for acetate ion, CH 3 CO 2 -. 2. Draw the features that are the same for all contributors Sigma and pi bonds, lone pairs, and formal charges 3. Add features that are not the same for all contributors 1. Draw contributors Partial (shared) pi bonds shown as ---- Partial (shared) charges shown as  + or  -    Resonance hybrid

10. Do All Contributors Have Equal Importance? Is a rhinoceros more unicorn or more dragon?  contributor “stability” =  resemblance to reality =  contribution to hybrid Therefore we need contributor preference (“stability”) rules: As the number and/or magnitude of rules violations , =  importance of individual contributor =  contribution to resonance hybrid

11. Resonance Contributor Preference Rules Rule #1: The most important contributor has the maximum number of atoms with full valence shells. Rule #1 is more influential than all the other preference rules. Rules #2-6 have no particular order of preference. Open valence shell on carbon Less important contributor Example: In some cases it may not be possible for all atoms to have full valence shells. For practice: see Klein Sec 2.8

12. Resonance Contributor Preference Rules Rule #2: The most significant contributor has the maximum number of covalent bonds. Three covalent bonds Less important contributor Example:

13. Resonance Contributor Preference Rules Rule #3: The most significant contributor has the least number of formal charges. No formal charges More important contributor Two formal charges Less important contributor Example:

14. Resonance Contributor Preference Rules Rule #4: If a contributor must have formal charge(s), the most important contributors has these charges on the atom(s) that can best accommodate them. Carbon EN = 2.5 Less important contributor Oxygen EN = 3.5 More important contributor Negative formal charges best on atoms of high electronegativity O - better than C - Positive formal charges best on atoms of low electronegativity C + better than O + Minimize formal charge magnitude +1 better than +2

15. Resonance Contributor Preference Rules Rule #5: Resonance interaction (i.e., pi bond) is strongest between atoms in the same row of the periodic table. Usually CNOF Usually outweighs electronegativity considerations (rule #4) F, C both 2nd row More important contributor Even though EN F > EN Cl C 2nd row; Cl 3rd row Less important contributor Example:

16. Resonance Contributor Preference Rules Rule #6: Other factors (such as aromaticity) that we will encounter later. Violations to the resonance contributor preference rules exist, but are uncommon.

19. Conjugated Molecules - Part 1 Lecture Supplement page 28

20. What is Conjugation? For any molecule best structure = lowest energy Lowest energy from minimizing electron repulsion and maximizing p orbital overlap Maximum p orbital overlap allows... Square planar or tetrahedral? Staggered or eclipsed? Methane What are these? } Resonance Conjugation Aromaticity Familiar examples: Which structure is lowest energy? Ethane

21. What is Conjugation? Case #1: Relative stability of C 4 H 6 isomers Restriction:  H o f comparisons only valid among isomers 4 C ( graphite ) + 3 H 2 ( g )  H o f = 29.9 kcal mol -1 4 C ( graphite ) + 3 H 2 ( g )  H o f = 47.7 kcal mol -1 More stable isomer Isomers Isomers : Same molecular formula, different structure Importance: Lower  H o f = more stable isomer Heat of formation (enthalpy of formation;  H o f ): Hypothetical enthalpy change when a substance is synthesized from elements in their standard states

22. What is Conjugation? Why this order? Ring strain? No ring strain Position of pi bonds? Two pi bonds One pi bond Ring strain Two pi bonds No ring strain Number of pi bonds? Case #1: Relative stability of C 4 H 6 isomers Alternating pi-sigma-pi bonds

23. What is Conjugation? Case #2: Catalytic Hydrogenation of 1,3-Dienes versus 1,4-Dienes Thermodynamics Lose: H-H sigma bond, C-C pi bond Gain: 2 x C-H sigma bond Sigma bonds usually stronger than pi bonds Therefore catalytic hydrogenation is exothermic (  H < 0)  H = -30 kcal mol -1 Example: Catalytic hydrogenation : Addition of H 2 to a pi bond with a catalyst

24. What is Conjugation? Case #2: Catalytic hydrogenation of 1,3-dienes versus 1,4-dienes  H = -65.1 kcal mol -1  H = -56.5 kcal mol -1 1,3-butadiene more stable than 2-butyne Same molecule = same enthalpy (H) + 2 H 2 Observation:  H (1,3-butadiene  butane) <  H (2-butyne  butane) by 8.6 kcal/mol Conclusion: Lowest  H (for catalytic hydrogenation) belongs to most stable isomer How can we use catalytic hydrogenation to probe C 4 H 6 isomer stability? Fact: 1,3-butadiene more stable than 2-butyne

