(BQ) Part 1 book Organic chemistry has contents: Chemical bonding, alkanes, conformations of alkanes and cycloalkanes, alcohols and alkyl halides, structure and preparation of alkenes elimination reactions, stereochemistry, nucleophilic substitution,... and other contents.
f o u r t h e d i t i o n ORGANIC CHEMISTRY Francis A Carey University of Virginia Boston Burr Ridge, IL Dubuque, IA Madison, WI New York San Francisco St Louis Bangkok Bogotá Caracas Lisbon London Madrid Mexico City Milan New Delhi Seoul Singapore Sydney Taipei Toronto McGraw-Hill Higher Education A Division of The McGraw-Hill Companies ORGANIC CHEMISTRY, FOURTH EDITION Copyright © 2000, 1996, 1992, 1987 by The McGraw-Hill Companies, Inc All rights reserved Printed in the United States of America Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher This book is printed on acid-free paper VNH/VNH 9 ISBN 0-07-290501-8 Vice president and editorial director: Kevin T Kane Publisher: James M Smith Sponsoring editor: Kent A Peterson Developmental editor: Terrance Stanton Editorial assistant: Jennifer Bensink Senior marketing manager: Martin J Lange Senior marketing assistant: Tami Petsche Senior project manager: Peggy J Selle Senior production supervisor: Sandra Hahn Designer: K Wayne Harms Photo research coordinator: John C Leland Senior supplement coordinator: David A Welsh Compositor: GTS Graphics, Inc Typeface: 10/12 Times Roman Printer: Von Hoffmann Press, Inc Cover/interior designer: Jamie O’Neal Photo research: Mary Reeg Photo Research The credits section for this book begins on page C-1 and is considered an extension of the copyright page Library of Congress Cataloging-in-Publication Data Carey, Francis A Organic chemistry / Francis A Carey — 4th ed p cm Includes index ISBN 0-07-290501-8 — ISBN 0-07-117499-0 (ISE) Chemistry, Organic I Title QD251.2.C364 547—dc21 2000 99-045791 CIP INTERNATIONAL EDITION ISBN 0-07-117499-0 Copyright © 2000 Exclusive rights by The McGraw-Hill Companies, Inc for manufacture and export This book cannot be re-exported from the country to which it is consigned by McGraw-Hill The International Edition is not available in North America www mhhe.com A B O U T T H E Francis A Carey is a native of Pennsylvania, educated in the public schools of Philadelphia, at Drexel University (B.S in chemistry, 1959), and at Penn State (Ph.D 1963) Following postdoctoral work at Harvard and military service, he joined the chemistry faculty of the University of Virginia in 1966 With his students, Professor Carey has published over 40 research papers in synthetic and mechanistic organic chemistry He is coauthor (with Richard J Sundberg) of Advanced Organic Chemistry, a two-volume treatment designed for graduate students and advanced undergraduates, and (with Robert C Atkins) of Organic Chemistry: A Brief Course, an introductory text for the one-semester organic course Since 1993, Professor Carey has been a member of the Committee of Examiners of the Graduate Record A U T H O R Examination in Chemistry Not only does he get to participate in writing the Chemistry GRE, but the annual working meetings provide a stimulating environment for sharing ideas about what should (and should not) be taught in college chemistry courses Professor Carey’s main interest shifted from research to undergraduate education in the early 1980s He regularly teaches both general chemistry and organic chemistry to classes of over 300 students He enthusiastically embraces applications of electronic media to chemistry teaching and sees multimedia presentations as the wave of the present Frank and his wife Jill, who is a teacher/director of a preschool and a church organist, are the parents of three grown sons and the grandparents of Riyad and Ava B R I E F C O N T E N T S Preface xxv Introduction 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 CHEMICAL BONDING ALKANES CONFORMATIONS OF ALKANES AND CYCLOALKANES ALCOHOLS AND ALKYL HALIDES STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS REACTIONS OF ALKENES: ADDITION REACTIONS STEREOCHEMISTRY NUCLEOPHILIC SUBSTITUTION ALKYNES CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS ARENES AND AROMATICITY REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION SPECTROSCOPY ORGANOMETALLIC COMPOUNDS ALCOHOLS, DIOLS, AND THIOLS ETHERS, EPOXIDES, AND SULFIDES ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP ENOLS AND ENOLATES CARBOXYLIC ACIDS CARBOXYLIC ACID DERIVATIVES: NUCLEOPHILIC ACYL SUBSTITUTION ESTER ENOLATES AMINES ARYL HALIDES PHENOLS CARBOHYDRATES LIPIDS AMINO ACIDS, PEPTIDES, AND PROTEINS NUCLEIC ACIDS APPENDIX APPENDIX APPENDIX