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Lecture 29 Organic Chemistry 1 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Lecture 29 Organic Chemistry 1 Professor Duncan Wardrop April 27, 2010 1 Today’s Lecture Topics Covered: 1. Phenol - Bonding, Physical Properties and Reactions 2. Electrophilic Aromatic Substitution: Halogen, Nitration, Nitrosation 3. O- and C-Acylation of Phenols: Fries Rearrangement 4. Kolbe-Schmitt Reaction: Carboxylation of Phenols 5. Preparation and Cleavage of Aryl Alkyl Ethers 2 Chapter 24 Phenols 3 24.1 Nomenclature 4 Nomenclature of Phenols 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 5 Nomenclature of Hydroxyphenols OH OH OH OH OH OH 1,2-Benzenediol 1,3-Benzenediol 1,4-Benzenediol (common name: (common name: (common name: pyrocatechol) resorcinol) hydroquinone) 6 Catechols are Biologically Important O OH NH2 Epinephrine HO is the principal hormone Tyrosine governing the "fight or flight" H response. This hormone also HO N triggers a variety of physiological H events, including increased heart rate. It is biosynthesized from HO Dopamine from the amino acid tyrosine by way of DOPA. OH H HO N http://www.ndrf.org/catechol.htm CH3 HO Epinephrine 7 Nomenclature of Hydroxyphenols OH p-Hydroxybenzoic acid CO2H name on basis of benzoic acid as parent higher oxidation states of carbon outrank hydroxyl group 8 24.2 Structure and Bonding 9 Structure of Phenol OH phenol is planar C—O bond distance is 136 pm, which is slightly shorter than that of CH3OH (142 pm) 10 24.3 Physical Properties 11 Hydrogen Bonding in Phenol O H O The hydroxyl group of phenols allows hydrogen bonding to other phenol molecules and to water. 12 Physical Properties of Phenol Compared to compounds of similar size and molecular weight, hydrogen bonding in phenol raises its melting point, boiling point, and solubility in water. 13 Physical Properties of Phenol C6H5CH3 C6H5OH C6H5F Molecular weight 92 94 96 Melting point (°C) –95 43 –41 Boiling 111 132 85 point (°C,1 atm) Solubility in 0.05 8.2 0.2 H2O (g/100 mL,25°C) 14 Carbolic Acid & the History of Antisepsis 15 24.4 Acidity of Phenols 16 Comparative Acidity of Phenol H O O Stabilized by H + solvation and resonance pKa = 10 phenoxide anion Stabilized by H + H C O H3C O H 3 solvation alone pKa = 16 alkoxide anion 17 The Phenoxide Anion is Stabilized Through Resonance O O O H H H H H H 18 Phenols are Converted to Phenoxide Ions in Aqueous Base H O O + HO H2O + weaker stronger stronger acid weaker acid base base 19 24.5 Substituent Effects on the Acidity of Phenols 20 Electron-Releasing Substituents have Little Effect on the pKa of Phenols OH OH OH H CH3 OCH3 pKa 10 pKa 10.3 pKa 10.2 21 Electron-Withdrawing Substituents Lower the pKa of Phenols OH OH OH H Cl N+ –O O pKa 10 pKa 9.4 pKa 7.2 22 Effect of Electron-Withdrawing Groups is Most Pronounced at Ortho and Para Positions OH O OH OH N+ O– O N+ O– N+ –O O pKa 7.2 pKa 8.4 pKa 7.2 23 Direct Conjugation of the Negatively Charged Group with the Nitro Substituents is Possible O O O O O O N+ N+ O– O– N N O O O O Consequence? Electron-withdrawing substituents (inc. NO2) have larger effect on the pKa of a phenol group when in an ortho or para relationship 24 Direct Conjugation of the Negatively Charged Group with the Nitro Substituents is not Possible O O O O O O + + N+ N N – – O– O O Consequence? Electron-withdrawing substituents (inc. NO2) have a smaller effect on the pKa of a phenol group when in a meta relationship 25 The Effect of Electron-Withdrawing Substituents on pKa of Phenols is Cumulative OH OH OH NO2 O2N NO2 NO2 NO2 NO2 pKa 7.2 pKa 4.0 pKa 0.4 Picric Acid 26 24.6 Sources of Phenols 27 Phenol - An Industrial Chemical Phenol is an important industrial chemical. Major use is in phenolic resins for adhesives and plastics. Annual U.S. production is about 4 billion pounds per year. 28 Industrial Preparations Benzene of Phenol SO3H Cl CH(CH3)2 1. NaOH 1. NaOH heat 1. O2 heat + 2. H 2. H2O, H2SO4 2. H+ OH 29 Preparation of Phenol from Diazonium Salts N H H N HSO4 N OH NaNO2 H2O H2SO4, H2O heat Me Me Me Me Me Me Primary Aryl Diazonium Phenol Arylamine Salt Remember - reaction proceeds via aryl cation 30 24.7 Naturally Occurring Phenols 31 An Antiseptic with a Pleasant Smell Thymol OH CH3 a major constituent of oil of thyme, was used CH in ancient Egyptian 3 religious ceremonies H3C 32 A Red Pigment Isolated from ‘Young’ Red Wine OCH3 OH Malvin this phenolic compound HO O belongs to a family of plant OCH3 p i g m e n t s c a l l e d anthocyanins OGlc OGlc 33 A Major Hormone of the