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ALCOHOLS and PHENOLS

ALCOHOLS and PHENOLS. Classification of hydroxylic compounds. sp 3. sp 2. an alcohol. a phenol. sp 2. an enol. Classification of alcohols. Molecular shape of alcohols. Methanol (methyl alcohol). Molecular shape of phenols. Phenol. Properties of hydroxyl group. Polarity

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ALCOHOLS and PHENOLS

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  1. ALCOHOLS and PHENOLS

  2. Classification of hydroxylic compounds sp3 sp2 an alcohol a phenol sp2 an enol

  3. Classification of alcohols

  4. Molecular shape of alcohols Methanol (methyl alcohol)

  5. Molecular shape of phenols Phenol

  6. Properties of hydroxyl group • Polarity • Hydrogen bonding • Solubility in water • Basicity • Acidity

  7. Boiling points of alkanes, chloroalkanes and alcohols

  8. Hydrogen bonding in alcohols δ- δ- δ- δ- δ+ δ+ δ+ δ+ δ+ δ+ δ+ δ+ δ- δ- δ- δ- Hydrogen bond strength: 21 kJ/mol (5 kcal/mol)

  9. Acid-base equilibria of alcohols and phenols Basicity base acid oxonium salt

  10. Acid-base equilibria of alcohols and phenols Acidity alkoxide anion acid base phenoxide anion

  11. Acidity constants of alcohols

  12. Effect of substituents on acidity of alcohols pKa = 16 pKa = 12.4 Ethoxide anion 2,2,2-Trifluoroethoxide anion Electron-withdrawing groups stabilize alkoxide anion and lower pKa (increase acidity of alcohol)

  13. Effect of substituents on acidity of alcohols pKa = 18 pKa = 5.4 Electron-withdrawing groups stabilize alkoxide anion and lower pKa (increase acidity of alcohol)

  14. Effect of anion size on acidity of alcohols pKa = 15.58 pKa = 18.00 Small anion, sterically accessible Easy solvation Weaker basicity of anion Greater acidity of alcohol Large anion, sterically hindered Difficult solvation Stronger basicity of anion Lower acidity of alcohol

  15. Acidity of phenols vs alcohols Phenols are stronger acids than alcohols Because of resonance stabilization of phenolate anion:

  16. Acidity constant of phenol pKa = 10.00 Phenol is stronger acid than aliphatic alcohols, and stronger acid than water pKa 15.5 – 18 pKa 15.7

  17. Acidity constants of some phenols

  18. Effect of substituents on acidity of phenols EWG –Electron Withdrawing Group Phenoxide anion stabilized by substituent Greater acidity of phenol pKa < 10 EWG: -NO2, -COOH, -CN, -Cl, -Br, EDG –Electron Donating Group Phenoxide anion destabilized by substituent Lower acidity of phenol pKa > 10 EDG: -OCH3, -NH2, alkyl (CH3)

  19. Alcohols in organic synthesis Alkene Ketone Carboxylic acid Ester Aldehyde Alkyl halide Ether

  20. Preparation of alcohols Hydration of alkenes (industrial method) Addition according to Markovnikov rule due to carbocation formation in the 1st step

  21. Mechanism of acid-catalyzed hydration of alkenes

  22. Preparation of alcohols Hydration of alkenes (laboratory methods) 1. Oxymercuration 2. Reduction Markovnikov product 1. Hydrobration 2. Oxidation Anti-Markovnikov product

  23. Preparation of alcohols Reduction of carbonyl group (hydrogenation) Reductive agents: NaBH4 LiAlH4 H2, metal catalyst

  24. Preparation of alcohols Preparation of Grignard’s reagents from alkyl halides + Electrophile Nucleophile - R = alkyl, aryl, alkenyl

  25. Preparation of alcohols Addition of Grignard’s reagents to carbonyl group New C-C bond From carbonyl compound and Grignard’s reagent, alcohol of larger hydrocarbon framework is formed

  26. Preparation of alcohols Addition of Grignard’s reagent to FORMALDEHYDE Primary alcohol is formed

  27. Preparation of alcohols Addition of Grignard’s reagent to OTHER ALDEHYDE Secondary alcohol is formed

  28. Preparation of alcohols Addition of Grignard’s reagent to KETONE Tertiary alcohol is formed

  29. Preparation of alcohols Addition of Grignard’s reagent to ESTER Tertiary alcohol with 2 chains from Grignard’s reagent is formed

  30. Reactions of alcohols Dehydration (elimination of water) REACTIVITY ORDER primary secondary tertiary alcohol

  31. Mechanism of acid-catalyzed dehydration of alcohols

  32. Regioselectivity of alcohol dehydration major product minor product (more stable) (less stable)

  33. Reactions of alcohols Conversion to alkyl halides (nucleophilic substitution) SOCl2 – thionyl chloride PBr3 – phosphorus tribromide

  34. Reactions of alcohols Oxidation of hydroxyl group (dehydrogenation) Oxidants: O2, catalyst CrO3, H2SO4 (Jones reagent) K2Cr2O7 (Na2Cr2O7), H2SO4 PCC (pyridinium chlorochromate) KMnO4

  35. Preparation of phenols Oxidation of cumene – industrial method of phenol production

  36. Preparation of phenols Thermal decomposition of arylsufonic acids

  37. Reactivity of phenols • Aromatic ring susceptible for electrophilic substitution (alkylation, nitration, halogenation etc.) • Hydroxyl group does not react in nucleophilic substitution • Hydroxyl proton more acidic than in aliphatic alcohols • Different way of oxidation

  38. Oxidation of phenols Oxidant – (KSO3)2NO Potassium nitrosodisulfonate

  39. Applications of some phenols

  40. ETHERS

  41. Ethers dialkyl ether alkyl aryl ether cyclic ether

  42. Ethers diethyl ether anisole tetrahydrofuran

  43. Ethers vs alkanes Ether b.p.  C Alkane b.p.  C

  44. Properties of ethers Comparison to alcohols and alkanes • More polar than alkanes, less than alcohols • No hydrogen bonding • Boiling points – higher than for alkanes, lower than • for alcohols • Solubility in water limited • Neutral - nor basic nor acidic • Used frequently as solvents

  45. Preparation of ethers Dehydration of alcohols – symmetric ethers only Williamson’s synthesis (SN2 reaction)

  46. Other example of Williamson’s synthesis

  47. Reactions of ethers Acid cleavage

  48. Claisen rearrangement of allyl ethers

  49. Cyclic ethers

  50. Preparation of oxiranes Oxidation of alkenes

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