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

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

3. Bonding Energy: Extra bonds between
Types of Dienes
Conjugated:
Requirements: Continuous _____ systems with adjacent ___ orbitals overlapping.
Bonding Energy: Extra bonds between
Reactivity: Reactivity differs depending on specific diene and other chemicals involved.
Continuous, overlapping p-orbitals. 6

4. _________ stable than conjugated.
Types of Dienes
Isolated:
_________ stable than conjugated.
Requirements: ____ systems separate and are isolated by an ________ center.
Bonding Energy: _______ bonding.
Reactivity: __________simple alkenes.
sp3 center
Alkene p-orbital overlap.
Alkene p-orbital overlap. 6

5. Reactivity: Same as simple alkynes.
Types of Dienes C
Cumulated:
_________stable.
Requirements: Double bonds _____________hybridization of middle carbon.
Bonding Energy:
Reactivity: Same as simple alkynes. 6

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

7. Dienes Name Line Diagram π system Type Resonance 6 1,3-cyclopentadiene
1,3-cyclohexadiene
1,4-cyclohexadiene
Benzene 6

8. Bonding in Allene
sp 2 sp
sp 2 19

9. The Double Bond as a Substituent
carbocation C +
radical • C
diene C 2

10. Allylic Carbocations Stability
The fact that a tertiary allylic halide undergoes solvolysis (SN1) _____ faster than a simple tertiary alkyl halide… Cl
CH3 C
H2C CH
CH3 C
CH3 Cl
CH3
relative rates: (ethanolysis, 45°C) 6

11. Allylic Carbocations Stability
Provides good evidence that allylic carbocations
are __________________________________.
CH3
H2C CH +
CH3 C
CH3 C +
CH3
stabilizes C+ better than does 6

12. More resonance leads to __________ stability:
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: 6

13. Stabilization of Allylic Carbocations
Delocalization of electrons in the double bond stabilizes the carbocation. 9

14. Resonance Model C
CH3
H2C CH
CH3
H2C CH + C C
CH3
H2C CH 10

15. Allylic Free Radicals are Stabilized by Electron Delocalization• C 22

16. Vinylic versus Allylic4

17. Vinylic versus AllylicH C C H C H 4

18. Vinylic versus Allylic
Allylic hydrogens are attached to allylic carbons. 4

19. Vinylic versus Allylic
Vinylic substituents are attached to vinylic carbons. 4

20. Vinylic versus Allylic
Allylic substituents are attached to allylic carbons. 4

21. Allylic Carbocations
Resonance
Molecular Orbitals
Resonance Hybrid 6

22. Allylic Radicals
Resonance
Molecular Orbitals
Resonance Hybrid 6

23. Allylic 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. 6

24. Allylic 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. 6

25. + H + H H H Radical Bond Energies
A comparison of bond energies associated with radicals and allylic radicals: K J / m o l H + H H K J / m o l + H 6

26. Chlorination of Propene
ClCH2CHCH3 Cl
CHCH3
H2C +
Cl2
CHCH2Cl
H2C
500 °C
+ HCl 25

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 26

28. B r B r Mechanism, Step 1 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. B r B r 6

29. The first is the radical abstraction of H by Br
Mechanism, Step 2
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. Br C H H Br C Br C C Br 6

30. Mechanism, Step 3 Step 3 (Termination): Br Br Br Br
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 Br Br C Br C Br C C 6

31. Reagent used (instead of Br2) for _______ bromination.
N-Bromosuccinimide
Reagent used (instead of Br2) for _______ bromination.
CCl4 +
heat O NH O
NBr 29

32. Allylic halogenation is only used when:
Limited Scope
Allylic halogenation is only used when:
all of the allylic hydrogens are ____________
and
the resonance forms of allylic radical are ________________. 30

33. Cyclohexene satisfies both requirements.
Example H
Cyclohexene satisfies both requirements. H • H H • H 31

34. Example
2-Butene
CH3CH
CHCH3
But •
CH3CH CH
CH2 31

35. All allylic hydrogens are equivalent. 2-Butene CH3CH CHCH3
Example
All allylic hydrogens are equivalent.
2-Butene
CH3CH
CHCH3
forms Br Br
CH3CH CH
CH2
and
CH3CH CH
CH2
Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. 31

36. Kinetic vs. Thermodynamic Control
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. 6

37. Kinetic vs. Thermodynamic Control
Pathway 1 vs. Pathway 2
Reaction 1 (solid) generates _______.
Transition State 1 (TS1) has a ______ activation barrier (ΔHact)
Product 1 (P1) is the ____________
Energy SM
Reaction 2 (dash) generates ______.
P2 is the ________ stable product.
P2 has ________ energy than P1
P2 is the _______________ product.
Reaction Coordinate 6

