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Understanding Alkyl Halides Substitution and Elimination Reactions: SN2 vs. SN1/E1 Mechanisms

Explore the mechanisms, stereochemistry, and reactivity of SN2 and SN1/E1 reactions of alkyl halides, with a focus on transition states, leaving groups, and carbocation stability. Learn about nucleophiles, solvents, and substrate reactivity in this comprehensive guide.

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Understanding Alkyl Halides Substitution and Elimination Reactions: SN2 vs. SN1/E1 Mechanisms

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  1. Substitution and Elimination Reactions of Alkyl Halides

  2. Substitution, Nucleophilic, Bimolecular – SN2

  3. Reaction Profile for SN2 Reaction (Wade)

  4. Stereochemistry of SN2 ReactionInversion of Configuration

  5. Proof of Inversion of Configuration at a Chiral Center

  6. Acetate Approaches from 180o 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. 1o > 2o >> 3oBulkiness 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. SN2 and E2

  17. Bimolecular Elimination - E2Nucleophile acts as Bronsted Base

  18. SN2 Competes with E2

  19. SN2 Competes with E2

  20. SN2 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. Hyperconjugationp bond associates with adjacent C-H s 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 – SN1 and E1

  36. SN1 mechanism(Wade)1st step is rate determining

  37. Reaction Profiles (Wade)SN1 SN2

  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 productExo- transition state looks like reactant

  40. SN1 Transition State

  41. SN1 Solvent Effects

  42. Partial Racemization in SN1

  43. Carbocation Stabilitymore highly substituted, lower energy

  44. Carbocation Stability

  45. Carbocations can Rearrange1,2-Hydride Shift

  46. Carbocations can Rearrange1,2-Methide Shift

  47. Hydride shift

  48. Ring Expansion

  49. Rings Contract, too

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