Organic chemistry 9e franics a carey

Organic Chemistry NINTH EDITION Francis A Carey University of Virginia Robert M Giuliano Villanova University TM ORGANIC CHEMISTRY, NINTH EDITION Published by McGraw-Hill, a business unit of The McGraw-Hill Companies, Inc., 1221 Avenue of the Americas, New York, NY 10020 Copyright © 2014 by The McGraw-Hill Companies, Inc All rights reserved Printed in the United States of America Previous editions © 2011, 2008, and 2006 No part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written consent of The McGraw-Hill Companies, Inc., including, but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning Some ancillaries, including electronic and print components, may not be available to customers outside the United States This book is printed on acid-free paper DOW/DOW ISBN 978_0_07_340274_1 MHID 0_07_340274_5 Senior Vice President, Products & 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has been requested from the Library of Congress The Internet addresses listed in the text were accurate at the time of publication The inclusion of a website does not indicate an endorsement by the authors or McGraw-Hill, and McGraw-Hill does not guarantee the accuracy of the information presented at these sites www.mhhe.com Each of the nine editions of this text has benefited from the individual and collective contributions of the staff at McGraw-Hill They are the ones who make it all possible We appreciate their professionalism and thank them for their continuing support About the Authors Prior to retiring in 2000, Frank Carey’s career teaching chemistry was spent entirely at the University of Virginia In addition to this text, he is coauthor (with Robert C Atkins) of Organic Chemistry: A Brief Course and (with Richard J Sundberg) of Advanced Organic Chemistry, a twovolume treatment designed for graduate students and advanced undergraduates Frank and his wife Jill, who is a teacher/director of a preschool and a church organist, are the parents of Andy, Bob, and Bill and the grandparents of Riyad, Ava, Juliana, Miles, and Wynne Robert M Giuliano was born in Altoona, Pennsylvania and attended Penn State (B.S in chemistry) and the University of Virginia (Ph.D., under the direction of Francis Carey) Following postdoctoral studies with Bert Fraser-Reid at the University of Maryland, he joined the chemistry department faculty of Villanova University in 1982, where he is currently Professor His research interests are in synthetic organic and carbohydrate chemistry, and in functionalized carbon nanomaterials Bob and his wife Margot, an elementary and preschool teacher he met while attending UVa, are the parents of Michael, Ellen, and Christopher and grandparents of Carina and Aurelia iv Brief Contents List of Important Features xvi Preface xx Acknowledgements xxvi 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Structure Determines Properties Alkanes and Cycloalkanes: Introduction to Hydrocarbons 52 Alkanes and Cycloalkanes: Conformations and cis–trans Stereoisomers 96 Alcohols and Alkyl Halides: Introduction to Reaction Mechanisms 132 Structure and Preparation of Alkenes: Elimination Reactions 176 Addition Reactions of Alkenes 216 Chirality 262 Nucleophilic Substitution 306 Alkynes 342 Conjugation in Alkadienes and Allylic Systems 370 Arenes and Aromaticity 406 Electrophilic and Nucleophilic Aromatic Substitution 456 Spectroscopy 510 Organometallic Compounds 578 Alcohols, Diols, and Thiols 614 Ethers, Epoxides, and Sulfides 650 Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group 686 Carboxylic Acids 736 Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution 770 Enols and Enolates 820 Amines 858 Phenols 914 Carbohydrates 946 Lipids 992 Amino Acids, Peptides, and Proteins 1030 Nucleosides, Nucleotides, and Nucleic Acids 1084 Synthetic Polymers 1122 Glossary G-1 Credits C-1 Index I-1 v This page intentionally left blank Contents List of Important Features xvi Preface xx Acknowledgements xxvi C H A P T E R 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 Structure Determines Properties 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 Atoms, Electrons, and Orbitals Organic Chemistry: The Early Days Ionic Bonds Covalent Bonds, Lewis Formulas, and the Octet Rule Double Bonds and Triple Bonds Polar Covalent Bonds, Electronegativity, and Bond Dipoles 10 Electrostatic Potential Maps 13 Formal Charge 13 Structural Formulas of Organic Molecules 15 Resonance 19 Sulfur and Phosphorus-Containing Organic Compounds and the Octet Rule 23 The Shapes of Some Simple Molecules 24 Molecular Models And Modeling 25 Molecular Dipole Moments 27 Curved Arrows and Chemical Reactions 28 Acids and Bases: The Brønsted–Lowry View 30 How Structure Affects Acid Strength 35 Acid–Base Equilibria 39 Lewis Acids and Lewis Bases 41 Summary 43 Problems 46 Descriptive Passage and Interpretive Problems 1: Amide Lewis Structural Formulas 51 C H A P T E R Alkanes and Cycloalkanes: Introduction to Hydrocarbons 52 2.1 2.2 2.3 2.4 2.5 Classes of Hydrocarbons 53 Electron Waves and Chemical Bonds 53 Bonding in H2: The Valence Bond Model 55 Bonding in H2: The Molecular Orbital Model 56 Introduction to Alkanes: Methane, Ethane, and Propane 57 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 sp3 Hybridization and Bonding in Methane 58 Methane and the Biosphere 59 Bonding in Ethane 61 sp2 Hybridization and Bonding in Ethylene 61 sp Hybridization and Bonding in Acetylene 63 Which Theory of Chemical Bonding Is Best? 64 Isomeric Alkanes: The Butanes 65 Higher n-Alkanes 66 The C5H12 Isomers 66 IUPAC Nomenclature of Unbranched Alkanes 68 Applying the IUPAC Rules: The Names of the C6H14 Isomers 69 What’s in a Name? Organic Nomenclature 70 Alkyl Groups 72 IUPAC Names of Highly Branched Alkanes 73 Cycloalkane Nomenclature 75 Sources of Alkanes and Cycloalkanes 76 Physical Properties of Alkanes and Cycloalkanes 77 Chemical Properties: Combustion of Alkanes 80 Thermochemistry 83 Oxidation–Reduction in Organic Chemistry 83 Summary 86 Problems 90 Descriptive Passage and Interpretive Problems 2: Some Biochemical Reactions of Alkanes 94 C H A P T E R Alkanes and Cycloalkanes: Conformations and cis–trans Stereoisomers 96 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 Conformational Analysis of Ethane 97 Conformational Analysis of Butane 101 Conformations of Higher Alkanes 102 Computational Chemistry: Molecular Mechanics and Quantum Mechanics 103 The Shapes of Cycloalkanes: Planar or Nonplanar? 