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Lectures 22 and 23

Lectures 22 and 23. Alcohols and Ethers Chapter 8. Alcohols and Phenols. Alcohols contain an OH group connected to a a saturated C (sp 3 ) They are important solvents and synthesis intermediates

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Lectures 22 and 23

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  1. Lectures 22 and 23 Alcohols and Ethers Chapter 8 Chemistry 243 - Lecture 18

  2. Alcohols and Phenols • Alcohols contain an OH group connected to a a saturated C (sp3) • They are important solvents and synthesis intermediates • Methanol, CH3OH, called methyl alcohol, is a common solvent, a fuel additive, produced in large quantities • Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beverage • Phenols contain an OH group connected to a carbon in a benzene ring • Phenol, C6H5OH (“phenyl alcohol”) has diverse uses - it gives its name to the general class of compounds Chemistry 243 - Lecture 18

  3. Naming Alcohols • General classifications of alcohols based on substitution on C to which OH is attached • Methyl (C has 3 H’s), Primary (1°) (C has two H’s, one R), secondary (2°) (C has one H, two R’s), tertiary (3°) (C has no H, 3 R’s), Chemistry 243 - Lecture 18

  4. IUPAC Rules for Naming Alcohols • Select the longest carbon chain containing the hydroxyl group, and derive the parent name by replacing the -e ending of the corresponding alkane with -ol • Number the chain from the end nearer the hydroxyl group • Number substituents according to position on chain, listing the substituents in alphabetical order Chemistry 243 - Lecture 18

  5. Properties of Alcohols and Phenols: Hydrogen Bonding • The structure around O of the alcohol or phenol is similar to that in water, sp3 hybridized • Alcohols and phenols have much higher boiling points than similar alkanes and alkyl halides Chemistry 243 - Lecture 18

  6. Alcohols Form Hydrogen Bonds • A positively polarized OH hydrogen atom from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another molecule • This produces a force that holds the two molecules together • These intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution Chemistry 243 - Lecture 18

  7. Properties of Alcohols and Phenols: Acidity and Basicity • Weakly basic and weakly acidic • Alcohols are weak Brønsted bases • Protonated by strong acids to yield oxonium ions, ROH2+ Chemistry 243 - Lecture 18

  8. Alchols and Phenols are Weak Brønsted Acids • Can transfer a proton to water to a very small extent • Produces H3O+ and an alkoxide ion, RO, or a phenoxide ion, ArO Chemistry 243 - Lecture 18

  9. Brønsted Acidity Measurements • The acidity constant, Ka, measure the extent to which a Brønsted acid transfers a proton to water and pKa = log Ka • Relative acidities are more conveniently presented on a logarithmic scale, pKa, which is directly proportional to the free energy of the equilibrium • Differences in pKa correspond to differences in free energy • Table 8.1 presents a range of acids and their pKa values Chemistry 243 - Lecture 18

  10. pKa Values for Typical OH Compounds Chemistry 243 - Lecture 18

  11. Generating Alkoxides from Alcohols • Alcohols are weak acids – requires a strong base to form an alkoxide such as NaH, sodium amide NaNH2, and Grignard reagents (RMgX) • Alkoxides are bases used as reagents in organic chemistry Chemistry 243 - Lecture 18

  12. Phenol Acidity • Phenols (pKa ~10) are much more acidic than alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion • Phenols react with NaOH solutions (but alcohols do not), forming soluble salts that are soluble in dilute aqueous • A phenolic component can be separated from an organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution Chemistry 243 - Lecture 18

  13. Nitro-Phenols • Phenols with nitro groups at the ortho and para positions are much stronger acids • The pKa of 2,4,6-trinitrophenol is 0.6, a very strong acid Chemistry 243 - Lecture 18

  14. Preparation of Alchols: an Overview • Alcohols are derived from many types of compounds • The alcohol hydroxyl can be converted to many other functional groups • This makes alcohols useful in synthesis Chemistry 243 - Lecture 18

  15. Review: Preparation of Alcohols by Regiospecific Hydration of Alkenes • Hydroboration/oxidation: syn, non-Markovnikov hydration • Oxymercuration/reduction: Markovnikov hydration Chemistry 243 - Lecture 18

