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Reactions of Alcohols

Reactions of Alcohols

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Reactions of Alcohols

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  1. Reactions of Alcohols Oxidation R-X, Ether, and Ester Preparation Protection of Alcohols Synthesis The Logic of Mechanisms

  2. Alcohols are Synthetically Versatile

  3. Oxidation levels of oxygen- halogen- and nitrogen-containing molecules

  4. Oxidation -Reduction

  5. Oxidation of 2o Alcohols with Cr(VI)

  6. Mechanism

  7. Oxidation of 1o Alcohols

  8. PCC oxidizes 1o Alcohols to Aldehydes

  9. Pyridinium Chlorochromate (PCC) • PCC is a complex of chromium trioxide, pyridine, and HCl. • Oxidizes primary alcohols to aldehydes. • Oxidizes secondary alcohols to ketones.

  10. Oxidation of 1o Alcohols to Aldehydes: PCC

  11. 3° Alcohols Cannot Be Oxidized • Carbon does not have hydrogen, so oxidation is difficult and involves the breakage of a C—C bond. • Chromic acid test is for primary and secondary alcohols because tertiary alcohols do not react. Orange color of Cr(VII) turns green - Cr(III); 3o alcohol is not oxidized, therefore no color change.

  12. Sodium Hypochlorite (NaOCl) • Sodium hypochlorite (household bleach) can oxidize alcohols without heavy metals or generating hazardous waste. • This is a much better option for acid-sensitive compounds.

  13. Swern Oxidation • Dimethylsulfoxide (DMSO), with oxalyl chloride and hindered base, oxidizes 2 alcohols to ketones and 1 alcohols to aldehydes (same as PCC).

  14. Oxidation Summary

  15. Reduction Summary

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

  17. Formation of p-Toluenesulfonate Esters

  18. Substitution and Elimination Reactions Using Tosylates

  19. Summary of Tosylate Reactions

  20. Best to use p-TsClwith pyridine

  21. Reactions of Tosylates:Reduction, Substitution, Elimination

  22. Reduction of Alcohols • Dehydrate with concentrated H2SO4, then add H2. • Make a tosylate, then reduce it with LiAlH4.

  23. Alcohols to Alkyl Halides

  24. Reaction of Alcohols with Acids • The hydroxyl group is protonated by an acid to convert it into a good leaving group (H2O). • Once the alcohol is protonated, a substitution or elimination reaction can take place.

  25. Reaction of Alcohols with HBr • –OH of alcohol is protonated. • –OH2+ is good leaving group. • 3° and 2° alcohols react with Br– via SN1. • 1° alcohols react via SN2.

  26. SN1 Mechanism Step 1: Protonation. Step 2: Formation of the carbocation. Step 3: Bromide attacks the carbocation.

  27. SN1: Carbocations can Rearrange

  28. Solved Problem 2 When 3-methyl-2-butanol is treated with concentrated HBr, the major product is 2-bromo-2-methylbutane. Propose a mechanism for the formation of this product. Solution The alcohol is protonated by the strong acid. This protonated secondary alcohol loses water to form a secondary carbocation.

  29. Solved Problem 2 (Continued) Solution (Continued) A hydride shift transforms the secondary carbocation into a more stable tertiary cation. Attack by bromide leads to the observed product.

  30. Lucas Test

  31. SN2 Reaction with the Lucas Reagent • Primary alcohols react with the Lucas reagent (HCl and ZnCl2) by the SN2 mechanism. • Reaction is very slow. The reaction can take from several minutes to several days.

  32. Qualitative test for Alcohol Characterization

  33. Other Simple Qualitative Tests

  34. 1o and 2o Alcohols: best to use SOCl2, PBr3,or P/I2

  35. Examples

  36. Thionyl chloride mechanism in Pyridine –SN2, Inversion

  37. Dehydration of Alcohols • Alcohol dehydration generally takes place through the E1 mechanism. • Rearrangements are possible. • The rate of the reaction follows the same rate as the ease of formation of carbocations: 3o > 2o > 1o. • Primary alcohols rearrange, so this is not a good reaction for converting 1° alcohols into alkenes.

  38. Dehydration of Alcohols – E1

  39. Methide Shift is Faster than Loss of H+

  40. Dimerization of Alcohols:Symmetrical Ethers

  41. Mechanism

  42. Esterification • Fischer: Alcohol + carboxylic acid • Tosylate esters • Sulfate esters • Nitrate esters • Phosphate esters

  43. Fischer Esterification • Reaction of an alcohol and a carboxylic acid produces an ester. • Sulfuric acid is a catalyst. • The reaction is an equilibrium between starting materials and products, and for this reason the Fischer esterification is seldom used to prepare esters.

  44. Nitrate Esters • The best-known nitrate ester is nitroglycerine, whose systematic name is glyceryl trinitrate. • Glyceryl nitrate results from the reaction of glycerol (1,2,3-propanetriol) with three molecules of nitric acid.

  45. Phosphate Esters

  46. Phosphate Esters in DNA

  47. Protection of Alcohols

  48. Alcohol is acidic enough to react preferentially.

  49. Chlorotrimethylsilane (TMS-Cl)