104 Small Rings: Cyclopropane and Cyclobutane 105 Cyclopentane 106 Conformations of Cyclohexane 107 Axial and Equatorial Bonds in Cyclohexane 108 Conformational Inversion in Cyclohexane 109 Conformational Analysis of Monosubstituted Cyclohexanes 110 Enthalpy, Free Energy, and Equilibrium Constant 113 Disubstituted Cycloalkanes: cis–trans Stereoisomers 114 Conformational Analysis of Disubstituted Cyclohexanes 115 vii viii 3.13 3.14 3.15 3.16 Contents Medium and Large Rings 119 Polycyclic Ring Systems 119 Heterocyclic Compounds 122 Summary 123 Problems 126 Descriptive Passage and Interpretive Problems 3: Cyclic Forms of Carbohydrates 131 C H A P T E R 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 Alcohols and Alkyl Halides: Introduction to Reaction Mechanisms 132 5.13 4.1 4.2 4.3 4.4 4.5 4.6 5.14 5.15 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 Functional Groups 133 IUPAC Nomenclature of Alkyl Halides 134 IUPAC Nomenclature of Alcohols 135 Classes of Alcohols and Alkyl Halides 136 Bonding in Alcohols and Alkyl Halides 136 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces 137 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides 141 Reaction of Alcohols with Hydrogen Halides: The SN1 Mechanism 142 Mechanism 4.1 Formation of tert-Butyl Chloride from tert-Butyl Alcohol and Hydrogen Chloride 143 Structure, Bonding, and Stability of Carbocations 149 Effect of Alcohol Structure on Reaction Rate 152 Reaction of Methyl and Primary Alcohols with Hydrogen Halides: The SN2 Mechanism 153 Mechanism 4.2 Formation of 1-Bromoheptane from 1-Heptanol and Hydrogen Bromide 154 Other Methods for Converting Alcohols to Alkyl Halides 155 Halogenation of Alkanes 156 Chlorination of Methane 156 Structure and Stability of Free Radicals 157 From Bond Enthalpies to Heats of Reaction 161 Mechanism of Methane Chlorination 161 Mechanism 4.3 Free-Radical Chlorination of Methane 162 Halogenation of Higher Alkanes 163 Summary 167 Problems 170 Descriptive Passage and Interpretive Problems 4: More About Potential Energy Diagrams 174 C H A P T E R Alkene Nomenclature 176 Structure and Bonding in Alkenes Ethylene 179 5.17 5.18 5.19 C H A P T E R 6.1 6.2 6.3 6.4 6.6 6.7 178 Addition Reactions of Alkenes 216 6.5 Structure and Preparation of Alkenes: Elimination Reactions 176 5.1 5.2 5.16 Isomerism in Alkenes 180 Naming Stereoisomeric Alkenes by the E–Z Notational System 181 Physical Properties of Alkenes 183 Relative Stabilities of Alkenes 184 Cycloalkenes 187 Preparation of Alkenes: Elimination Reactions 188 Dehydration of Alcohols 189 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule 190 Stereoselectivity in Alcohol Dehydration 191 The E1 and E2 Mechanisms of Alcohol Dehydration 191 Mechanism 5.1 The E1 Mechanism for Acid-Catalyzed Dehydration of tert-Butyl Alcohol 192 Rearrangements in Alcohol Dehydration 193 Mechanism 5.2 Carbocation Rearrangement in Dehydration of 3,3-Dimethyl-2-butanol 194 Mechanism 5.3 Hydride Shift in Dehydration of 1-Butanol 196 Dehydrohalogenation of Alkyl Halides 197 The E2 Mechanism of Dehydrohalogenation of Alkyl Halides 199 Mechanism 5.4 E2 Elimination of 1-Chlorooctadecane 200 Anti Elimination in E2 Reactions: Stereoelectronic Effects 202 Isotope Effects and the E2 Mechanism 204 The E1 Mechanism of Dehydrohalogenation of Alkyl Halides 205 Mechanism 5.5 The E1 Mechanism for Dehydrohalogenation of 2-Bromo-2-methylbutane 205 Summary 207 Problems 210 Descriptive Passage and Interpretive Problems 5: A Mechanistic Preview of Addition Reactions 215 Hydrogenation of Alkenes 216 Stereochemistry of Alkene Hydrogenation 217 Mechanism 6.1 Hydrogenation of Alkenes 218 Heats of Hydrogenation 219 Electrophilic Addition of Hydrogen Halides to Alkenes 221 Mechanism 6.2 Electrophilic Addition of Hydrogen Bromide to 2-Methylpropene 223 Rules, Laws, Theories, and the Scientific Method 225 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes 225 Acid-Catalyzed Hydration of Alkenes 226 Mechanism 6.3 Acid-Catalyzed Hydration of 2-Methylpropene 227 Thermodynamics of Addition–Elimination Equilibria 228 Contents 6.8 6.9 6.10 6.11 6.12 6.13 6.14 6.15 6.16 Hydroboration–Oxidation of Alkenes 231 Mechanism of Hydroboration–Oxidation 233 Mechanism 6.4 Hydroboration of 1-Methylcyclopentene 233 Mechanism 6.5 Oxidation of an Organoborane 235 Addition of Halogens to Alkenes 234 Mechanism 6.6 Bromine Addition to Cyclopentene 237 Epoxidation of Alkenes 239 Mechanism 6.7 Epoxidation of Bicyclo[2.2.1]-2heptene 240 Ozonolysis of Alkenes 241 Free-Radical Addition of Hydrogen Bromide to Alkenes 242 Mechanism 6.8 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243 Free-Radical Polymerization of Alkenes 245 Mechanism 6.9 Free-Radical Polymerization of Ethylene 245 Introduction to Organic Chemical Synthesis: Retrosynthetic Analysis 246 Ethylene and Propene: The Most Important Industrial Organic Chemicals 248 Summary 249 Problems 252 Descriptive Passage and Interpretive Problems 6: Oxymercuration 258 C H A P T E R Chirality 