Conjugation in Alkadienes and Allylic Systems

1. Conjugation in Alkadienes andAllylic Systems • conjugare is a Latin verb meaning "to link or yoke together"

2. Isolated – p system on a single pair of adjacent atoms. Extended – p system on a longer series of atoms. This gives extended chemical reactivity. Classification of Allylic Systems

3. Conjugated: Requirements: Continuous _____ systems with adjacent ___ orbitals overlapping. Bonding Energy: Extra bonds between Reactivity: Reactivity differs depending on specific diene and other chemicals involved. Types of Dienes Continuous, overlapping p-orbitals.

4. Isolated: _________ stable than conjugated. Requirements: ____ systems separate and are isolated by an ________ center. Bonding Energy: _______ bonding. Reactivity: __________simple alkenes. Types of Dienes sp3 center Alkene p-orbital overlap. Alkene p-orbital overlap.

5. Cumulated: _________stable. Requirements: Double bonds _____________hybridization of middle carbon. Bonding Energy: Reactivity: Same as simple alkynes. Types of Dienes C C C

6. Name Line Diagram π system Type Resonance Propene 1,2-propadiene 1,3-butadiene 1,4-pentadiene Dienes

7. Name Line Diagram π system Type Resonance 1,3-cyclopentadiene 1,3-cyclohexadiene 1,4-cyclohexadiene Benzene Dienes

8. Bonding in Allene sp sp 2 sp 2

9. C C C C C C + • carbocation radical C C C C diene The Double Bond as a Substituent

10. CH3 Cl H2C C C CH CH3 Allylic Carbocations Stability • The fact that a tertiary allylic halide undergoessolvolysis (SN1) _____ faster than a simple tertiaryalkyl halide… CH3 Cl CH3 CH3 relative rates: (ethanolysis, 45°C)

11. CH3 + H2C CH CH3 Allylic Carbocations Stability • Provides good evidence that allylic carbocations • are __________________________________. CH3 + C C CH3 CH3 stabilizes C+ better than does

12. Must have π systems – __________change positions in resonance contributors shown by ______________. Molecular structure is composite of all the resonance contributors, with the most favorable contributing the most character. More resonance leads to __________ stability: Resonance

13. Stabilization of Allylic Carbocations • Delocalization of electrons in the doublebond stabilizes the carbocation.

14. CH3 + C H2C CH CH3 CH3 CH3 C C H2C H2C CH CH CH3 CH3 Resonance Model

15. Allylic Free Radicals are Stabilized byElectron Delocalization • C C C C C C

16. Vinylic versus Allylic C C C

17. Vinylic versus Allylic H C C H C H

18. C C C Vinylic versus Allylic Allylic hydrogens are attached to allylic carbons.

19. Vinylic versus Allylic C C C Vinylic substituents are attached to vinylic carbons.

20. Vinylic versus Allylic C C C Allylic substituents are attached to allylic carbons.

21. Resonance Molecular Orbitals Resonance Hybrid Allylic Carbocations

22. Resonance Molecular Orbitals Resonance Hybrid Allylic Radicals

23. Allylic Carbocations/Radicals Carbocations Radicals Stabilization Double bonds ______ electron density. Position On __________l C’s, never on a ______ C. On _______l C’s, never on a ______ C. Delocalized ________ of charge is stabilizing. _______ radical is stabilizing. Reaction Site Either ____________ by nucleophiles Either _____________ by a radical.

24. Allylic Carbocations/Radicals Carbocations Radicals Intermediates _________ energy then alkyl Carbocation intermediates. _________ energy than alkyl Radical intermediates. Stabilization One π= _____ R groups ~ _____-propyl cation One π= ____ R groups ~ ____-propyl radical Bond Dissociation Energies Allylic bonds are often ________ and are _______ broken.

25. A comparison of bond energies associated with radicals and allylic radicals: Radical Bond Energies K J / m o l + H K J / m o l + H H H

26. ClCH2CHCH3 Cl CHCH3 H2C CHCH2Cl H2C Chlorination of Propene + Cl2 500 °C + HCl

27. Allylic Halogenation • Reaction Type: • Overall Reaction: Alkene  • Reactivity Order: • Regioselectivity: Substitution at the ______position due to the stability of the ______ radical (resonance). • Stereoselectivity: • Requirements: Br2 or Cl2 with ________, or • N-bromosuccinimide (NBS) which can act as a source of Br2

28. Step 1 (Initiation): First step in radical halogenation of an allylic system is to perform homolytic cleavage of a diatomic halogen by heat or UV light. Mechanism, Step 1 B r B r

