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Chapter 11 Reactions of Alcohols

Organic Chemistry , 6 th Edition L. G. Wade, Jr. Chapter 11 Reactions of Alcohols. Types of Alcohol Reactions. Dehydration to alkene Oxidation to aldehyde, ketone Substitution to form alkyl halide Reduction to alkane Esterification Tosylation Williamson synthesis of ether =>.

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Chapter 11 Reactions of Alcohols

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  1. Organic Chemistry, 6th EditionL. G. Wade, Jr. Chapter 11Reactions of Alcohols

  2. Types of Alcohol Reactions • Dehydration to alkene • Oxidation to aldehyde, ketone • Substitution to form alkyl halide • Reduction to alkane • Esterification • Tosylation • Williamson synthesis of ether => Chapter 11

  3. Summary Table => Chapter 11

  4. Oxidation States • Easy for inorganic salts • CrO42- reduced to Cr2O3 • KMnO4 reduced to MnO2 • Oxidation: loss of H2, gain of O, O2, or X2 • Reduction: gain of H2 or H-, loss of O, O2, or X2 • Neither: gain or loss of H+, H2O, HX=> Chapter 11

  5. 1º, 2º, 3º Carbons => Chapter 11

  6. => Oxidation of 2° Alcohols • 2° alcohol becomes a ketone • Reagent is Na2Cr2O7/H2SO4 • Active reagent probably H2CrO4 • Color change: orange to greenish-blue Chapter 11

  7. => Oxidation of 1° Alcohols • 1° alcohol to aldehyde to carboxylic acid • Difficult to stop at aldehyde • Use pyridinium chlorochromate (PCC) to limit the oxidation. • PCC can also be used to oxidize 2° alcohols to ketones. Chapter 11

  8. => 3° Alcohols Don’t Oxidize • Cannot lose 2 H’s • Basis for chromic acid test Chapter 11

  9. Other Oxidation Reagents • Collins reagent: Cr2O3 in pyridine • Jones reagent: chromic acid in acetone • KMnO4 (strong oxidizer) • Nitric acid (strong oxidizer) • CuO, 300°C (industrial dehydrogenation) • Swern oxidation: dimethylsulfoxide, with oxalyl chloride and hindered base, oxidizes 2 alcohols to ketones and 1 alcohols to aldehydes. => Chapter 11

  10. Biological Oxidation • Catalyzed by ADH, alcohol dehydrogenase. • Oxidizing agent is NAD+, nicotinamide adenine dinucleotide. • Ethanol oxidizes to acetaldehyde, then acetic acid, a normal metabolite. • Methanol oxidizes to formaldehyde, then formic acid, more toxic than methanol. • Ethylene glycol oxidizes to oxalic acid, toxic. • Treatment for poisoning is excess ethanol.=> Chapter 11

  11. Alcohol as a Nucleophile • ROH is weak nucleophile • RO- is strong nucleophile • New O-C bond forms, O-H bond breaks.=> Chapter 11

  12. Alcohol as an Electrophile • OH- is not a good leaving group unless it is protonated, but most nucleophiles are strong bases which would remove H+. • Convert to tosylate (good leaving group) to react with strong nucleophile (base).=>  + C-Nuc bond forms, C-O bond breaks Chapter 11

  13. Formation of Tosylate Ester p-toluenesulfonyl chloride TsCl, “tosyl chloride” ROTs, a tosylate ester => Chapter 11

  14. SN2 Reactions of Tosylates • With hydroxide produces alcohol • With cyanide produces nitrile • With halide ion produces alkyl halide • With alkoxide ion produces ether • With ammonia produces amine salt • With LiAlH4 produces alkane => Chapter 11

  15. Summary of Tosylate Reactions => Chapter 11

  16. => Reaction 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 Chapter 11

  17. Reaction with HCl • Chloride is a weaker nucleophile than bromide. • Add ZnCl2, which bonds strongly with -OH, to promote the reaction. • The chloride product is insoluble. • Lucas test: ZnCl2 in conc. HCl • 1° alcohols react slowly or not at all. • 2 alcohols react in 1-5 minutes. • 3 alcohols react in less than 1 minute. => Chapter 11

  18. Limitations of HX Reactions • HI does not react • Poor yields of 1° and 2° chlorides • May get alkene instead of alkyl halide • Carbocation intermediate may rearrange. => Chapter 11

  19. Reactions with Phosphorus Halides • Good yields with 1° and 2° alcohols • PCl3 for alkyl chloride (but SOCl2 better) • PBr3 for alkyl bromide • P and I2 for alkyl iodide (PI3 not stable) => Chapter 11

  20. Mechanism with PBr3 • P bonds to -OH as Br- leaves • Br- attacks backside (SN2) • HOPBr2 leaves => Chapter 11

  21. Reaction with Thionyl Chloride • Produces alkyl chloride, SO2, HCl • S bonds to -OH, Cl-leaves • Cl- abstracts H+ from OH • C-O bond breaks as Cl- transferred to C=> Chapter 11

  22. Dehydration Reactions • Conc. H2SO4 produces alkene • Carbocation intermediate • Zaitsev product • Bimolecular dehydration produces ether • Low temp, 140°C and below, favors ether • High temp, 180°C and above, favors alkene => Chapter 11

  23. => Dehydration Mechanisms Chapter 11

  24. Energy Diagram, E1 => Chapter 11

  25. => Alkoxide Ions • ROH + Na (or NaH) yields sodium alkoxide • RO- + 1° alkyl halide yields ether (Williamson ether synthesis) Chapter 11

  26. End of Chapter 11 Chapter 11

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