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 C H A P T E R Nucleophilic Substitution 306 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 262 Molecular Chirality: Enantiomers 263 The Chirality Center 265 Symmetry in Achiral Structures 266 Optical Activity 268 Absolute and Relative Configuration 269 The Cahn–Ingold–Prelog R–S Notational System 271 Fischer Projections 273 Properties of Enantiomers 275 The Chirality Axis 276 Chiral Drugs 277 Reactions That Create a Chirality Center 279 Chiral Molecules with Two Chirality Centers 282 Achiral Molecules with Two Chirality Centers 284 Chirality of Disubstituted Cyclohexanes 286 Molecules with Multiple Chirality Centers 287 Reactions That Produce Diastereomers 288 Resolution of Enantiomers 290 Stereoregular Polymers 293 Chirality Centers Other Than Carbon 294 Summary 295 Problems 298 Descriptive Passage and Interpretive Problems 7: Prochirality 304 ix Functional Group Transformation by Nucleophilic Substitution 307 Relative Reactivity of Halide Leaving Groups 309 The SN2 Mechanism of Nucleophilic Substitution 310 Mechanism 8.1 The SN2 Mechanism of Nucleophilic Substitution 311 Steric Effects and SN2 Reaction Rates 313 Nucleophiles and Nucleophilicity 315 Enzyme-Catalyzed Nucleophilic Substitutions of Alkyl Halides 317 The SN1 Mechanism of Nucleophilic Substitution 317 Mechanism 8.2 The SN1 Mechanism of Nucleophilic Substitution 318 Stereochemistry of SN1 Reactions 320 Carbocation Rearrangements in SN1 Reactions 321 Mechanism 8.3 Carbocation Rearrangement in the SN1 Hydrolysis of 2-Bromo-3-methylbutane 322 Effect of Solvent on the Rate of Nucleophilic Substitution 322 Substitution and Elimination as Competing Reactions 326 Nucleophilic Substitution of Alkyl Sulfonates 329 Nucleophilic Substitution and Retrosynthetic Analysis 332 Summary 333 Problems 335 Descriptive Passage and Interpretive Problems 8: Nucleophilic Substitution 340 C H A P T E R Alkynes 342 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 Sources of Alkynes 342 Nomenclature 344 Physical Properties of Alkynes 344 Structure and Bonding in Alkynes: sp Hybridization 344 Acidity of Acetylene and Terminal Alkynes 347 Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes 348 Preparation of Alkynes by Elimination Reactions 350 Reactions of Alkynes 352 Hydrogenation of Alkynes 352 Metal–Ammonia Reduction of Alkynes 354 Addition of Hydrogen Halides to Alkynes 354 Mechanism 9.1 Sodium–Ammonia Reduction of an Alkyne 355 Hydration of Alkynes 357 Mechanism 9.2 Conversion of an Enol to a Ketone 357 1130 Chapter 27 Synthetic Polymers restricting the movement of the polymer chains Vulcanized rubber is a lightly cross-linked elastomer; Bakelite can be so highly cross-linked as to be considered a single molecule 27.6 Classification of Polymers: Properties How a polymer responds to changes in temperature is important not only with respect to the conditions under which it can be used, but also in the methods by which it is transformed into a commercial product Thermoplastic polymers are the most common and are those that soften when heated At their glass transition temperature (Tg), thermoplastic polymers change from a glass to a flexible, rubbery state Past this point amorphous polymers are gradually transformed to a liquid as the temperature is raised Crystalline polymers undergo a second transition, liquefying only when the melting temperature (Tm) is reached Compare the behaviors of atactic, isotactic, and syndiotactic poly(methyl methacrylate) on being heated Poly(methyl methacrylate) Tg(؇C) Tm(؇C) atactic 114 — isotactic 48 160 syndiotactic 126 200 CO2CH3 C CH2 CH3 n The atactic form of poly(methyl methacrylate) is amorphous and exhibits only one transition temperature (Tg) The stereoregular isotactic and syndiotactic forms are partially crystalline and undergo both a glass transition and melting The process that takes place at Tg is an increase in the conformational mobility of the polymer chains At Tm, attractive forces in crystallites are broken and individual chains separate Melting temperature is an important factor in respect to how polymers are used The relatively low Tm for low-density polyethylene (115ЊC) makes it an easy polymer to cast into the desired shape when melted, but at the same time limits its applications When, for example, a container is required that must be sterilized by heating, the higher Tm of HDPE (137ЊC) makes it a better choice than LDPE Unlike thermoplastic polymers that soften on heating, thermosetting polymers (also called thermosetting resins) pass through a liquid state then solidify (“cure”) on continued heating The solidified material is a thermoset It is formed by irreversible chemical reactions that create cross links as the thermosetting polymer is heated Bakelite, a highly crosslinked thermoset made from phenol and formaldehyde, is prepared in two stages In the first stage, condensation between phenol and formaldehyde gives a polymer, which, in its fluid state, is cast in molds and heated, whereupon it solidifies to a hard, rigid mass The chemical reactions that form the fluid polymer and the solid thermoset are the same kind of condensations; the difference is that there are more cross links in the thermoset Melamine (used in plastic dinnerware) is another example of a thermoset Elastomers are flexible polymers that can be stretched but return to their original state when the stretching force is released Most amorphous polymers become rubbery beyond their glass transition temperature, but not all rubbery polymers are elastic Cross links in elastomers limit the extent to which elastomers can be deformed then encourage them to return to their original shape when they are relaxed 27.7 Addition Polymers: A Review and a Preview Addition