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Chapter 11

Chapter 11. Alcohols & Ethers. Structure & Nomenclature. Alcohols have a hydroxyl (–OH) group bonded to a saturated carbon atom (sp 3 hybridized). 1 o. 2 o. 3 o. Ethanol. 2-Propanol (isopropyl alcohol). 2-Methyl- 2-propanol ( tert -butyl alcohol). Phenols

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Chapter 11

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  1. Chapter 11 Alcohols & Ethers

  2. Structure & Nomenclature • Alcohols have a hydroxyl (–OH) group bonded to a saturated carbon atom (sp3 hybridized). 1o 2o 3o Ethanol 2-Propanol (isopropyl alcohol) 2-Methyl- 2-propanol (tert-butyl alcohol)

  3. Phenols • Compounds that have a hydroxyl group attached directly to a benzene or other aromatic ring.

  4. Ethers • The oxygen atom of an ether is bonded to two carbon atoms.

  5. 1A. Nomenclature of Alcohols • Rules of naming alcohols • Identify the longest carbon chain that includes the carbon to which the –OH group is attached. • Use the lowest number for the carbon to which the –OH group is attached. • Alcohol as parent (suffix) • ending with “ol”.

  6. Examples

  7. Example: 3-Propyl-2-heptanol Correct longest chain Incorrect longest chain

  8. 1B. Nomenclature of Ethers • Rules of naming ethers: • Similar to those with alkyl halides • CH3O– Methoxy • CH3CH2O– Ethoxy • Example

  9. Cyclic ethers

  10. Physical Properties ofAlcohols and Ethers • Ethers have boiling points that are roughly comparable with those of hydrocarbons of the same molecular weight (MW). • Alcohols have much higher boiling points than comparable ethers or hydrocarbons.

  11. For example • Alcohol molecules can associate with each other through hydrogen bonding, whereas those of ethers and hydrocarbons cannot.

  12. Water solubility of ethers and alcohols • Both ethers and alcohols are able to form hydrogen bonds with water. • Ethers have solubilities in water that are similar to those of alcohols of the same molecular weight and are very different from solubilities those of hydrocarbons. • The solubility of alcohols in water gradually decreases as the hydrocarbon portion of the molecule lengthens; long-chain alcohols are more “alkane-like” and are, therefore, less like water.

  13. Physical Properties of Ethers Name Formula mp (oC) bp (oC) (1 atm)

  14. Physical Properties of Alcohols Name Formula mp (oC) bp (oC) (1 atm) * * Water solubility (g/100 mL H2O)

  15. 4. Synthesis of Alcohols from Alkenes • Acid-catalyzed Hydration of Alkenes H⊕

  16. Acid-Catalyzed Hydration of Alkenes • Markovnikov regioselectivity. • Free carbocation intermediate. • Rearrangement of carbocation possible.

  17. Oxymercuration–Demercuration • Markovnikov regioselectivity. • Anti stereoselectivity. • Generally takes place without the complication of rearrangements. • Mechanism is an anti addition. • Discussed in Section 8.6

  18. Hydroboration–Oxidation • Anti-Markovnikov regioselectivity. • Syn-stereoselectivity. • Mechanism is a syn addition. • Discussed in Section 8.7

  19. Markovnikov regioselectivity H+, H2O or 1. Hg(OAc)2, H2O, THF 2. NaBH4, NaOH 1. BH3• THF 2. H2O2, NaOH Anti-Markovnikov regioselectivity

  20. Example

  21. Synthesis (1) • Need anti-Markovnikov addition of H–OH. • Use hydroboration-oxidation. 1. BH3• THF 2. H2O2, NaOH

  22. Synthesis (2) • Need Markovnikov addition of H–OH. • Use either: • acid-catalyzed hydration or • oxymercuration-demercuration. • Acid-catalyzed hydration is NOT desired due to rearrangement of carbocation.

  23. Acid-catalyzed hydration H⊕ Rearrangement of carbocation (2o cation)

  24. Oxymercuration-demercuration

  25. Reactions of Alcohols • The reactions of alcohols have mainly to do with the following: • The oxygen atom of the –OH group is nucleophilic and weakly basic. • The hydrogen atom of the –OH group is weakly acidic. • The –OH group can be converted to a good leaving group so as to allow substitution or elimination reactions.

  26. C–O & O–H bonds of an alcohol are polarized. • Protonation of the alcohol converts a poor leaving group (OH⊖) into a good one (H2O).

  27. Once the alcohol is protonated substitution reactions become possible. The protonated –OH group is a good leaving group (H2O).

  28. Alcohols as Acids • Alcohols have acidities similar to that of water.

  29. Relative Acidity H2O & alcohols are the strongest acids in this series Increasing acidity • Relative Basicity OH⊖ is the weakest acid in this series Increasing basicity

  30. Conversion of Alcohols intoAlkyl Halides • HX (X = Cl, Br, I) • PBr3 • SOCl2

  31. Examples

  32. Alkyl Halides from the Reaction ofAlcohols with Hydrogen Halides • The order of reactivity of alcohols: • 3o • The order of reactivity of the hydrogen halides: • HI > HBr > HCl (HF is generally unreactive) > 2o > 1o < methyl

  33. OH⊖ is a poor leaving group H3O⊕ is a good leaving group

  34. 8A. Mechanisms of the Reactions ofAlcohols with HX • Secondary, tertiary, allylic, and benzylic alcohols appear to react by a mechanism that involves the formation of a carbocation (SN1). • Step 1

  35. Step 2 • Step 3

  36. Primary alcohols and methanol react to form alkyl halides under acidic conditions by an SN2 mechanism.

  37. Alkyl Halides from the Reaction of Alcohols with PBr3 or SOCl2 • Reaction of alcohols with PBr3 • The reaction does not involve the formation of a carbocation and usually occurs without rearrangement of the carbon skeleton (especially if the temperature is kept below 0°C).

  38. Reaction of alcohols with PBr3: • Phosphorus tribromide is often preferred as a reagent for the transformation of an alcohol to the corresponding alkyl bromide.

  39. Mechanism

  40. Reaction of alcohols with SOCl2: • SOCl2 converts 1o and 2o alcohols to alkyl chlorides. • As with PBr3, the reaction does not involve the formation of a carbocation and usually occurs without rearrangement of the carbon skeleton (especially if the temperature is kept below 0°C). • Pyridine (C5H5N) is often included to promote the reaction.

  41. Mechanism

  42. Mechanism Cl⊖

  43. Tosylates, Mesylates, & Triflates:Leaving Group Derivatives of Alcohols Good leaving groups

  44. Direct displacement of the –OH group with a nucleophile via an SN2 reaction is not possible since OH⊖ is a very poor leaving group. • As indicated previously, the OH⊖ needs to be converted to a better leaving group first.

  45. Mesylates (OMs) and Tosylates (OTs) are good leaving groups and they can be prepared easily from an alcohol. (methane sulfonyl chloride)

  46. Preparation of Tosylates (OTs) from an alcohol: (p-toluene sulfonyl chloride)

  47. SN2 displacement of the mesylate or tosylate with a nucleophile is possible.

  48. Example Retention of configuration Inversion of configuration

  49. Example Retention of configuration Inversion of configuration

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