38. Control and Temperature
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: 6

39. Control and Temperature
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 6

40. Control and Temperature
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: 6

41. Preparation of Conjugated Dienes
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. OH Br
KOH
heat
KHSO4
heat 6

42. Dienes undergo electrophilic addition reactions similar to alkenes:
Reactions of Dienes
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 _______________. Nu E 6

43. Three types of electrophilic addition of dienes: Reaction with H-X:
Reactions of Dienes
Three types of electrophilic addition of dienes:
Reaction with H-X:
Reaction with X2: + H X + + X 2 + 6

44. Reaction with other C=C (Diels Alder):
Reactions of Dienes
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): 6

45. Introduction to 1,2 and 1,4 Addition+ H X H
Proton adds to ________ of diene system.
Carbocation formed is __________. 8

46. Example: H
HCl H ? H ? 9

47. Protonation of the end of the diene unit gives an ________________.
via: H H H X H
Protonation of the end of the diene unit gives an ________________. 10

48. and: H H H 10

49. 1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Via resonance 12

50. Addition of Hydrogen Halides to Dienes
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 H B r - 8 C + o 2 C + 6

51. Addition of Hydrogen Halides to Dienes
Conditions
Low Temp
Room Temp
Control
Reversibility
Determination
Control
Structure 6

52. Overall Reaction: Diene + Dienophile (alkene) 
Diels-Alder Reaction
Reaction Type:
Overall Reaction: Diene + Dienophile (alkene) 
Stereoselectivity: Syn and Endo or Exo
Requirements: Diene + Dienophile, high temp or EDG on diene/EWG on dienophile. 6

53. Mechanism ____________ process:
Aromatic like transition state.
____________ process:
This makes the reaction very __________ and ____________selective.
Thermodynamically favorable: 2

54. Diels-Alder Reaction Simple Diels Alder Examples:
1,3-butadiene + ethene 
1,3-butadiene + ethyne  6

55. Diels-Alder Reactivity
The most reactive dienes have an electron-___________ group (E__G) directly attached to nucleophilic diene.
Typical E___Gs
E___G 5

56. Effect of Electron Donor/Acceptors
A molecular orbital look at the effect of electron donor/acceptors B e t r D o n G u p s B e t r A c p o G u s L U M O O r b i t a l e n e r g y H O M O D i e n e D i e n e o p h i l e 5

57. Example CH O
H2C
CHCH
CH2 +
H2C CH
solvent
100°C 6

58. Example O
CHC
CH2
H2C
CH3 +
solvent
100°C 6

59. Example Diels-Alder Questions
1. Rank the relative reactivity towards 1,3-cyclopentadiene of the following: i ii
iii 5

60. Example Diels-Alder Questions
2. Rank the relative reactivity towards dimethyl cis-butendioate of the following: i ii
iii 5

61. Example Diels-Alder Questions
3. Rank the order of the relative reactivity towards 3-buten-2-one of the following 5

62. Common Diels-Alder Reactants
Common Dienes:
Common Dienophiles: 5

63. Reactions with Cyclic Dienes
Two different conformations are possible:
Endo: Dienophile is ‘_________’ diene.
___________________ product.
Exo: Dienophile is ________.
__________________ product.
______ conformations are generally the major product with _______ being a minor product. O O + O O + O 5

64. Reactions with Cyclic Dienes
____________________________favors the endo transition state. C H O R C H O R H O R 5

65. Diels-Alder Reaction is Stereospecific*
*A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other; characterized by terms like syn addition, anti elimination, inversion of configuration, etc.
Diels-Alder: Both the diene and the dienophile are _______
Cis-dienophile: __________ substituted product.
Trans-dienophile: __________ substituted product.
Both diene and alkene are Z (or E) both on the _________side of the product.
Dienes and alkene are E and Z  Are on ________ side of the product. 9

66. Example C +
H2C
CHCH
CH2 10

67. Example C H +
H2C
CHCH
CH2 10

68. Diels-Alder Reaction is Stereospecific Examples2 C O M e 2 C O M e + 2 C O M e 2 C O M e 2 C O M e + 2 C O M e 2 C O M e 2 C O M e + 2 C O M e 2 C O M e 2 C O M e + 2 C O M e 9

69. Diels-Alder Reaction is Stereospecific Examples
Predict the reactants:
Product has the two ___________groups ___________
– Dienophile has to be _______________ 9

70. Regiochemistry
Determined by the position of the electron donating group (EDG) on the diene.
Common EDG groups include ethers, amines, sulfides
(Using the the nonbonding electron pair). CH 3 O CH 3 O CH 3 O H O H O H O + 5

71. Regiochemistry C H O + H O
Determined by the position of the electron donating group (EDG) on the diene.
Common EDG groups include ethers, amines, sulfides (the nonbonding electron pair). C H O 3 + H O 5

72. Example Problems
What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene? H 2 N H 2 N O O O 5

73. Example Problems
What product might you expect when 2-amino-1,3-butadiene reacts with 3-oxo-1-butene? O + H N 2 5