Alcohols contain an OH group connected to a a saturated C sp 3 They are important solvents and synthesis intermediates Phenols contain an OH group connected to a carbon in a benzene ring Methanol CH ID: 1011605
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1. ALCOHOLS &PHENOLS
2. Alcohols and PhenolsAlcohols contain an OH group connected to a a saturated C (sp3)They are important solvents and synthesis intermediatesPhenols contain an OH group connected to a carbon in a benzene ringMethanol, CH3OH, called methyl alcohol, is a common solvent, a fuel additive, produced in large quantitiesEthanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beveragePhenol, C6H5OH (“phenyl alcohol”) has diverse uses - it gives its name to the general class of compoundsOH groups bonded to vinylic, sp2-hybridized carbons are called enols
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4. Naming Alcohols and PhenolsGeneral classifications of alcohols based on substitution on C to which OH is attachedMethyl (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),
5. IUPAC Rules for Naming AlcoholsSelect 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 groupNumber substituents according to position on chain, listing the substituents in alphabetical order
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7. Naming PhenolsUse “phenol” (the French name for benzene) as the parent hydrocarbon name, not benzeneName substituents on aromatic ring by their position from OH
8. Properties of Alcohols and PhenolsThe structure around O of the alcohol or phenol is similar to that in water, sp3 hybridizedAlcohols and phenols have much higher boiling points than similar alkanes and alkyl halidesA positively polarized OH hydrogen atom from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another moleculeThis produces a force that holds the two molecules togetherThese intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution
9. Properties of Alcohols and Phenols: Acidity and Basicity Weakly basic and weakly acidicAlcohols are weak Brønsted basesProtonated by strong acids to yield oxonium ions, ROH2+
10. Alcohols and Phenols are Weak Brønsted AcidsCan transfer a proton to water to a very small extentProduces H3O+ and an alkoxide ion, RO, or a phenoxide ion, ArO
11. Acidity MeasurementsThe acidity constant, Ka, measures the extent to which a Brønsted acid transfers a proton to water [A] [H3O+] Ka = ————— and pKa = log Ka [HA] Relative acidities are more conveniently presented on a logarithmic scale, pKa, which is directly proportional to the free energy of the equilibriumDifferences in pKa correspond to differences in free energyFollowing Table presents a range of acids and their pKa values
12. pKa Values for Typical OH Compounds
13. Relative Acidities of AlcoholsSimple alcohols are about as acidic as waterAlkyl groups make an alcohol a weaker acidThe more easily the alkoxide ion is solvated by water the more its formation is energetically favoredSteric effects are important
14. Inductive EffectsElectron-withdrawing groups make an alcohol a stronger acid by stabilizing the conjugate base (alkoxide)
15. Generating Alkoxides from AlcoholsAlcohols 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
16. Phenol AcidityPhenols (pKa ~10) are much more acidic than alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ionPhenols react with NaOH solutions (but alcohols do not), forming salts that are soluble in dilute aqueous solutionA 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
17. Nitro-PhenolsPhenols with nitro groups at the ortho and para positions are much stronger acids
18. Preparation of Alcohols: A ReviewThe alcohol hydroxyl can be converted to many other functional groupsThis makes alcohols useful in synthesis
19. Preparation of Alcohols by Regiospecific Hydration of AlkenesHydroboration/oxidation: syn, non-Markovnikov hydration Oxymercuration/reduction: Markovnikov hydration.
