8Chapter 22

8Chapter 22 - Chapter 22 Phenols Dr. Wolf's CHM 201...

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Unformatted text preview: Chapter 22 Phenols Dr. Wolf's CHM 201 & 202 22 - 1 22.1 Nomenclature Dr. Wolf's CHM 201 & 202 22 - 2 Nomenclature Nomenclature OH OH CH3 5-Chloro-2-methylphenol Cl named on basis of phenol as parent substituents listed in alphabetical order lowest numerical sequence: first point of difference rule Dr. Wolf's CHM 201 & 202 22 - 3 Nomenclature Nomenclature OH OH OH OH OH OH OH OH OH 1,2-Benzenediol (common name: pyrocatechol) Dr. Wolf's CHM 201 & 202 1,3-Benzenediol (common name: resorcinol) 1,4-Benzenediol (common name: hydroquinone) 22 - 4 Nomenclature Nomenclature OH OH p-Hydroxybenzoic acid CO2H name on basis of benzoic acid as parent higher oxidation states of carbon outrank hydroxyl group Dr. Wolf's CHM 201 & 202 22 - 5 22.2 Structure and Bonding Dr. Wolf's CHM 201 & 202 22 - 6 Structure of Phenol Structure of Phenol phenol is planar C—O bond distance is 136 pm, which is slightly shorter than that of CH3OH (142 pm) Dr. Wolf's CHM 201 & 202 22 - 7 22.3 Physical Properties The OH group of phenols allows hydrogen bonding to other phenol molecules and to water. Dr. Wolf's CHM 201 & 202 22 - 8 Hydrogen Bonding in Phenols Hydrogen Bonding in Phenols OH Dr. Wolf's CHM 201 & 202 O 22 - 9 Physical Properties (Table 24.1) Physical Properties (Table 24.1) Compared to compounds of similar size and molecular weight, hydrogen bonding in phenol molecular raises its melting point, boiling point, and raises solubility in water. Dr. Wolf's CHM 201 & 202 22 - 10 Physical Properties (Table 24.1) Physical Properties (Table 24.1) C6H5CH3 C6H5OH C6H5F Molecular weight 92 94 96 Melting point (°C) –95 43 –41 Boiling point (°C,1 atm) 111 132 85 Solubility in H2O (g/100 mL,25°C) 0.05 8.2 0.2 Dr. Wolf's CHM 201 & 202 22 - 11 22.4 Acidity of Phenols most characteristic property of most phenols is their acidity phenols Dr. Wolf's CHM 201 & 202 22 - 12 Compare Compare •• •O • H Ka = 10-10 •• CH3CH2O •• Dr. Wolf's CHM 201 & 202 •• – •O • •• ++ H Ka = 10-16 H ++ H •• – CH3CH2O • ••• 22 - 13 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion – •• •O • H • H • H H H Dr. Wolf's CHM 201 & 202 22 - 14 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion – •• •O • H • H • •• H H H Dr. Wolf's CHM 201 & 202 H •O • –H •• H H H 22 - 15 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion •• H •O • –H •• H H H Dr. Wolf's CHM 201 & 202 22 - 16 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion •• H •O • •• H •• H –H H Dr. Wolf's CHM 201 & 202 H •O • –H •• H H H 22 - 17 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion •• H •O • H •• H –H H Dr. Wolf's CHM 201 & 202 22 - 18 Delocalized negative charge in phenoxide ion Delocalized negative charge in phenoxide ion Delocalized •• H •O • •• H H –H H •• H H Dr. Wolf's CHM 201 & 202 •O • – •• H H H 22 - 19 Phenols are converted to phenoxide ions Phenols are converted to phenoxide ions in aqueous base in aqueous base •• •O • •• – •O • •• H – + HO