  16. Alcohols from Reduction of Carbonyl Compounds • Reduction of a carbonyl compound in general gives an alcohol • Note that organic reduction reactions add the equivalent of H2 to a molecule Chemistry 243 - Lecture 18

  17. Reduction of Aldehydes and Ketones • Aldehydes gives primary alcohols • Ketones gives secondary alcohols Chemistry 243 - Lecture 18

  18. Reduction Reagent: Sodium Borohydride • NaBH4 is not sensitive to moisture and it does not reduce other common functional groups • Lithium aluminum hydride (LiAlH4) is more powerful, less specific, and very reactive with water • Both add the equivalent of “H–” Chemistry 243 - Lecture 18

  19. Mechanism of Reduction • The reagent adds the equivalent of hydride to the carbon of C=O and polarizes the group as well Chemistry 243 - Lecture 18

  20. Reduction of Carboxylic Acids and Esters • Carboxylic acids and esters are reduced to give primary alcohols • LiAlH4 is used because NaBH4 is not effective Chemistry 243 - Lecture 18

  21. Alcohols from Reaction of Carbonyl Compounds with Grignard Reagents • Alkyl, aryl, and vinylic halides react with magnesium in ether or tetrahydrofuran to generate Grignard reagents, RMgX • Grignard reagents react with carbonyl compounds to yield alcohols Chemistry 243 - Lecture 18

  22. Examples of Reactions of Grignard Reagents with Carbonyl Compounds Chemistry 243 - Lecture 18

  23. Mechanism of the Addition of a Grignard Reagent • Grignard reagents act as nucleophilic carbon anions (carbanions, : R) in adding to a carbonyl group • The intermediate alkoxide is then protonated to produce the alcohol Chemistry 243 - Lecture 18

  24. Some Reactions of Alcohols • Two general classes of reaction • At the carbon of the C–O bond • At the proton of the O–H bond Chemistry 243 - Lecture 18

  25. Dehydration of Alcohols to Yield Alkenes • The general reaction: forming an alkene from an alcohol through loss of O-H and H (hence dehydration) of the neighboring C–H to give  bond • Specific reagents are needed Chemistry 243 - Lecture 18

  26. Acid- Catalyzed Dehydration • Tertiary alcohols are readily dehydrated with acid • Secondary alcohols require severe conditions (75% H2SO4, 100°C) - sensitive molecules don't survive • Primary alcohols require very harsh conditions – impractical • Reactivity is the result of the nature of the carbocation intermediate Chemistry 243 - Lecture 18

  27. Conversion of Alcohols into Alkyl Halides • 3° alcohols are converted by HCl or HBr at low temperature • 1° and alcohols are resistant to acid – use SOCl2 or PBr3 by an SN2 mechanism Chemistry 243 - Lecture 18

  28. Conversion of Alcohols into Tosylates • Reaction with p-toluenesulfonyl chloride (tosyl chloride, p-TosCl) in pyridine yields alkyl tosylates, ROTos • Formation of the tosylate does not involve the C–O bond so configuration at a chirality center is maintained • Alkyl tosylates react like alkyl halides Chemistry 243 - Lecture 18

  29. Stereochemical Uses of Tosylates • The SN2 reaction of an alcohol via a tosylate, produces inversion at the chirality center • The SN2 reaction of an alcohol via an alkyl halide proceeds with two inversions, giving product with same arrangement as starting alcohol Chemistry 243 - Lecture 18

  30. Oxidation of Alcohols • Can be accomplished by inorganic reagents, such as KMnO4, CrO3, and Na2Cr2O7 or by more selective, expensive reagents Chemistry 243 - Lecture 18

  31. Oxidation of Primary Alcohols • To aldehyde: pyridinium chlorochromate (PCC, C5H6NCrO3Cl) in dichloromethane • Other reagents produce carboxylic acids Chemistry 243 - Lecture 18

  32. Oxidation of Secondary Alcohols • Effective with inexpensive reagents such as Na2Cr2O7 in acetic acid • PCC is used for sensitive alcohols at lower temperatures Chemistry 243 - Lecture 18

  33. Mechanism of Chromic Acid Oxidation • Alcohol forms a chromate ester followed by elimination with electron transfer to give ketone. • The mechanism was determined by observing the effects of isotopes on rates Chemistry 243 - Lecture 18