29. Step 2 (Propagation): Step 2 has two steps. The first is the radical abstraction of H by Br The second step adds Br to the radical and creates another Br radical. Mechanism, Step 2 H H Br C Br C Br C Br Br Br C

30. Step 3 (Termination): Step 3 has three steps which ends the radical reaction. Three different products are made. The first product forms Br2 again. The second product forms the expected allyl bromide. The third product is a byproduct of the two radical carbons linking together Mechanism, Step 3 Br Br Br Br Br C Br C C C C C

31. O O heat + + NBr NH CCl4 O O N-Bromosuccinimide • Reagent used (instead of Br2) for _______ bromination.

32. Limited Scope Allylic halogenation is only used when: • all of the allylic hydrogens are ____________ • and • the resonance forms of allylic radicalare ________________.

33. H H H H H H • H Example Cyclohexene satisfies both requirements. H H • H

34. CH3CH CHCH3 But • • CH3CH CH CH2 CH3CH CH CH2 Example 2-Butene

35. CH3CH CHCH3 Example All allylichydrogens areequivalent. 2-Butene forms Br Br CH3CH CH CH2 and CH3CH CH CH2 Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.

36. Thermodynamic Factors: Corresponds to the relative ____________of the products. Kinetic Factors: Is the ______ at which the product is formed. It is possible to start off with the same material and receive two different products via different pathways. Kinetic vs. Thermodynamic Control

37. Pathway 1 vs. Pathway 2 Kinetic vs. Thermodynamic Control SM Reaction 1 (solid) generates _______. Transition State 1 (TS1) has a ______ activation barrier (ΔHact) Product 1 (P1) is the ____________ Energy Reaction 2 (dash) generates ______. P2 is the ________ stable product. P2 has ________ energy than P1 P2 is the _______________ product. Reaction Coordinate

38. Increase in temperature: Average energy of the molecules increases. Low Temperatures: Preferred Path: Path similar to ______ (on previous slide.) Reaction 1: Reaction 2: Product Ratio: Is determined by the Control: Control and Temperature

39. Intermediate Temperatures: Preferred Path: Path similar to __________ Reaction 1: Reaction 2: Product Ratio: Dependent on ______________ (a ________ of reaction results in more product ____ ______ forms initially then over time goes back to starting material, then forms the ____________. Major product: Depends on time of reaction Short (time): Long (time): Control: Variable Control and Temperature

40. High Temperatures: Preferred Path: ___________ is preferred, but then goes through ___________. Reaction 1: Reaction 2: Product Ratio: Dependent on _______________ between P1 and P2 Major product: Depends on time of reaction, but end result is more _______ Short (time): Long (time): Control: Control and Temperature

41. Dienes can be prepared by elimination reactions of unsaturated alkyl halides and alcohols. Elimination favors the most stable product. Conjugated dienes major product are more stable than isolated dienes unless structure doesn’t allow. Preparation of Conjugated Dienes Br KOH KHSO4 heat heat OH

42. Dienes undergo electrophilic addition reactions similar to alkenes: Isolated dienes: Double bonds react ___________ one another, and therefore react like ___________. Cumulated dienes: React more ___________ Conjugated dienes: Conjugated C=C changes the reactivity. Dienes act as ____________, reacting with _______________. Reactions of Dienes Nu E

43. Three types of electrophilic addition of dienes: Reaction with H-X: Reaction with X2: H X + + + X + 2 Reactions of Dienes

44. Note the numbering scheme from the previous slide. The 1,2 and 1,4 addition will be discussed in detail in upcoming notes. Third Reaction type: Reaction with other C=C (Diels Alder): Reactions of Dienes

45. Proton adds to ________ of diene system. Carbocation formed is __________. Introduction to 1,2 and 1,4 Addition + H X H

46. Example: H H H H H H HCl H H H H ? ? H H H H H H H H

47. H H H H H H H H H H H H H H H H H H via: H X Protonation of the end ofthe diene unit gives an________________.

48. H H H H H H H H H H H H H H H H H H H H H H H H and:

49. 1,2-Addition versus 1,4-Addition 1,4-addition of XY Via resonance 1,2-addition of XY

50. Two types of addition: Direct: H-X adds directly across the ends of a C=C (1,2-addition) Conjugate: H-X adds across the ends of a conjugated system (1,4-addition). Distribution of product depends on conditions: o - 8 0 C + r H B 2 0 C o + Addition of Hydrogen Halides to Dienes