polymers are most familiar to us in connection with the polymerization of alkenes C C C C n 27.7 1131 Addition Polymers: A Review and a Preview Table 27.2 reviews alkene polymerizations that proceed by free radicals and by coordination complexes of the Ziegler–Natta type Both are chain-growth processes; their propagation steps were outlined in Mechanisms 6.9 and 14.3, respectively The present section examines two other significant factors in alkene polymerization: initiation and termination Initiators of Alkene Polymerization: Whether free-radical or coordination polymerization occurs depends primarily on the substance used to initiate the reaction Freeradical polymerization occurs when a compound is present that undergoes homolytic bond cleavage when heated Two examples include O 100°C O O Di-tert-butyl peroxide N N N N 50°C Azobisisobutyronitrile (AIBN) TABLE 27.2 + O Two tert-butoxy radicals N + N + N N Two 1-cyano-1-methylethyl radicals Nitrogen Summary of Alkene Polymerizations Discussed in Earlier Chapters Reaction (section) and comments Example Free-radical polymerization of alkenes (see Section 6.14) Many alkenes polymerize when treated with free-radical initiators A free-radical chain mechanism is followed and was illustrated for the case of ethylene in Mechanism 6.9 H2C CH2 200°C, 2000 atm O2 or peroxides n Ethylene Free-radical polymerization of dienes (see Section 10.10) Conjugated dienes undergo free-radical polymerization under conditions similar to those of alkenes The major product corresponds to 1,4-addition Polyethylene Cl free-radical initiator Cl n Polychloroprene 2-Chloro-1,3-butadiene (Chloroprene) Free-radical polymerization of styrene (see Section 11.14) Styrene can be polymerized under free-radical, cationic, anionic, and Ziegler–Natta conditions The mechanism of the free-radical polymerization was shown in Mechanism 11.2 benzoyl peroxide n Polystyrene Styrene Ring-opening metathesis polymerization (see Section 14.13) The double bonds of strained cyclic alkenes are cleaved by certain carbene complexes of tungsten and, in the process, undergo polymerization catalyst −80°C n Bicyclo[2.2.1]-2-heptene (Norbornene) Coordination polymerization (see Section 14.14) Organometallic compounds such as bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2) catalyze the polymerization of ethylene by the sequence of steps shown in Mechanism 14.3 H2C CH2 Cp2ZrCl2 methalumoxane Ethylene Polynorbornene n Polyethylene 1132 Chapter 27 Synthetic Polymers Problem 27.5 (a) Write a chemical equation for the reaction in which tert-butoxy radical adds to vinyl chloride to initiate polymerization Show the flow of electrons with curved arrows (b) Repeat part (a) for the polymerization of styrene using AIBN as an initiator Sample Solution (a) tert-Butoxy radical adds to the CH2 group of vinyl chloride The free radical formed in this process has its unpaired electron on the carbon bonded to chlorine + O tert-Butoxy radical O Cl Cl Vinyl chloride 2-tert-Butoxy-1chloroethyl radical Coordination polymerization catalysts are complexes of transition metals The original Ziegler–Natta catalyst, a mixture of titanium tetrachloride and diethylaluminum chloride, has been joined by numerous organometallic complexes such as the widely used bis(cyclopentadienyl)zirconium dichloride Cl Zr Cl Bis(cyclopentadienyl)zirconium dichloride Termination Steps in Alkene Polymerization: The main chain-terminating processes in free-radical polymerization are combination and disproportionation In a combination, the pairing of the odd electron of one growing radical chain with that of another gives a stable macromolecule RO CH2CH2 x CH2CH2 ϩ H2CCH2 CH2CH2 OR y Two growing polyethylene chains RO CH2CH2 x CH2CH2 CH2CH2 CH2CH2 OR y Terminated polyethylene In disproportionation, two alkyl radicals react by hydrogen-atom transfer Two stable molecules result; one terminates in a methyl group, the other in a double bond H RO CH2CH2 x CH2 CH2 ϩ H2C CH CH2CH2 OR y Two growing polyethylene chains H RO CH2CH2 x CH2 CH2 ϩ Methyl-terminated polyethylene H2C CH CH2CH2 OR y Double-bond-terminated polyethylene 27.8 Chain Branching in Free-Radical Polymerization Both combination and disproportionation consume free radicals and decrease the number of growing chains Because they require a reaction between two free radicals, each of which is present in low concentration, they have a low probability compared with chain growth, in which a radical reacts with a monomer Combination involves only bond making and has a low activation energy; disproportionation has a higher activation energy because bond breaking accompanies bond making Disproportionation has a more adverse effect on chain length and molecular weight than combination Problem 27.6 Other than combination, a macromolecule of the type RO CH2CH2 x CH2 CH2 OR can arise by a different process, one which also terminates chain growth Show a reasonable reaction and represent the flow of electrons by curved arrows Among several chain terminating reactions that can occur in coordination polymerization, a common one is an elimination in which a ␤-hydrogen is transferred to the metal R H ϩ ϩ CH Zr Zr H ϩ H2C CHR CH2 27.8 Chain Branching in Free-Radical Polymerization Even with the same monomer, the properties of a polymer can vary significantly depending on how it is prepared Free-radical polymerization of ethylene gives low-density polyethylene; coordination polymerization gives high-density polyethylene The properties are different because the structures are different, and the difference in the structures comes from the mechanisms by which the polymerizations take place Free-radical polymerization of ethylene gives a branched polymer, coordination polymerization