25. What is Conjugation? Case #2: Catalytic hydrogenation of 1,3-dienes versus 1,4-dienes Use  H (cat H 2 ) to compare pi-sigma-pi (1,3-diene) versus pi-sigma-sigma-pi (1,4-diene)  H = -30 kcal mol -1 Predict:  H = 2 x (-30) = -60 kcal mol -1 Observe:  H = -60 kcal mol -1 Conclusion: No special stability for 1,4-diene 1,4-Pentadiene (a 1,4-diene) 1-Pentene (alkene energy benchmark) 1,3-Butadiene (a 1,3-diene) Predict:  H = -60 kcal mol -1 if stability = 1,4-diene Observe:  H = -56.5 kcal mol -1 3.5 kcal mol -1 less than expected Conclusion: 1,3-diene more stable than 1,4-diene. General observation: 1,3-dienes more stable than similar 1,4-dienes The experiment

26. Lecture from Wed, 4/11/12 -Conformations of 1,3-butadiene (s-cis vs. s-trans) -Discussion of how dihedral angle relates to extent of p-orbital overlap -Resonance picture of 1,3-butadiene correlates well to physical properties -Conjugated molecules and color

27. 1,3-Butadiene: A Closer Look What is origin of special 1,3-diene stability? Major conformation? Torsional strain: s -trans < s -cis Stability: s -trans > s -cis Planarity: Nearly always planar s -cis* 5% s -trans 95% 2.21 Å 2.50 Å K eq > 1 * s-cis = two pi bonds, separated by sigma bond, in cis arrangement

28. 1,3-Butadiene: A Closer Look Rotation around C sp 2 - C sp 2 bond: Pi bonds perpendicular Barrier to rotation Highest energy point Conclusion: More than just torsional strain is at work Angle between planes formed by three atoms each Energy difference between most and least stable conformations

29. 1,3-Butadiene: Resonance Model What is the origin of this extra stability, planarity, and barrier to rotation? Resonance hybrid  explains barrier to rotation  explains planarity Resonance hybrid observations Partial C2-C3 pi bond C2-C3 p z orbital overlap Resonance is often a strong influence on molecular structure, so start there

30. 1,3-Butadiene: Resonance Model The resonance model looks useful, but simplistic. Is it accurate? C2-C3 barrier to rotation (kcal mol -1 ) C2-C3 bond length (Å) 1,3-ButadieneModel molecules 1.54 1.33 4.5 ~60 Conclusion: 1.487.5 Resonance model is accurate despite its simplicity The resonance model predicts...

31. Molecule is more stable Molecule is less stable Pi electrons have longer wavelength No significant resonance Has some resonance 1,3-Butadiene: Resonance Model How does resonance explain why a 1,3-diene is more stable than a 1,4-diene? 1,3-diene1,4-diene Pi electrons have shorter wavelength Pi electrons confined between two carbons Pi electrons roam over four carbons E = h c/ so  wavelength =  energy

32. Discussion of Handout: “p orbital overlap: Resonance, Conjugation, and Aromaticity”

33. Conjugated Molecules - Part 2 Lecture Supplement page 37 chlorophyll a lycopene  -carotene

34. Part 1 Summary...more resonance...more electron delocalization...lower electron energy Consequences of p orbital overlap Atoms with p orbitals must be planar Partial pi bond(s) Barrier to rotation more stable than Example: Adjacent, overlapping p orbitals allows for... Greater stability

35. Extra Stability Limited to 1,3-Dienes? Amide resonance contributors: Pi electron delocalization provides increased stability Amide resonance hybrid: 1,3-diene has four adjacent p orbitals Three adjacent p orbitals is enough to provide extra stability Example: An amide

36. Extra Stability Limited to 1,3-Dienes? Another special stabilization example: An amide Predict: Four attachments = sp 3 sp 3 lacks p orbital needed for resonance Therefore sp 2 to accommodate resonance Therefore sp 2 to increase stability I thought hybridization is controlled only by the number of attachments?! Energy causes geometry; geometry causes hybridization Influenced by electron repulsion, resonance, etc. Resonance hybrid Nitrogen hybridization?

