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Synthesis and Reactions of Alcohols: Mechanisms, Oxidation, and Ester Preparation

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joettan
60 diapositives · 932 Views · Jan 01, 2020

This comprehensive overview covers the synthesis and reactions of alcohols, focusing on oxidation reactions, conversion mechanisms, and ester preparation. It discusses the oxidation of primary (1°) and secondary (2°) alcohols using reagents like PCC and sodium hypochlorite, the formation of leaving groups, and various substitution and elimination reactions. Additionally, it delves into Fischer esterification, highlighting the role of acid catalysts. The content serves as a valuable resource for understanding the versatile nature of alcohols in organic synthesis.

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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)