gives a linear one What is the mechanism responsible for the branching that occurs in the free-radical polymerization of ethylene? By itself, the propagation step in the free-radical polymerization of ethylene cannot produce branches Polymer CH2 CH2 H2C CH2 Polymer CH2 CH2 CH2 CH2 In order for the polymer to be branched, an additional process must occur involving a radical site somewhere other than at the end of the chain The two main ways this can happen both involve hydrogen abstraction from within the polymer chain Intramolecular hydrogen atom abstraction Intermolecular hydrogen atom abstraction (chain transfer) Intramolecular Hydrogen Atom Abstraction: Mechanism 27.1 shows how intramolecular hydrogen atom abstraction can lead to the formation of a four-carbon branch Recall that an intramolecular process takes place within a molecule, not between molecules As the mechanism shows, the radical at the end of the growing polymer abstracts a hydrogen atom from the fifth carbon Five carbons and one hydrogen comprise six atoms of a cyclic transition state When a hydrogen atom is removed from the fifth carbon, a secondary radical is generated at that site This, then, is the carbon that becomes the origin for further chain growth Analogous mechanisms apply to branches shorter or longer than four carbons 1133 1134 Chapter 27 Synthetic Polymers Mechanism 27.1 Branching in Polyethylene Caused by Intramolecular Hydrogen Transfer THE OVERALL REACTION: H2CœCH2 Polymer ±CHCH2CH2(CH2CH2)nCH2CH2 ± ±±±£ Polymer ±CH2CH2CH2CH2CH2 ±± CH2CH2CH2CH3 THE MECHANISM: Step 1: The carbon at the end of the chain––the one with the unpaired electron––abstracts a hydrogen atom from the fifth carbon The transition state is a cyclic arrangement of six atoms H Polymer CH2± H ±£ Polymer H CH2± H The resulting radical is secondary and more stable than the original primary radical Therefore, the hydrogen atom abstraction is exothermic Polymer ±CH±CH2CH2CH2CH3 Step 2: When the radical reacts with ethylene, chain extension takes place at the newly formed radical site The product of this step has a four-carbon branch attached to the propagating chain ± ϩ H2CœCH2 ±£ Polymer ±CH±CH2±CH2 ± Polymer ±CH CH2CH2CH2CH3 CH2CH2CH2CH3 Step 3: Reaction with additional ethylene molecules extends the growing chain H2CœCH2 ±±±±£ ±± CH2CH2CH2CH3 Polymer ±CHCH2CH2±(CH2CH2)n±CH2CH2 ± ± Polymer ±CHCH2CH2 CH2CH2CH2CH3 Problem 27.7 Suggest an explanation for the observation that branches shorter or longer than four carbons are found infrequently in polyethylene Frame your explanation in terms of how ⌬H and ⌬S affect the activation energy for intramolecular hydrogen atom abstraction A comparable process cannot occur when Ziegler–Natta catalysts are used because free radicals are not intermediates in coordination polymerization Intermolecular Hydrogen Atom Abstraction (Chain Transfer): Mechanism 27.2 shows how a growing polymer chain abstracts a hydrogen atom from a terminated chain The original growing chain is now terminated, and the original terminated chain is activated toward further growth Chain growth, however, occurs at the branch point, not at the end of the chain An already long chain adds a branch while terminating a (presumably shorter) growing chain Chain transfer not only leads to branching, but also encourages disparity in chain lengths—more short chains and more long branched chains Both decrease the crystallinity of the polymer and reduce its strength As in the case of intramolecular hydrogen abstraction, branching by chain transfer is not a problem when alkenes are polymerized under Ziegler–Natta conditions because free radicals are not intermediates in coordination polymerization 27.9 Anionic Polymerization: Living Polymers Mechanism 27.2 Branching in Polyethylene Caused by Intermolecular Hydrogen Transfer Step 1: A growing polymer chain abstracts a hydrogen atom from a terminated chain This step terminates the growing chain and activates the terminated one H Polymer ±CH2±CH2 ϩ Polymer ±CH± Polymer ±£ Growing chain Terminated chain Polymer ±CH2±CH3 ϩ Polymer ±CH± Polymer Terminated chain Growing chain Step 2: Reaction of the new chain with monomer molecules produces a branch at which future growth occurs ± ± CH2 CH2 Polymer ±CH± Polymer ϩ Growing chain H2CœCH2 ±£ Polymer ±CH± Polymer Ethylene Growing branched chain 27.9 Anionic Polymerization: Living Polymers Anionic polymerization is a useful alternative to free-radical and Ziegler–Natta procedures for certain polymers Adding butyllithium to a solution of styrene in tetrahydrofuran (THF), for example, gives polystyrene CH CH2 CH3CH2CH2CH2Li THF CH Styrene CH2 n Polystyrene Mechanism 27.3 shows how addition of butyllithium to the double bond of styrene initiates polymerization The product of this step is a benzylic carbanion that then adds to a second molecule of styrene to give another benzylic carbanion, and so on by a chaingrowth process Polystyrene formed under these conditions has a narrower range of molecular weights than provided by other methods Initiation of polymerization by addition of butyllithium to styrene is much faster than subsequent chain growth Thus, all the butyllithium is consumed and the number of chains is equal to the number of molecules of butyllithium used These starter chains then grow at similar rates to produce similar chain lengths Problem 27.8 How will the average chain length of polystyrene vary with the amount of butyllithium used to initiate polymerization? 