37. Extra Stability Limited to 1,3-Dienes? More adjacent p orbitals = larger electron “playground” An amide has three adjacent, parallel p orbitals: Compare with 1,3-butadiene: Four adjacent, parallel p orbitals: Build your own model p orbital overlap forms pi bonds In general: Adjacent, parallel p orbitals improve molecular stability Sigma bonds p orbital overlap gives delocalized pi bonds

38. Conjugation: Special stability provided by electron delocalization in three or more adjacent, parallel, overlapping p orbitals. Conjugation: A Definition Finally! Decrease in electron energy Not limited to pi-sigma-pi (four carbon p z orbitals)

39. Consequences of Conjugation Conjugation influences widespread Chemical reactivity: Is molecule reactive or inert? Molecular structure: Is a molecule (or portion of molecule) flat? Is bond rotation hindered? Physical properties: Color Etc.

40. Consequences of Conjugation Influences distribution of products in chemical reaction Reaction products:  stability =  amount produced Example: Determine major product of this reaction: Ten conjugated p orbitals Six conjugated p orbitals More stable Less stable Produced in greatest amount or Which product isomer is more stable? Consequence #1: More extensive conjugation = greater stability

41. Consequences of Conjugation Resistance to conformational change (barrier to rotation) More stable Less stable Barrier to rotation and planarity critical to protein function. Resonance hybrid Barrier to rotation Planar Less torsional strain More torsional strain Consequence #2: Partial pi bond character Example: Amide linkage between two amino acids in a protein Causes planarity of atoms conjugated p orbitals

42. Consequences of Conjugation Consequence #3: Highly conjugated molecules may be colored Examples: ChlorophyllLycopene  -Carotene Origin of color: Some portion of visible (white) light spectrum is absorbed Brain perceives remaining light as color So how does molecular structure control energy of photons absorbed?

44. Consequences of Conjugation How does molecular structure control energy of photons absorbed? Molecule absorbs photon ( h ) Electron is excited to higher energy molecular orbital Energy of photon absorbed must equal HOMO/LUMO orbital energy difference (  E) Controlled by electron energies

45. Consequences of Conjugation How does molecular structure control energy of photons absorbed?  number of conjugated p orbitals   E When  E low enough, photons of visible light absorbed Unabsorbed portion of visible light spectrum perceived as color Photon absorbedObserved color Ultraviolet (UV) Visible light Yellow Orange Red Violet Indigo Blue Green Colorless Violet Indigo Blue Green Yellow Orange Red

46. Consequences of Conjugation How does molecular structure control energy of photons absorbed? Example Molecules Four conjugated p orbitals  E = ultraviolet Perceived color = colorless 1,3-Butadiene Six conjugated p orbitals  E = ultraviolet Perceived color = colorless 1,3,5-Hexatriene Eight conjugated p orbitals  E = ultraviolet Perceived color = colorless Styrene Yellow Orange Red Violet Indigo Blue Green Violet Indigo Blue Green Yellow Orange Red

47. Consequences of Conjugation How does molecular structure control energy of photons absorbed? Example Molecules Twelve conjugated p orbitals Photons of h =  E are indigo Perceived color = orange Ten conjugated p orbitals Photons of h =  E are violet Perceived color = yellow Retinal Retinol (vitamin A) Yellow Orange Red Violet Indigo Blue Green Violet Indigo Blue Green Yellow Orange Red

48. Consequences of Conjugation How does molecular structure control energy of photons absorbed? Example Molecules 22 conjugated p orbitals Photons of h =  E are blue Perceived color = red Lycopene Not conjugated Yellow Orange Red Violet Indigo Blue Green Violet Indigo Blue Green Yellow Orange Red

49. Friday’s topic: Aromaticity (pre-read!!!)

51. 1,3-Butadiene: A Closer Look Rotation around the C2-C3 bond The s -cis conformation is planar Indicates movie to play Filename: s-cis change perspective.mov

52. 1,3-Butadiene: A Closer Look Rotation around the C2-C3 bond Perpendicular conformation has lowest torsional strain Filename: s-cis to perpendicular.mov

53. 1,3-Butadiene: A Closer Look Rotation around the C2-C3 bond The s -trans conformation is planar Filename: perpendicular to s-trans_prof.mov