stronger acid Dr. Wolf's CHM 201 & 202 + H2O weaker acid 22 - 20 22.5 Substituent Effects on the Acidity of Phenols Dr. Wolf's CHM 201 & 202 22 - 21 Electron-releasing groups have little or no effect Electron-releasing groups have little or no effect Ka: 1 x 10-10 Dr. Wolf's CHM 201 & 202 OH OH OH OH CH3 OH OH OCH3 5 x 10-11 6 x 10-11 22 - 22 Electron-withdrawing groups increase acidity Electron-withdrawing groups increase acidity Ka: 1 x 10-10 Dr. Wolf's CHM 201 & 202 OH OH OH OH Cl OH OH NO2 4 x 10-9 7 x 10-8 22 - 23 Effect of electron-withdrawing groups is most Effect of electron-withdrawing groups is most pronounced at ortho and para positions pronounced at ortho and para positions OH OH OH OH OH OH NO2 NO2 NO2 Ka: 6 x 10-8 Dr. Wolf's CHM 201 & 202 4 x 10-9 7 x 10-8 22 - 24 Effect of strong electron-withdrawing groups Effect of strong electron-withdrawing groups is cumulative is cumulative OH OH OH OH OH OH NO2 NO2 Ka: 7 x 10-8 Dr. Wolf's CHM 201 & 202 NO2 1 x 10-4 NO2 O2 N NO2 4 x 10-1 22 - 25 Resonance Depiction Resonance Depiction – •• •O • • • H •O • •• N + Dr. Wolf's CHM 201 & 202 H H H H •• •O • •• H •• O• ••• – H H •• N •• + •O – •• • •• O• ••• – 22 - 26 22.6 Sources of Phenols Phenol is an important industrial chemical. Major use is in phenolic resins for adhesives Major and plastics. and Annual U.S. production is about 4 billion Annual pounds per year. pounds Dr. Wolf's CHM 201 & 202 22 - 27 Industrial Industrial Preparations Preparations of Phenol of Phenol SO3H Cll C 1. NaOH 2. H+ 1. heat 1. NaOH heat OH OH 2. H+ Dr. Wolf's CHM 201 & 202 CH(CH3)2 CH(CH 1. O2 2. H2O H2SO4 22 - 28 Laboratory Synthesis of Phenols Laboratory Synthesis of Phenols from arylamines via diazonium ions O2N NH2 1. NaNO2, H2SO4, H2O O2N OH 2. H2O, heat (81-86%) Dr. Wolf's CHM 201 & 202 22 - 29 22.7 Naturally Occurring Phenols Many phenols occur naturally Dr. Wolf's CHM 201 & 202 22 - 30 Example: Thymol Example: Thymol OH CH3 CH(CH3)2 Thymol (major constituent of oil of thyme) Dr. Wolf's CHM 201 & 202 22 - 31 Example: 2,5-Dichlorophenol Example: 2,5-Dichlorophenol OH Cl Cl 2,5-Dichlorophenol (from defensive secretion of a species of grasshopper) Dr. Wolf's CHM 201 & 202 22 - 32 22.8 Reactions of Phenols: Electrophilic Aromatic Electrophilic Substitution Substitution Hydroxyl group strongly activates the ring toward electrophilic aromatic substitution Dr. Wolf's CHM 201 & 202 22 - 33 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 34 Halogenation Halogenation OH OH + Br2 ClCH2CH2Cl 0°C Br Br (93%) monohalogenation in nonpolar solvent (1,2-dichloroethane) Dr. Wolf's CHM 201 & 202 22 - 35 Halogenation Halogenation OH OH + 3Br2 F H2O Br Br Br 25°C F Br (95%) multiple halogenation in polar solvent (water) Dr. Wolf's CHM 201 & 202 22 - 36 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 37 Nitration Nitration OH OH NO2 HNO3 acetic acid 5°C CH3 CH OH group controls OH regiochemistry regiochemistry Dr. Wolf's CHM 201 & 202 CH3 (73-77%) 22 - 38 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 39 Nitrosation Nitrosation NO OH