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Substitution and Elimination Reactions of Alkyl Halides.

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Presentation on theme: "Substitution and Elimination Reactions of Alkyl Halides."— Presentation transcript:

1 Substitution and Elimination Reactions of Alkyl Halides

2 Substitution, Nucleophilic, Bimolecular – S N 2

3 Reaction Profile for S N 2 Reaction (Wade)

4 Stereochemistry of S N 2 Reaction Inversion of Configuration

5 Proof of Inversion of Configuration at a Chiral Center

6 Acetate Approaches from 180 o Behind Leaving Group

7 Inversion on a Ring is often more Obvious: Cis Trans

8 Substrate Reactivity Since the energy of the transition state is significant in determining the rate of the reaction, a primary substrate will react more rapidly than secondary (which is much more rapid than tertiary).

9 1 o > 2 o >> 3 o Bulkiness of Substrate

10 Polar, Aprotic Solvents

11 Nucleophilicity

12 Iodide vs. Fluoride as Nucleophiles

13 Nucleophiles (preferably non-basic)

14 Good Leaving Groups are Weak Bases

15 Common Leaving Groups

16 S N 2 and E2

17 Bimolecular Elimination - E2 Nucleophile acts as Bronsted Base

18 S N 2 Competes with E2



21 Stereochemistry of E2

22 Anti-Coplanar Conformation

23 3(R),4(R) 3-Bromo-3,4- dimethylhexane

24 H and Br Anti-coplanar orientation

25 In a Cyclohexane, Leaving Group must be Axial

26 Zaitsev’s Rule

27 More Stable Alkene Predominates

28 Hyperconjugation  bond associates with adjacent C-H  bond

29 With Bulky Base, Hofmann Product Forms

30 Which will react more rapidly?

31 Reactive Conformations

32 E2 Reaction of (R,R) 2-iodo-3-methylpentane

33 Stereochemistry is Important

34 E2 Reaction of a Vicinal Dibromide using Zn dust or Iodide

35 Unimolecular Substitution and Elimination – S N 1 and E1

36 S N 1 mechanism (Wade) 1 st step is rate determining

37 Reaction Profiles (Wade) S N 1 S N 2

38 Hammond’s Postulate Related species that are close in energy are close in structure. In an endothermic reaction, the transition state is similar to the product in structure and stability. In an exothermic reaction, the transition state is similar to the reactant in structure and stability. i.e. the structure of the transition state resembles the structure of the most stable species.

39 Endo- transition state looks like product Exo- transition state looks like reactant

40 S N 1 Transition State

41 S N 1 Solvent Effects

42 Partial Racemization in S N 1

43 Carbocation Stability more highly substituted, lower energy


45 Carbocation Stability

46 Carbocations can Rearrange 1,2-Hydride Shift

47 Carbocations can Rearrange 1,2-Methide Shift

48 Hydride shift

49 Ring Expansion

50 Rings Contract, too

51 E1 Mechanism

52 E1 and S N 1 Compete

53 Synthetic Chemist’s Nightmare

54 Ring Expansion to a More Stable 6-membered Ring

55 Dehydration of Alcohols – E1

56 Methide Shift is Faster than Loss of H +

57 Provide a Mechanism



60 Give the Major Product & Predict the Mechanism


















78 Which Reacts More Rapidly in E2 Reaction?

79 Cis Reacts more Rapidly

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