Alkyl Halides - PowerPoint PPT Presentation

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alkyl halides n.
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Alkyl Halides

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  1. Alkyl Halides

  2. Name the following alkyl halides

  3. How are alkyl halides prepared? • From alkanes • Free radical halogenation • From alkenes • Hydrohalogenation • Halogenation • From alcohols

  4. Free Radical Chlorination


  5. Experimental Evidence Helps to Determine Mechanism • Chlorination does not occur at room temperature in the dark. • The most effective wavelength of light is blue that is strongly absorbed by Cl2 gas. • The light-initiated reaction has a high quantum yield (many molecules of product are formed from each photon of light).

  6. Free Radical Species are Constantly Generated Throughout the ReactionPropagation

  7. Termination: Reaction of any 2 Radicals

  8. Chlorination of Propane

  9. Chlorinationof Methylpropane

  10. 3o Radicals are Easiest to Form

  11. Stability of Free Radicals

  12. (1) Consider the free radical monochlorination of 2,2,5-trimethylhexane. Draw all of the unique products (Ignore stereoisomers). (2) Consider the free radical monochlorination of 1,4-dimethylcyclohexane. The reaction affords three different products (all C8H15Cl). Draw the three products (Ignore stereoisomers).

  13. Conversion of Alkenes into Alkyl Halides

  14. Conversion of Alcohols into Alkyl Halides Reactions with HX, SOCl2, PBr3

  15. Conversion of Alcohol into a Leaving Group • Form Tosylate (p-TsCl, pyridine) • Use strong acid (H3O+) • Convert to Alkyl Halide (HX, SOCl2, PBr3)

  16. Alcohols to Alkyl Halides

  17. What the **** is SN1? Unimolecular Nucleophilic Substitution

  18. Lucas Test

  19. Qualitative test for Alcohol Characterization

  20. 1o and 2o Alcohols: best to use SOCl2, PBr3,or P/I2

  21. Thionyl chloride mechanism

  22. How would you prepare the following RX from the appropriate alcohols?

  23. 7.25] Draw and name the monochlorination products of the reaction of 2-methylpentane with chlorine gas. Which are chiral? 7.36] How would you prepare the ff cpds, starting from cyclopentene and any other reagents needed? • Chlorocyclopentene • Cyclopentanol • Cyclopentylmagnesium chloride • cyclopentane

  24. Reactions of Alkyl Halides Grignard Reagents

  25. What’s special with Grignard reagents?

  26. Outline a synthetic scheme from the appropriate alkyl halide

  27. Reactions of Alkyl Halides Substitution and Elimination

  28. Substitution, Nucleophilic, Bimolecular – SN2

  29. Reaction Profile for SN2 Reaction (Wade)

  30. Stereochemistry of SN2 ReactionInversion of Configuration

  31. Proof of Inversion of Configuration at a Chiral Center

  32. Acetate Approaches from 180o Behind Leaving Group

  33. Inversion on a Ring is often more Obvious: Cis Trans

  34. 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).

  35. 1o > 2o >> 3oBulkiness of Substrate

  36. Polar, Aprotic Solvents favor SN2

  37. Nucleophilicity

  38. Iodide vs. Fluoride as Nucleophiles

  39. Nucleophiles(preferably non-basic)

  40. Good Leaving Groups are Weak Bases

  41. Common Leaving Groups

  42. SN2 and E2

  43. Bimolecular Elimination - E2Nucleophile acts as Bronsted Base

  44. SN2 Competes with E2

  45. SN2 Competes with E2

  46. SN2 Competes with E2

  47. Stereochemistry of E2

  48. Anti-Coplanar Conformation