OH NaNO2 H2SO4, H2O 0°C only strongly activated only rings undergo nitrosation when treated with nitrous acid acid Dr. Wolf's CHM 201 & 202 (99%) 22 - 40 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 41 Sulfonation Sulfonation OH H3C OH CH3 H2SO4 H3C CH3 100°C SO3H OH group controls regiochemistry regiochemistry Dr. Wolf's CHM 201 & 202 (69%) 22 - 42 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 43 Friedel-Crafts Alkylation Friedel-Crafts Alkylation OH OH CH3 CH3 CH (CH3)3COH H3PO4 60°C H3C (CH3)3COH reacts COH with H3PO4 to give with (CH3)3C+ (CH Dr. Wolf's CHM 201 & 202 C CH3 CH3 (63%) 22 - 44 Electrophilic Aromatic Substitution in Phenols Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 22 - 45 22.9 Acylation of Phenols Acylation can take place either on the ring by electrophilic aromatic substitution or on oxygen by nucleophilic acyl substitution Dr. Wolf's CHM 201 & 202 22 - 46 Friedel-Crafts Acylation Friedel-Crafts Acylation OH OH O CH3CCl + ortho isomer AlCl3 under Friedel-Crafts under conditions, acylation of the ring occurs of (C-acylation) Dr. Wolf's CHM 201 & 202 O C CH3 (74%) (16%) 22 - 47 O-Acylation O-Acylation O OH OC(CH2)6CH3 O + CH3(CH2)6CCl (95%) in the absence of AlCl3, acylation of the acylation hydroxyl group occurs (O-acylation) hydroxyl Dr. Wolf's CHM 201 & 202 22 - 48 O- versus C-Acylation O- versus C-Acylation O OH OC(CH2)6CH3 AlCl3 formed faster O C (CH2)6CH3 more stable O-Acylation is kinetically controlled process; C-acylation is thermodynamically controlled AlCl3 catalyzes the conversion of the aryl ester to the aryl alkyl ketones; this is called the Fries rearrangement Dr. Wolf's CHM 201 & 202 22 - 49 2 2.10 Carboxylation of Phenols O Aspirin and Aspirin the Kolbe-Schmitt Reaction Reaction OCCH3 COH O Dr. Wolf's CHM 201 & 202 22 - 50 Aspirin is prepared from salicylic acid Aspirin is prepared from salicylic acid OO OH COH COH CH3COCCH3 H2SO4 O O OCCH3 COH O how is salicylic acid prepared? Dr. Wolf's CHM 201 & 202 22 - 51 Preparation of Salicylic Acid Preparation of Salicylic Acid ONa CO2 125°C, 100 atm OH CONa CONa O called the Kolbe-Schmitt reaction acidification converts the sodium salt shown above to salicylic acid Dr. Wolf's CHM 201 & 202 22 - 52 What Drives the Reaction? What Drives the Reaction? acid-base considerations provide an explanation: stronger base on left; weaker base on right ••– O• •• •+ •• CO2 H C stronger base: pKa of conjugate acidCHM 201 Dr.acid = 10& 202 Wolf's O ••– O• •• •• •O • •• weaker base: • pKa of conjugate acid = 3 acid 22 - 53 Preparation of Salicylic Acid Preparation of Salicylic Acid ONa CO2 OH 125°C, 100 atm CONa CONa O how does carbon-carbon bond form? recall electron delocalization in phenoxide ion negative charge shared by oxygen and by the ring carbons that are ortho and para to oxygen Dr. Wolf's CHM 201 & 202 22 - 54 – •• •O • • • •• H H H H H H •O • H Dr. Wolf's CHM 201 & 202 H H H •• H •• H •• •• – –H H H H •O • •O • H H •• –H H 22 - 55 Mechanism of ortho Carboxylation Mechanism of ortho Carboxylation •• •O – •• C • O• •• • H