1135 1136 Chapter 27 Synthetic Polymers Mechanism 27.3 Anionic Polymerization of Styrene Step 1: Anionic polymerization of styrene is initiated by addition of butyllithium to the double bond The regioselectivity of addition is governed by formation of the more stable carbanion, which in this case is benzylic Ϫ ±CHœCH2 ϩ CH2(CH2)2CH3 Liϩ Styrene ±CH±CH2±CH2(CH2)2CH3 Ϫ Liϩ Butyllithium 1-Phenylhexyllithium Step 2: The product of the first step adds to a second molecule of styrene ±CHœCH2 ± ± ±CH±CH2 Ϫ Liϩ CH±CH2(CH2)3CH3 ±CH±CH2(CH2)3CH3 ϩ Ϫ Liϩ Styrene ϩ 1-Phenylhexyllithium 1,3-Diphenyloctyllithium Step 3: The product of the second step adds to a third molecule of styrene, then a fourth, and so on to give a macromolecule Reaction continues until all of the styrene is consumed At this point the polystyrene exists as an organolithium reagent CH±CH2±(CH2)3CH3 Ϫ ± ±CH±CH2 ϩ Li n The organolithium reagent is stable, but easily protonated by water to give polystyrene Alternatively, another monomer can be added to continue extending the chain As shown in step of Mechanism 27.3 once all of the monomer is consumed the polymer is present as its organolithium derivative This material is referred to as a living polymer because more monomer can be added and anionic polymerization will continue until the added monomer is also consumed Adding 1,3-butadiene, for example, to a living polymer of styrene gives a new living polymer containing sections (“blocks”) of polystyrene and poly(1,3-butadiene) CH Ϫ CH2 CH (CH2)3CH3 CH2 Liϩ ϩ H2C CH CH CH2 n “Living” polystyrene Ϫ Liϩ CH2 CH CH CH2 CH2 1,3-Butadiene CH CH CH2 CH m CH2 CH (CH2)3CH3 CH2 n “Living” styrene-butadiene copolymer 27.10 Cationic Polymerization Living polymerizations are characterized by the absence of efficient termination processes They are normally terminated by intentionally adding a substance that reacts with carbanions such as an alcohol or carbon dioxide The kinds of vinyl monomers that are susceptible to anionic polymerization are O those that bear electron-withdrawing groups such as bond N and C O C on the double OCH3 O N Acrylonitrile OCH3 Methyl acrylate N Methyl 2-cyanoacrylate When a carbonyl and a cyano group are attached to the same carbon as in methyl 2-cyanoacrylate, the monomer that constitutes Super Glue, anionic polymerization can be initiated by even weak bases such as atmospheric moisture or normal skin dampness Problem 27.9 Write a structural formula for the carbanion formed by addition of hydroxide ion to methyl 2-cyanoacrylate Accompany this structural formula by a contributing resonance structure that shows delocalization of the negative charge to oxygen, and another to nitrogen 27.10 Cationic Polymerization Analogous to the initiation of anionic polymerization by addition of nucleophiles to alkenes, cationic polymerization can be initiated by the addition of electrophiles The alkenes that respond well to cationic polymerization are those that form relatively stable 1137 1138 Chapter 27 Synthetic Polymers carbocations when protonated Of these, the one used most often is 2-methylpropene, better known in polymer chemistry by its common name isobutylene Mechanism 27.4 outlines the mechanism of this polymerization as catalyzed by boron trifluoride to which a small amount of water has been added The active catalyst is believed to be a Lewis acid/Lewis base complex formed from them by the reaction: H H O ϩ ϩ BF3 O Ϫ BF3 H H Water Boron trifluoride Water/Boron trifluoride complex This complex is a strong Brønsted acid and protonates the double bond of 2-methylpropene in step of the mechanism Mechanism 27.4 Cationic Polymerization of 2-Methylpropene THE OVERALL REACTION BF3 H2O n Polyisobutylene 2-Methylpropene THE MECHANISM: Step 1: The alkene is protonated, forming a carbocation H + H + O BF3 H O BF3 H 2-Methylpropene tert-Butyl cation Step 2: The carbocation formed in the preceding step reacts with a molecule of the alkene, forming a new carbocation + 2-Methylpropene tert-Butyl cation 1,1,3,3-Tetramethylbutyl cation Step 3: The process shown in step continues, forming a chain-extended carbocation n Step 4: One mechanism for chain termination is loss of a proton H H O + O H H n H H + n 27.11 1139 Polyamides Polyisobutylene is the “butyl” in butyl rubber, one of the first synthetic rubber substitutes Most inner tubes are a copolymer of 2-methylpropene (isobutylene) and 2-methyl1,3-butadiene (isoprene) 27.11 Polyamides The polyamide nylon 66 takes its name from the fact that it is prepared from a six-carbon dicarboxylic acid and a six-carbon diamine The acid–base reaction between adipic acid and hexamethylenediamine gives a salt, which on heating undergoes condensation polymerization in which the two monomers are joined by amide bonds O O O Ϫ OC(CH2)4CO Ϫ ϩ ϩ H3N(CH2)6NH3 280–300ЊC ϪH2O O NH(CH2)6NHC(CH2)4C n Salt of adipic acid and hexamethylenediamine The systematic names of adipic acid and hexamethylenediamine are hexanedioic acid and 1,6-hexanediamine, respectively Nylon 66 Nylon 66 was the first and remains the most commercially successful synthetic polyamide (Figure 27.7) Others have been developed by varying the number of carbons in the chains of the diamine and the dicarboxylic acid Nylon 66 resembles silk in both structure and properties Both are polyamides in which hydrogen bonds provide an ordered arrangement of adjacent chains H H O N N N O O H H H O N N N O O H A variation on the diamine/dicarboxylic acid theme is to incorporate the amino and carboxylic acid groups into the same molecule, much as Nature does in amino acids Nylon is a polyamide derived by heating 6-aminohexanoic acid H O heat H3N O 6-Aminohexanoic acid Skydivers’ parachutes are made of nylon 66 O + N H2O n Nylon Figure 27.7 Water Problem 27.10 Nylon is normally prepared from the lactam derived from 6-aminohexanoic acid, called ε-caprolactam Do you remember what a lactam is? Write the structure of ε-caprolactam Problem 27.11 Nomex is an aramid fiber used for fire-resistant protective clothing It is a polyamide prepared by condensation of 1,3-benzenediamine (m-phenylenediamine) and 1,3-benzenedicarboxylic acid (isophthalic acid) What is the repeating unit of Nomex? 