  34. Reactions of Phenols • The hydroxyl group is a strongly activating, making phenols substrates for electrophilic halogenation, nitration, sulfonation, and Friedel–Crafts reactions • Reaction of a phenol with strong oxidizing agents yields a quinone • Fremy's salt [(KSO3)2NO] works under mild conditions through a radical mechanism Chemistry 243 - Lecture 18

  35. Quinones in Nature • Ubiquinones mediate electron-transfer processes involved in energy production through their redox reactions Chemistry 243 - Lecture 18

  36. Summary -Alcohols • Synthesis • Reduction of aldehydes and ketones • Addition of Grignard reagents to aldehydes and ketones • Reactions • Conversion to alkyl halides • Dehydration • Oxidation Chemistry 243 - Lecture 18

  37. Summary - Phenols • Much more acidic (pKa 10) than alcohols • Substitution of the aromatic ring by an electron-withdrawing group increases phenol acidity • Substitution by an electron-donating group decreases acidity • Oxidized to quinones • Quinones are reduced to hydroquinones Chemistry 243 - Lecture 18

  38. Ethers and Their Relatives • An ether has two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R–O–R • Diethyl ether is used industrially as a solvent • Tetrahydrofuran (THF) is a solvent that is a cyclic ether • Thiols (R–S–H) and sulfides (R–S–R) are sulfur (for oxygen) analogs of alcohols and ethers Chemistry 243 - Lecture 18

  39. Naming Ethers • Simple ethers are named by identifying the two organic substituents and adding the word ether • If other functional groups are present, the ether part is considered an alkoxy substituent Chemistry 243 - Lecture 18

  40. Structure, Properties, and Sources of Ethers • R–O–R ~ tetrahedral bond angle (112° in dimethyl ether) • Oxygen is sp3-hybridized • Oxygen atom gives ethers a slight dipole moment • Diethyl ether prepared industrially by sulfuric acid–catalyzed dehydration of ethanol – also with other primary alcohols Chemistry 243 - Lecture 18

  41. The Williamson Ether Synthesis • Reaction of metal alkoxides and primary alkyl halides and tosylates • Best method for the preparation of ethers • Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH Chemistry 243 - Lecture 18

  42. Cyclic Ethers • Cyclic ethers behave like acyclic ethers, except if ring is 3-membered • Dioxane and tetrahydrofuran are used as solvents Chemistry 243 - Lecture 18

  43. Epoxides (Oxiranes) • Three membered ring ether is called an oxirane (root “ir” from “tri” for 3-membered; prefix “ox” for oxygen; “ane” for saturated) • Also called epoxides • Ethylene oxide (oxirane; 1,2-epoxyethane) is industrially important as an intermediate • Prepared by reaction of ethylene with oxygen at 300 °C and silver oxide catalyst Chemistry 243 - Lecture 18

  44. Preparation of Epoxides Using a Peroxyacid • Treat an alkene with a peroxyacid Chemistry 243 - Lecture 18

  45. Ring-Opening Reactions of Epoxides • Water adds to epoxides with dilute acid at room temperature • Product is a 1,2-diol (on adjacent C’s: vicinal) • Mechanism: acid protonates oxygen and water adds to opposite side (trans addition) Chemistry 243 - Lecture 18

  46. Ethylene Glycol • 1,2-ethanediol from acid catalyzed hydration of ethylene • Widely used as automobile antifreeze (lowers freezing point of water solutions) Chemistry 243 - Lecture 18

  47. Base-Catalyzed Epoxide Opening • Strain of the three-membered ring is relieved on ring-opening • Hydroxide cleaves epoxides at elevated temperatures to give trans 1,2-diols Chemistry 243 - Lecture 18

  48. Addition of Grignards to Ethylene Oxide • Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react Chemistry 243 - Lecture 18

  49. Thiols and Sulfides • Thiols (RSH), are sulfur analogs of alcohols • Named with the suffix -thiol • SH group is called “mercapto group” (“capturer of mercury”) Chemistry 243 - Lecture 18

  50. Sulfides • Sulfides (RSR), are sulfur analogs of ethers • Named by rules used for ethers, with sulfide in place of ether for simple compounds and alkylthio in place of alkoxy Chemistry 243 - Lecture 18

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