Dr. Wolf's CHM 201 & 202 O• •• •• O• ••– O• • •• C • • H •O • •• 22 - 56 Mechanism of ortho Carboxylation Mechanism of ortho Carboxylation •• •O – •• C • O• •• • H •• O• ••– O• • •• C • O• •• • •• H O H C ••– O• •• •• Dr. Wolf's CHM 201 & 202 •O • •• •O • • 22 - 57 Why ortho? Why ortho? Why Why not para? Why not para? Why Why •• •• – O• •• • •• OH •• O H C ••– O• •• •• •O • •• Dr. Wolf's CHM 201 & 202 • – •• •O C •• • •O • • • 22 - 58 Why ortho? Why ortho? Why Why not para? Why not para? Why Why •• H C ••– O• •• •O • •• • weaker base: pKa of conjugate acid = 3 Dr. Wolf's CHM 201 & 202 • •• OH •• O •• •• – O• •• – •• •O C •• • •O • • stronger •base: pKa of conjugate acid = 4.5 22 - 59 Intramolecular Hydrogen Bonding Intramolecular Hydrogen Bonding in Salicylate Ion in Salicylate Ion O H C O– O Hydrogen bonding between carboxylate and hydroxyl group stabilizes salicylate ion. Salicylate is less basic group than para isomer and predominates under conditions than of thermodynamic control. Dr. Wolf's CHM 201 & 202 22 - 60 22.11 Preparation of Aryl Ethers Dr. Wolf's CHM 201 & 202 22 - 61 Typical Preparation is by Williamson Synthesis Typical Preparation is by Williamson Synthesis ONa + RX Dr. Wolf's CHM 201 & 202 SN2 OR + NaX 22 - 62 Typical Preparation is by Williamson Synthesis Typical Preparation is by Williamson Synthesis ONa + RX SN2 OR + NaX but the other combination X + RONa fails because aryl halides are normally unreactive toward nucleophilic substitution Dr. Wolf's CHM 201 & 202 22 - 63 Example Example ONa + CH3I acetone heat OCH3 (95%) Dr. Wolf's CHM 201 & 202 22 - 64 Example Example OH K2CO3 + H2C acetone, heat OCH2CH Dr. Wolf's CHM 201 & 202 CHCH2Br (86%) CH2 22 - 65 Aryl Ethers from Aryl Halides Aryl Ethers from Aryl Halides F OCH3 + KOCH3 NO2 CH3OH + KF 25°C NO2 (93%) nucleophilic aromatic substitution is effective nucleophilic with nitro-substituted (ortho and/or para) aryl halides Dr. Wolf's CHM 201 & 202 22 - 66 22.12 Cleavage of Aryl Ethers by Hydrogen Halides Dr. Wolf's CHM 201 & 202 22 - 67 Cleavage of Alkyl Aryl Ethers Cleavage of Alkyl Aryl Ethers Cleavage Alkyl Aryl Cleavage Alkyl Aryl Ar R •• •• O• + H • Dr. Wolf's CHM 201 & 202 •• Br • •• • •• – •Br • + •• • • Ar •• +O H R 22 - 68 Cleavage of Alkyl Aryl Ethers Cleavage of Alkyl Aryl Ethers Cleavage Alkyl Aryl Cleavage Alkyl Aryl Ar R •• •• O• + H • Br • •• • An alkyl halide is An alkyl formed; never an aryl halide! R aryl Dr. Wolf's CHM 201 & 202 •• – •Br • + •• • • •• Ar •• Br • •• • + Ar •• +O H R •• O •• H 22 - 69 Example Example OCH3 OH HBr heat OH OH + CH3Br OH (85-87%) Dr. Wolf's CHM 201 & 202 (57-72%) 22 - 70 22.13 Claisen Rearrangement of Allyl Aryl Ethers Dr. Wolf's CHM 201 & 202 22 - 71 Allyl Aryl Ethers Rearrange on Heating Allyl Aryl Ethers Rearrange on Heating OCH2CH OCH CH2 200°C OH OH CH2CH allyl group allyl migrates to ortho position ortho CH2 (73%) Dr. Wolf's CHM 201 & 202 22 - 72 Mechanism Mechanism Mechanism OCH2CH OCH CH2 O rewrite as rewrite OH OH keto-to-enol keto-to-enol isomerization O H Dr. Wolf's CHM 201 & 202 22 - 73 