Chapter 14: Conjugated Dienes

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1 Chapter 14: Conjugated Dienes Coverage: 1. Conjugated vs Nonconjugated dienes and Stability 2. MO picture of 1,3-butadiene 3. Electrophilic addition to Dienes 4. Kinetic vs Thermodynamic Control 5. Diels-Alder Reaction Problems: 20,21,24,25,26,31,32,34,35 Goals: 1. Know relative stabilites of conjugated vs nonconjugated dienes 2. Be able to draw an orbital picture showing pi system (overlapping p orbitals) for 1,3- butadiene. 3. Know the difference between the s-cis and s-trans conformation. Know which conformation is preferred and why. 4. Be able to draw a energy diagram for the MO system of 1-3-butadiene. 5. Understand the definitions of kinetic and thermodynamic products as they apply to the electrophilic addition products of 1,3-butadiene and related compounds. 6. Be able to predict the products of Diels-Alder reaction. Know the mechanism of this reaction. Also be able to predict what reactants are required to synthesise a Diels-Alder adduct.

2 Dienes two double bonds a. Nonconjugated diene double bonds are separated by at least two C-C single bonds. 1,4-pentadiene b. Conjugated diene double bond separated by only one C-C single bond. 1,3-pentadiene c. Allenes - cumulated double bonds. C C C C 2 1,2-pentadiene

3 Stabilites of dienes measured by heats of hydrogenation, - 2, Pt -, kcal/mol , Pt , Pt , Pt 53.7 Are the values for dienes what you expect?

4 1,4-pentadiene contains two monosubstituted double bonds. Predicted: - = 2 x 30.0 = 60.0 kcal/mol Actual: - = 60.2 kcal/mol 1,3-pentadiene contains one monosubstituted double bond and one disubstituted double bond. Predicted: - = 1 x x 27.4 = 57.4 kcal/mol Actual: - = 53.7 kcal/mol Conclusion: 1,3-pentadiene is more stable than predicted. Why? Answer: Conjugation sp 2 sp 2 sp 2 sp 2 sp 3

5 Why? Conjugation the two double bonds form a continuous overlap of the p orbitals. This results in delocalization of the pi electrons and extra stability sp 2 sp 2 sp 2 sp 2 sp 3 Conformations of 1,3-butadiene 2 C C C C ,3-butadiene exists in two conformations that are in equilibrium 180 o s-trans s-cis

6 s - single bond s-cis - double bonds are cis with respect to single bond s-trans double bonds are trans with respect to single bond All atoms (including C and ) lie in the same plane for these conformations. Any nonplanar conformations results in disruption of the continuous overlap of p orbitals and raises the energy. Bond Rotation of Butadiene S-Trans S-Cis S-Trans kcal/mol kcal/mol 5.35 kcal/mol S-Cis kcal/mol 2.80 kcal/mol 3.46 kcal/mol

7 6 Energy, kcal/mol S-Trans Ea S-Cis Dihedral Angle

8 Space-Filling Model of S-Cis Conformation Steric Repulsion of ydrogen Atoms Steric repulsion makes the s-cis conformation adopt a slightly nonplanar conformation.

9 1,2- and 1,4-addition reactions to 1,3-butadiene Br 1,2-product 2 C C C C 2 2 C C C C 2 Br 2 C C C C 2 Br 1,4-product Mechanism: Markovnikov Addtion of Br (see Alkenes) Br 2 C C C C 2 2 C C + C C C C C C 2 Allylic carbocation resonance stabilized Br-

10 2 C C C C 2 Br 2 C C C C 2 Br -80 C 80% 20% +40 C 15% 85% The product distribution is temperature dependent. Low Temperature: The product that forms the fastest will be the major product. This product is termed the kinetic product. Thus, 1,2-product is the kinetic product and forms faster than than the 1,4-product. igh Temperature: The product that is more stable will be the major product. This product is termed the thermodynamic product. The 1,4-product is the thermodynamic product and is more stable.

11 Further explanation: At low temperature, the reaction is not reversible. The 1,2-product forms faster because attack by Br- at the 20 carbon, which bears a larger positive charge, has a lower energy of activation, Ea. So the 1,2-product builds up and it does not revert back to reactant. At high temperature, the reaction is reversible. Although the 1,2-product forms faster, once it forms, it reverts back to reactant, which then reacts to form 1,4 product, which is more stable. The 1,2- and 1,4-product are in equilibrium at high temperature, with the 1,4-product predominating.

12 Molecular Orbital Picture of 1,3-butadiene Nodes π 4 * Antibonding 3 E π 3 * Antibonding LUMO 2 π 2 Bonding OMO 1 π 1 Bonding 0

13 UV-Visible Spectroscopy UV Visible nm nm Electronic transitions of π or nonbonding electrons Follows Beer s Law A = ε c l A = absorbance ε = molar absorptivity of molecule c = concentration l = path length of light Absorption characterized by λ max wavelength of maximum absorption λ max increases with 1. Conjugation - 30 nm per double bond 2. Alkyl substitution - 5 nm per alkyl group

14 π 4 * Antibonding E π 3 * Antibonding UV 217 nm π 2 Bonding π 1 Bonding Ground State Excited State

15

16 495 nm 603 nm 483 nm

17 Why are my jeans so blue? Oxidized form of Indigo responsible for blue color Reduced water-soluble yellow form of Indigo applied to jeans

18 Phenolphthalein Acidic and Basic Forms Phenolphthalein is an p indicator dye used in titrations. It is also an ingredient in Ex-lax! When the solution is acidic, the molecule is colorless. Under basic conditions, it turns red. Increased conjugation is responsible for the red color. Acid Form less conjugation, absorbs in UV and colorless sp 3 O - Basic Form more flat and conjugated, absorbs in visible region and red sp 2 3-D structures

19 Otto Diels Diels-Alder Reaction Method for synthesis of 6-membered ring One-step, conerted reaction Termed [4+2] cycloaddition reaction where 4π and 2π electrons react. Kurt Alder + π σ σ Diene electron-rich nucleophile electron-donating groups make it more reactive. 3 C 3 C MeO

20 Dienophile electron poor electrophile electron-withdrawing groups make it more reactive Reaction: CN acrylonitrile O O O maleic anhydride C C MeO + CN MeO CN O + C C O

21 Mechanism of Diels-Alder: one-step, simultaneous (concerted) bond-making and bond-breaking involving 6 π electrons Diels Alder Movie

22 Stereochemical Requirements of Diels-Alder Reaction 1. Diene must be in the s-cis conformation in order to react. + s-trans unreactive s-cis Explain the following reactivities of dienes. Very Unreactive Reactive! 100% s-cis Cannot adopt s-cis

23 2. Syn Stereochemistry due to its concerted nature, the reaction is syn with respect to both the diene and dienophile. A T T A + A A T T A A T T A T T A T = toward you in product A = away from you in product + cis cis 3 C + trans trans Conclusion: The reaction stereospecific with respect to the dienophile. cis cis trans trans

24 What about the diene? + trans, trans cis + cis, trans trans severe steric crowding s-cis 3 C s-trans - much more stable cis, cis Unreactive!!!! Why?

25 Conclusion: Diels-Alder is stereospecific with respect to the diene. trans, trans cis product trans, cis trans product cis, trans trans product cis, cis unreactive 3. Endo Rule when substituted bicyclic structures form,the endo product is favored over the exo product. + CO OC CO Endo favored Exo

26 Explanation of Endo Rule: There are two possible approaches between the diene and dienophile in this reaction. OMO diene LUMO dienophile O O Addition interaction lowers E a reacts faster OC Endo - major CO Exo - minor

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