Thyroid Gland I I O NH2 O HO I I OH Thyroxine is one of the major hormones secreted by the human thyroid gland. Its principle function is to stimulate the metabolism of cells 34 An Antibiotic OH Me NMe2 H H OH NH2 OH OH O OH O O Tetracycline the tetracyclines are a family of antibiotics produced by various strains of microorganisms of the genus Streptomyces. Bacterial resistance to these antibiotics is a major problem 35 24.8 Reactions of Phenols: Electrophilic Aromatic Substitution 36 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 37 Electrophilic Aromatic Substitution H H E -H+ E E Sigma Complex or Wheland Intermediate OH groups on benzene rings are ortho, para- directing and strongly activating 38 Halogenation of Phenols - Non-Polar Solvents OH OH ClCH2CH2Cl + Br2 0°C (93%) Br monohalogenation occurs in non-polar solvents (1,2-dichloroethane) 39 Halogenation of Phenols - Polar Solvents OH OH Br Br H2O + 3 x Br2 25°C F F (95%) Br multiple halogenation in polar solvent (water) 40 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 41 Nitration of Phenols OH OH NO2 HNO3 acetic acid 5°C (73-77%) CH3 CH3 the OH group is more electron donating than the methyl group and consequently controls the Active Electrophile O N O regiochemistry of this reaction 42 Nitration of Phenols OH OH NO2 HNO3 O OH O OH Hydroxyl groups are ortho, para-directing, while carboxylate groups are meta-directing. In this example, these effects reinforce each other and a single product is obtained 43 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 44 Nitrosation of Phenols NO OH OH NaNO2 H2SO4, H2O 0°C (99%) only strongly activated rings Active Electrophile O N undergo nitrosation when treated with nitrous acid 45 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 46 Sulfonation of Phenols OH OH H3C CH3 H3C CH3 H2SO4 100°C (69%) O S O OH O O Possible H H Active S O O S O Electrophiles O O H 47 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 48 Friedel-Crafts Alkylation OH OH Me OH Me Me Me Me Me Me Me Me H3PO4 60°C (63%) Me Me BHT (butylated hydroxy toluene) OH Me Active H+ Electrophile Me Me Me Me Me 49 Phenol-Formaldehyde Resins OH OH H H H O HCl O OH H H AcOH H H Friedel-Crafts Alkylation Formaldehyde H+ -H2O OH OH OH OH OH H H H H H H Friedel-Crafts Alkylations Polymer 50 Bakelite - The First Commerical Synthetic Polymer OH OH OH OH HO OH HO HO http://www.bakelite.de/eng/ 51 Electrophilic Aromatic Substitution in Phenols Halogenation Nitration Nitrosation Sulfonation Friedel-Crafts Alkylation Friedel-Crafts Acylation 52 24.9 Acylation of Phenols 53 Phenol Acylation: A Question of Regioselectivity and Chemoselectivity OH O O R R O O OH O O-Acylation R X R X via Electrophilic OH2 Aromatic acylating acylating Subsitution agent agent O-Acylation via Nucleophilic Acyl Subsitution O R Acylation of phenolic compounds can take place either on the ring by electrophilic aromatic substitution or on oxygen by nucleophilic acyl substitution 54 Friedel-Crafts Conditions Yield Aryl Ketones via C-Acylation O OH OH OH O H C Cl 3 CH3 AlCl3 (17%) O CH3 (74%) Active Electrophile H3C C O AlCl4 acylium ion 55 ‘Unactivated’ Acylating Reagents Provide Aryl Esters via O-Acylation O O OH O CH3 H15C7 Cl (95%) in the absence of AlCl3, acylation of the hydroxyl group occurs (O-acylation) 56 O-Acylation vs. C-Acylation O O CH3 O-Acylation is kinetically controlled process; Formed faster C-acylation is ('Kinetic Product') thermodynamically controlled AlCl3 OH AlCl3 catalyzes the conversion of the aryl ester to the aryl alkyl More stable ketones; this is called ('Thermodynamic Product') O 57 Mechanism of Fries Rearrangement O Cl O C Cl O R Al C Cl O R AlCl3 Cl Al OH O Cl O H O O R O Cl R R C 58 Albuterol Mimics Epinephrine Alleviates the Symptoms of Asthma OH H OH H N N CH3 HO CH3 HO H CH3 HO HO Epinephrine Isoprotenerol OH H N CH3 HO CH3 CH3 HO Salbutamol 59 Synthesis of Albuterol O O O HO AlCl3 HO CH3 O Fries Rearrangement HO H C O 3 H+ (cat) Aspirin O O O OH Br HO HBr HO HO HO 60 Synthesis of Albuterol Ph Ph O O O O HN Br N HO HO Substitution HO HO Carboxylic Acid LiAlH Ketone 4 Reduction Ph OH H OH N N HO H2 HO Pd-C HO Hydrogenolysis HO Salbutamol 61 Difference Between Hydrogenation and Hydrogenolysis 62 24.10 Carboxylation of Phenols 63 Aspirin is Prepared Through O-Acylation of Salicylic Acid O CH3 O O OH O H3C O CH3 OH OH H2SO4 O O Salicylic Acid Aspirin how is salicylic acid prepared? 64 Aspirin & the Kolbe-Schmitt Reaction O Na OH OH + CO2 H3O 125 °C O Na OH 100 atm O O
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