Sigmatropic Rearrangement Sigmatropic Rearrangement Claisen rearrangement is an example of a Claisen sigmatropic rearrangement. A σ bond migrates from one end of a conjugated π electron system from to the other. this σ bond breaks this O O “conjugated π conjugated electron system” is the allyl group is H this σ bond forms this Dr. Wolf's CHM 201 & 202 22 - 74 22.14 Oxidation of Phenols: Quinones Dr. Wolf's CHM 201 & 202 22 - 75 Quinones Quinones The most common examples of phenol oxidations are the oxidations of 1,2- and 1,4-benzenediols to give quinones. to OH O OH Na2Cr2O7, H2SO4 H2O OH O (76-81%) (76-81%) Dr. Wolf's CHM 201 & 202 22 - 76 Quinones Quinones The most common examples of phenol oxidations are the oxidations of 1,2- and 1,4-benzenediols to give quinones. to OH O OH O Ag2O diethyl ether CH3 CH CH3 CH (68%) (68%) Dr. Wolf's CHM 201 & 202 22 - 77 Some quinones are dyes Some quinones are dyes O OH OH O Alizarin (red pigment) Dr. Wolf's CHM 201 & 202 22 - 78 Some quinones are important biomolecules Some quinones are important biomolecules O CH3 CH3O CH3O n O Ubiquinone (Coenzyme Q) n = 6-10 involved in biological electron transport Dr. Wolf's CHM 201 & 202 22 - 79 Some quinones are important biomolecules Some quinones are important biomolecules O CH3 CH3 O CH3 CH3 CH3 CH3 Vitamin K (blood-clotting factor) Dr. Wolf's CHM 201 & 202 22 - 80 Section 22.15 Spectroscopic Analysis of Phenols Dr. Wolf's CHM 201 & 202 22 - 81 Infrared Spectroscopy Infrared Spectroscopy iinfrared spectra of phenols combine features nfrared of alcohols and aromatic compounds of O—H stretch analogous to alcohols near 3600 cm-1 C—O stretch at 1200-1250 cm-1 Dr. Wolf's CHM 201 & 202 22 - 82 Figure 24.3: Infrared Spectrum of p-Cresol Figure 24.3: Infrared Spectrum of p-Cresol CH3 C—H OH C—O O—H 3500 3000 2500 2000 1500 1000 500 Wave number, cm-1 Dr. Wolf's CHM 201 & 202 22 - 83 H NMR H NMR 11 Hydroxyl proton of OH group lies between alcohols and carboxylic acids; range is ca. δ 4-12 ppm and (depends on concentration). For p-cresol the OH (depends proton appears at δ 5.1 ppm (Figure 24.4). proton H H CH3 CH HO H Dr. Wolf's CHM 201 & 202 H 22 - 84 H H HO CH3 H 10.0 9.0 8.0 7.0 6.0 H 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (δ, ppm) Dr. Wolf's CHM 201 & 202 22 - 85 C NMR C NMR 13 13 OH 155.1 112..3 116.1 139.8 129.4 121.7 CH3 21.3 CH Oxygen of hydroxyl group deshields carbon to which it is directly attached. The most shielded carbons of the ring are those that are ortho and para to the oxygen. Dr. Wolf's CHM 201 & 202 22 - 86 UV-VIS UV-VIS Oxygen substitution on ring shifts λmax to longer Oxygen wavelength; effect is greater in phenoxide ion. OH OH O λmax λmax λmax 204 nm 210 nm 235 nm 256 nm 270 nm – 287 nm Dr. Wolf's CHM 201 & 202 22 - 87 Mass Spectrometry Mass Spectrometry Prominent peak for molecular ion. Most intense Prominent peak in phenol is for molecular ion. peak •+ OH OH •• m/z 94 m/z 94 Dr. Wolf's CHM 201 & 202 22 - 88 End of Chapter 22 Dr. Wolf's CHM 201 & 202 22 - 89 ...
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