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OXIDATIONS OF ALCOHOLS

OXIDATIONS OF ALCOHOLS. OXIDATION OF AN ALCOHOL. ( LOSS OF 2H + and 2e - ). a- carbon. - 2H. a. OXIDATION. C. O. C. O. REDUCTION. + 2H. H. H. LOSS OF TWO HYDROGENS. a- hydrogen. one an a -H. The alcohol must. have a - hydrogens. Oxidations. no reaction.

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OXIDATIONS OF ALCOHOLS

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  1. OXIDATIONS OF ALCOHOLS

  2. OXIDATION OF AN ALCOHOL ( LOSS OF 2H+ and 2e- ) a-carbon - 2H a OXIDATION C O C O REDUCTION + 2H H H LOSS OF TWO HYDROGENS a-hydrogen one an a-H

  3. The alcohol must have a-hydrogens. Oxidations no reaction Remember: A dehydrogenation (loss of hydrogen) is also a form of oxidation!

  4. MANTRA • Primary alcohols oxidize to give • Carboxylic acids(via aldehyde)* • Seconday alcohols oxidize to give • Ketones • Tertiary alcohols do not oxidize • no oxidation • Aldehydes oxidize easily to give • Carboxylic acids *With special reagents, the oxidation of a primary alcohol can be stopped at the aldehyde.

  5. PRIMARY ALCOHOLS

  6. Oxidation of Primary Alcohols with KMnO4 REQUIRES HEAT twoa-hydrogens + MnO2 precipitate C=C Double bonds are also oxidized by this reagent.

  7. Oxidation of Primary Alcohols with K2Cr2O7 CHROMIC ACID TEST

  8. SECONDARY ALCOHOLS

  9. Oxidation of Secondary Alcohols CHROMIC ACID TEST Jones Oxidation

  10. JONES REAGENT

  11. CHROMIC ACID EQUILIBRIA Chromic acid Dichromic acid ALL OF THESE ARE CHROMIC ACID SPECIES CrO3 H2CrO4 NaCr2O7 oxidizing agents H2SO4 H2SO4 H2SO4

  12. Primary alcohol has two a-H MECHANISM H2O H2SO4 Alcohols react with chromic acid to form chromate esters. : + .. .. .. : .. .. Chromate ester

  13. MECHANISM ( continued ) .. aldehyde : .. .. .. .. : .. .. The two lost electrons end up here. .. : : Loss of two electrons + Loss of a-hydrogen Oxidation continues FIRST OXIDATION (next slide)

  14. MECHANISM ( continued ) Requires water and acid hydrate (an alcohol !) Oxidation continues because the aldehyde forms a hydrate. The hydrate is an alcohol (diol) that has an a-hydrogen. Oxidation continues

  15. MECHANISM ( continued ) : .. Chromate ester : + .. .. .. : .. .. : .. Loss of a-hydrogen, loss of 2 electrons. hydrate : Carboxylic acid .. .. : .. .. SECOND OXIDATION +

  16. HOW CAN WE STOP OXIDATION OF THE ALDEHYDE ? Oxidation of the aldehyde requires the hydrate to form. Formation of the hydrate requires acid and water. What if we do the reaction in basic medium with no water ?

  17. SARRETT AND PCC REAGENTS

  18. Oxidation with Chromic Oxide and Pyridine aldehydes do not oxidize further Sarett Oxidation

  19. Oxidation with Pyridinium Chlorochromate aldehydes do not oxidize further except in DMF which enhances reactivity “PCC” Oxidation

  20. MEERWEIN-PONNDORF-VERLEY REDUCTION

  21. MEERWEIN-PONNDORF-VERLEY REDUCTION aluminum isopropoxide Al(OiPr)3 + + isopropyl alcohol acetone EQUILIBRIUM • Use an excess of isopropyl alcohol to reduce a ketone. • Use an excess of acetone to oxidize an alcohol.

  22. HYDRIDE DONOR The alcohol complexes with the aluminum isopropoxide and acts as a hydride donor to the ketone.

  23. TOLLEN’S TEST

  24. The Tollens Test metallic silver aldehyde Ketones do not react. silver mirror Remember that aldehydes are easily oxidized.

  25. CARBOHYDRATES AND THE TOLLEN’S TEST aldehyde The open chain form is an aldehyde and gives a Tollen’s test. “REDUCING SUGAR” b-D-(+)-glucopyranose Pyranose and open-chain forms are in equilibrium in solution. D-(+)-glucose

  26. CARBOHYDRATES AND THE TOLLEN’S TEST ketone TAUTOMERIZATION aldehyde * D-(-)-fructose “enediol” * both diastereomers “REDUCING SUGAR” a-D-(-)-fructofuranose Ketoses also give the test because they tautomerize! in solution.

  27. CARBOHYDRATES AND THE TOLLEN’S TEST hemiacetals open in solution = REDUCING SUGAR aldehyde ketoses tautomerize to reducing sugars X acetals do not open in solution unless hydrolyzed in acid = NONREDUCING SUGAR

  28. CARBOHYDRATES AND THE TOLLEN’S TEST Any polysaccharide which has a hemiacetal ring will give a positive Tollen’s test = reducing sugar. (+)-Sucrose non-reducing Neither sugar is in a hemiacetal link. Maltose reducing sugar Hemiacetal link at a.

  29. SYNTHESIS PROBLEMS

  30. A Simple Synthesis Problem Make from benzyl alcohol complete synthesis on board

  31. A Synthesis Problem ? complete synthesis on board

  32. Yet Another Conversion ? complete synthesis on board

  33. SYNTHESIS OF TRIPHENYLMETHANOL • via benzoyl chloride • via benzaldehyde • via benzyl alcohol • via benzophenone

  34. CONVERSIONS

  35. Another Synthesis Problem ? complete synthesis on board

  36. C D A B PHARMACEUTICALS Mexican Yams “the pill” must be injected better absorbed through the stomach and intestines than progesterone STEROID progesterone

  37. Mexican Yams CrO3 pyridine H+ (-H2O) acetal protecting group Na NH3(liq) Na H3O+ norethisterone

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