20. Alcohols from Reduction of Carbonyl Compounds Reduction of a carbonyl compound in general gives an alcoholNote that organic reduction reactions add the equivalent of H2 to a molecule
21. Reduction of Aldehydes and Ketones Aldehydes gives primary alcoholsKetones gives secondary alcohols
22. Reduction Reagent: Sodium BorohydrideNaBH4 is not sensitive to moisture and it does not reduce other common functional groupsLithium aluminum hydride (LiAlH4) is more powerful, less specific, and very reactive with waterBoth add the equivalent of “H-”
23. Mechanism of ReductionThe reagent adds the equivalent of hydride to the carbon of C=O and polarizes the group as well
24. Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are reduced to give primary alcoholsLiAlH4 is used because NaBH4 is not effective
25. Alcohols from Reaction of Carbonyl Compounds with Grignard ReagentsAlkyl, aryl, and vinylic halides react with magnesium in ether or tetrahydrofuran to generate Grignard reagents, RMgXGrignard reagents react with carbonyl compounds to yield alcohols
26. Mechanism of the Addition of a Grignard ReagentGrignard reagents act as nucleophilic carbon anions (carbanions, : R) in adding to a carbonyl groupThe intermediate alkoxide is then protonated to produce the alcohol
27. Reactions of AlcoholsConversion of alcohols into alkyl halides:3˚ alcohols react with HCl or HBr by SN1 through carbocation intermediate1˚ and 2˚ alcohols converted into halides by treatment with SOCl2 or PBr3 via SN2 mechanism
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29. 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 bondSpecific reagents are needed
30. Acid- Catalyzed DehydrationTertiary alcohols are readily dehydrated with acidSecondary alcohols require severe conditions (75% H2SO4, 100°C) - sensitive molecules don't survivePrimary alcohols require very harsh conditions – impracticalReactivity is the result of the nature of the carbocation intermediate
31. Dehydration with POCl3Phosphorus oxychloride in the amine solvent pyridine can lead to dehydration of secondary and tertiary alcohols at low temperaturesAn E2 via an intermediate ester of POCl2
32. Conversion of Alcohols into Esters
33. Oxidation of Alcohols Can be accomplished by inorganic reagents, such as KMnO4, CrO3, and Na2Cr2O7 or by more selective, expensive reagents
34. Oxidation of Primary AlcoholsTo aldehyde: pyridinium chlorochromate (PCC, C5H6NCrO3Cl) in dichloromethaneOther reagents produce carboxylic acids
35. Oxidation of Secondary AlcoholsEffective with inexpensive reagents such as Na2Cr2O7 in acetic acidPCC is used for sensitive alcohols at lower temperatures
36. Mechanism of Chromic Acid OxidationAlcohol forms a chromate ester followed by elimination with electron transfer to give ketoneThe mechanism was determined by observing the effects of isotopes on rates
37. Phenols and Their UsesIndustrial process from readily available cumeneForms cumene hydroperoxide with oxygen at high temperatureConverted into phenol and acetone by acid
38. Reactions of PhenolsThe hydroxyl group is a strongly activating, making phenols substrates for electrophilic halogenation, nitration, sulfonation, and Friedel–Crafts reactionsReaction of a phenol with strong oxidizing agents yields a quinoneFremy's salt [(KSO3)2NO] works under mild conditions through a radical mechanism
39. Spectroscopy of Alcohols and PhenolsCharacteristic O–H stretching absorption at 3300 to 3600 cm1 in the infraredSharp absorption near 3600 cm-1 except if H-bonded: then broad absorption 3300 to 3400 cm1 rangeStrong C–O stretching absorption near 1050 cm1 (See Figure 17.11)Phenol OH absorbs near 3500 cm-1
40. Preparation of DiolsVicinal diols have hydroxyl groups on adjacent carbons (1,2-diols, vic-diols, glycols)Dihydroxylation: formal addition of HO-OH across the -bond of an alkene to give a 1,2-diol. This is an overall oxidation.
41. Oxidative Cleavage of Vicinal DiolsOxidative Cleavage of 1,2-diols to aldehydes and ketones withsodium periodate (NaIO4) or periodic acid (HIO4)
42. ThiolsThiols (mercaptans) are sulfur analogues of alcohols.Thiols have a pKa ~ 10 and are stronger acids than alcohols. RS-H + HO– RS– + H-OH (pKa ~10) (pKa ~15.7)
43. RS– and HS – are weakly basic and strong nucleophiles.Throats react with 1° and 2° alkyl halides to yield sulfides (SN2).
44. Thiols can be oxidized to disulfides