1140 Chapter 27 Synthetic Polymers 27.12 Polyesters The usual synthetic route to a polyester is by condensation of a dicarboxylic acid with a diol The best known polyester is poly(ethylene terephthalate) prepared from ethylene glycol and terephthalic acid The dimethyl ester of terephthalic acid is used in an analogous method O HOC O 200–300ЊC ϪH2O COH ϩ HOCH2CH2OH OCH2CH2O O O C C n Terephthalic acid (Benzene-1,4-dicarboxylic acid) Ethylene glycol Poly(ethylene terephthalate) The popularity of clothing made of polyester-cotton blends testifies to the economic impact of this polymer Poly(ethylene terephthalate) is the PETE referred to in the recycling codes listed in Table 27.1 Plastic bottles for juice, ketchup, and soft drinks are usually made of PETE, as is Mylar film and Dacron sails for boats Alkyd resins number in the hundreds and are used in glossy paints and enamels— house, car, and artist’s—as illustrated in Figure 27.9 Most are derived from benzene-1,2dicarboxylic acid (o-phthalic acid) and 1,2,3-propanetriol (glycerol) Two of the hydroxyl groups of glycerol are converted to esters of o-phthalic acid; the third is esterified with an unsaturated fatty acid that forms cross links to other chains Figure 27.8 O Dacron is widely used as the material in surgical sutures OCH2CHCH2O O C O C C O R n An alkyd resin With both a hydroxyl group and a carboxylic acid function in the same molecule, glycolic acid and lactic acid have the potential to form polyesters Heating the ␣-hydroxy acid gives a cyclic diester, which, on treatment with a Lewis acid catalyst (SnCl2 or SbF3) yields the polymer O HOCHCOH R RϭH heat ϪH2O R O O O O R O Lewis acid OCHC R n Glycolic acid Glycolide Poly(glycolic acid) Lactic acid Lactide Poly(lactic acid) R ϭ CH3 Surgical sutures made from poly(glycolic acid) and poly(lactic acid), while durable enough to substitute for ordinary stitches, are slowly degraded by ester hydrolysis and don’t require a return visit for their removal Poly(glycolic acid) fibers also hold promise as a scaffold upon which to grow skin cells This “artificial skin” is then applied to a wound to promote healing Problem 27.12 Another monomer from which surgical sutures are made is ε-caprolactone What is the repeating unit of poly(ε-caprolactone)? O O Figure 27.9 Alkyds are used for more than painting rooms Artists use them too ε-Caprolactone 27.14 1141 Polyurethanes Polyesters are also used in controlled-release forms of drugs and agricultural products such as fertilizers and herbicides By coating the active material with a polyester selected so as to degrade over time, the material is released gradually rather than all at once 27.13 Polycarbonates Polycarbonates are polyesters of carbonic acid Lexan is the most important of the polycarbonates and is prepared from the diphenolic compound bisphenol A CH3 NaO CH3 O ϪNaCl ONa ϩ ClCCl C O O C CH3 OC n CH3 Disodium salt of bisphenol A Phosgene Bisphenol A polycarbonate (Lexan) Problem 27.13 Bisphenol A is made from phenol and acetone Industrial processes are usually very efficient One process, described in Chapter 22, gives both phenol and acetone as products of the same reaction Can you find it? Write a mechanism for the reaction of one molecule of the disodium salt of bisphenol A with one molecule of phosgene Lexan is a clear, transparent, strong, and impact-resistant plastic with literally countless applications It is used in both protective and everyday eyeglasses as illustrated in Figure 27.10 The Apollo 11 astronauts wore Lexan helmets with Lexan visors on their 1969 trip to the moon CDs and DVDs are Lexan polycarbonate, as are many cell phones, automobile dashpanels, and headlight and taillight lenses 27.14 Polyurethanes A urethane, also called a carbamate, is a compound that contains the functional group O OCNH Urethanes are normally prepared by the reaction of an alcohol and an isocyanate O ROH ϩ RЈN Alcohol C O ROCNHRЈ Isocyanate Urethane Polyurethanes are the macromolecules formed from a diol and a diisocyanate In most cases the diol is polymeric and the diisocyanate is a mixture of the “toluene diisocyanate” isomers CH3 CH3 N HOCH2 C O O C N N C O CH2OH Polymer N Polymeric diol C O Mixture of “toluene diisocyanate” isomers If, for example, only the 2,6-diisocyanate were present, the repeating unit of the resulting polyurethane would be O OCH2 Polymer CH2OCNH CH3 O NHC n Figure 27.10 The polycarbonate lenses in these protective glasses are lightweight, yet shatterproof 1142 Chapter 27 Synthetic Polymers Because a mixture of diisocyanate isomers is actually used, a random mixture of 2,4- and 2,6-substitution patterns results Problem 27.14 Write the repeating unit of the “polymeric diol” if it is derived from 1,2-epoxypropane Figure 27.11 Spandex skinsuits make speedskaters more aerodynamic The reaction of an alcohol with an isocyanate is addition, not condensation Therefore, polyurethanes are classified as addition polymers But because the monomers are difunctional, the molecular weight increases by step growth rather than chain growth A major use of polyurethanes is in spandex fibers Spandex, even when stretched several times its length, has the ability to return to its original state and is a superior substitute for rubber in elastic garments Its most recognizable application is in athletic wear (swimming, cycling, running) where it is the fabric of choice for high-performance athletes (Figure 27.11) Polyurethanes have many other applications, especially in paints, adhesives, and foams Polyurethane foams, which can be rigid (insulation panels) or flexible (pillows, cushions, and mattresses) depending on their degree of cross linking, are prepared by adding foaming agents to the polymerization mixture One method takes advantage of the reaction between isocyanates and water O RN C O ϩ Isocyanate H2O RNH Water C RNH2 ϩ OH Carbamic acid Amine CO2 Carbon dioxide Although esters of carbamic acid (urethanes) are stable compounds, carbamic acid itself rapidly dissociates to an amine and carbon dioxide Adding some water to the reactants during polymerization generates carbon dioxide bubbles which are trapped within the polymer 27.15 Copolymers Copolymers, polymers made from more than one monomer, are as common as homopolymers The presence of more than one monomer in a chain makes some control of properties possible Some structural units stiffen the chain, others make it more flexible Often a second monomer is added to allow cross linking Copolymers are classified according to the distribution of monomers in the macromolecule Random Block Graft Random Copolymers: As the name implies, there is no pattern to the distribution of monomer units in a random copolymer A A B A B B A A B B A B A B Styrene–butadiene rubber (SBR) for automobile tires is a random copolymer It is prepared by two methods, free-radical and anionic polymerization, both of which are carried out on a mixture of styrene and 1,3-butadiene Free-radical initiation is essentially nonselective and gives the random copolymer Anionic initiation is carried out under conditions designed to equalize the reactivity of the two monomers so as to ensure randomness Block Copolymers: The main chain contains sections (blocks) of repeating units derived from different monomers The sequence: A A A A A B B B B B B B B B 27.15 1143 Copolymers shows only two blocks, one derived from A and the other from B A macromolecule derived from A and B can contain many blocks The living polymers generated by anionic polymerization are well suited to the preparation of block polymers Adding 1,3-butadiene to a living polystyrene block sets the stage for attaching a poly(1,3-butadiene) block Polystyrene H2C CH Liϩ CH2 CH CH Ϫ CH2 CH2 Polystyrene Ϫ CH CH2 Further reaction with H2C Ϫ Polystyrene CH2 Poly(1,3-butadiene) CH CH2 CH CH Graft Copolymer: ferent monomer The main chain bears branches (grafts) that are derived from a difB A B A B A B A A B A B A B A A B A A B A B A B B A B A graft copolymer of styrene and 1,3-butadiene is called “high-impact polystyrene” and is used, for example, in laptop computer cases It is prepared by free-radical polymerization of styrene in the presence of poly(1,3-butadiene) Instead of reacting with styrene, the free-radical initiator abstracts an allylic hydrogen from poly(1,3-butadiene) Poly(1,3-butadiene) CH CH CH CH2 Poly(1,3-butadiene) CH CH CH2 Poly(1,3-butadiene) H Initiator Poly(1,3-butadiene) CH Polystyrene chain growth begins at the allylic radical site and proceeds in the usual way at this and random other allylic carbons of poly(1,3-butadiene) Poly(1,3-butadiene) CH Poly(1,3-butadiene) CH CH CH CH CH2 Poly(1,3-butadiene) CH CH CH2 Poly(1,3-butadiene) CH2 CH CH2 CHCH CH2 Liϩ The properties of the block copolymer prepared by anionic living polymerization are different from the random styrene–butadiene copolymer CH CH CH2 CH2 Liϩ 1144 Chapter 27 Synthetic Polymers Polystyrene grafts on a poly(1,3-butadiene) chain are the result Poly(1,3-butadiene) CH Poly(1,3-butadiene) CH CH CH CH2 CH CH Poly(1,3-butadiene) CH2 Further reaction with styrene CH CH Poly(1,3-butadiene) CH2 CH2 Polystyrene Polystyrene alone is brittle; poly(1,3-butadiene) alone is rubbery The graft copolymer is strong, but absorbs shock without cracking because of the elasticity provided by its poly(1,3-butadiene) structural units Conducting Polymers T he notion that polymers can conduct electricity seems strange to most of us After all, the plastic wrapped around the wires in our homes and automobiles serves as insulation Do polymers exist that can conduct electricity? Even if such materials could be made, why would we be interested in them? Henry Letheby, a lecturer in chemistry and toxicology at the College of London Hospital, obtained a partially conducting material in 1862 by the anodic oxidation of aniline in sulfuric acid The material Letheby synthesized was a form of polyaniline In the 1980s, Alan MacDiarmid of the University of Pennsylvania reinvestigated polyaniline, which is now a widely used conducting polymer Polyaniline exists in a variety of oxidation states (Figure 27.12), each with different properties The emaraldine salt is a conductor without the use of additives that enhance conductivity, but its conductivity is enhanced by adding a Brønsted acid that protonates the nitrogen atoms Figure 27.12 Polyaniline exists in different forms with varying states of oxidation One of the forms is a conductor H N H N H N H N Leucoemaraldine, colorless, fully reduced, insulating H N H N N N Emaraldine base, green, partially oxidized, insulating H N H N ϩ N H N Emaraldine salt, blue, partially oxidized, conducting N N N Pernigraniline, purple, fully oxidized, insulating N ... connectivities as: A A A OCOCONO A A and A A OCONOCO A A A Place a hydrogen on each of the seven available bonds of each framework H H H A A A HOCOCONOH A A H H and H H A A HOCONOCOH A A A H H H The... transformation was remarkable at the time because an inorganic salt, ammonium cyanate, was converted to urea, a known organic substance earlier isolated from urine It is now recognized as a significant... reactivity are reinforced when a reaction used to prepare a particular functional–group family reappears as a characteristic reaction of another Mechanism 5.1 The E1 Mechanism for Acid-Catalyzed