GLOSSARY CREDITS INDEX PHYSICAL PROPERTIES ANSWERS TO IN-TEXT PROBLEMS LEARNING CHEMISTRY WITH MOLECULAR MODELS: Using SpartanBuild and SpartanView 53 89 126 167 208 259 302 339 365 398 443 487 546 579 619 654 701 736 774 831 858 917 939 972 1015 1051 A-1 A-9 A-64 G-1 C-1 I-1 ix C O N T E N T S Preface xxv INTRODUCTION The Origins of Organic Chemistry Berzelius, Wöhler, and Vitalism The Structural Theory Electronic Theories of Structure and Reactivity The Influence of Organic Chemistry Computers and Organic Chemistry Challenges and Opportunities Where Did the Carbon Come From? CHAPTER CHEMICAL BONDING 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 Atoms, Electrons, and Orbitals Ionic Bonds 11 Covalent Bonds 12 Double Bonds and Triple Bonds 14 Polar Covalent Bonds and Electronegativity 15 Formal Charge 16 Structural Formulas of Organic Molecules 19 Constitutional Isomers 22 Resonance 23 The Shapes of Some Simple Molecules 26 Learning By Modeling 27 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 Molecular Dipole Moments 30 Electron Waves and Chemical Bonds 31 Bonding in H2: The Valence Bond Model 32 Bonding in H2: The Molecular Orbital Model 34 Bonding in Methane and Orbital Hybridization 35 sp3 Hybridization and Bonding in Ethane 37 sp2 Hybridization and Bonding in Ethylene 38 sp Hybridization and Bonding in Acetylene 40 Which Theory of Chemical Bonding Is Best? 42 1.20 SUMMARY 43 PROBLEMS 47 CHAPTER ALKANES 53 2.1 2.2 2.3 2.4 2.5 Classes of Hydrocarbons 53 Reactive Sites in Hydrocarbons 54 The Key Functional Groups 55 Introduction to Alkanes: Methane, Ethane, and Propane Isomeric Alkanes: The Butanes 57 56 Methane and the Biosphere 58 xi xii CONTENTS 2.6 2.7 2.8 2.9 Higher n-Alkanes 59 The C5H12 Isomers 59 IUPAC Nomenclature of Unbranched Alkanes 61 Applying the IUPAC Rules: The Names of the C6H14 Isomers 62 A Brief History of Systematic Organic Nomenclature 63 2.10 2.11 2.12 2.13 2.14 2.15 Alkyl Groups 65 IUPAC Names of Highly Branched Alkanes 66 Cycloalkane Nomenclature 68 Sources of Alkanes and Cycloalkanes 69 Physical Properties of Alkanes and Cycloalkanes 71 Chemical Properties Combustion of Alkanes 74 Thermochemistry 77 2.16 2.17 Oxidation–Reduction in Organic Chemistry SUMMARY PROBLEMS 78 80 83 CHAPTER CONFORMATIONS OF ALKANES AND CYCLOALKANES 3.1 3.2 89 Conformational Analysis of Ethane 90 Conformational Analysis of Butane 94 Molecular Mechanics Applied to Alkanes and Cycloalkanes 3.3 3.4 3.5 3.6 3.7 3.8 Conformations of Higher Alkanes 97 The Shapes of Cycloalkanes: Planar or Nonplanar? 98 Conformations of Cyclohexane 99 Axial and Equatorial Bonds in Cyclohexane 100 Conformational Inversion (Ring Flipping) in Cyclohexane 103 Conformational Analysis of Monosubstituted Cyclohexanes 104 Enthalpy, Free Energy, and Equilibrium Constant 106 3.9 3.10 3.11 3.12 3.13 3.14 3.15 Small Rings: Cyclopropane and Cyclobutane 106 Cyclopentane 108 Medium and Large Rings 108 Disubstituted Cycloalkanes: Stereoisomers 108 Conformational Analysis of Disubstituted Cyclohexanes Polycyclic Ring Systems 114 Heterocyclic Compounds 116 3.16 SUMMARY PROBLEMS 110 117 120 CHAPTER ALCOHOLS AND ALKYL HALIDES 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 96 126 IUPAC Nomenclature of Alkyl Halides 127 IUPAC Nomenclature of Alcohols 127 Classes of Alcohols and Alkyl Halides 128 Bonding in Alcohols and Alkyl Halides 129 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces 130 Acids and Bases: General Principles 133 Acid–Base Reactions: A Mechanism for Proton Transfer 136 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides 137 Mechanism of the Reaction of Alcohols with Hydrogen Halides 139 Structure, Bonding, and Stability of Carbocations 140 CONTENTS 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 xiii Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism 143 Effect of Alcohol Structure on Reaction Rate 145 Reaction of Primary Alcohols with Hydrogen Halides: The SN2 Mechanism 146 Other Methods for Converting Alcohols to Alkyl Halides 147 Halogenation of Alkanes 148 Chlorination of Methane 148 Structure and Stability of Free Radicals 149 Mechanism of Methane Chlorination 153 From Bond Energies to Heats of Reaction 4.19 Halogenation of Higher Alkanes 4.20 SUMMARY 155 156 159 PROBLEMS 163 CHAPTER STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 167 5.1 Alkene Nomenclature 167 Ethylene 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 168 Structure and Bonding in Alkenes 170 Isomerism in Alkenes 172 Naming Stereoisomeric Alkenes by the E–Z Notational System 173 Physical Properties of Alkenes 174 Relative Stabilities of Alkenes 176 Cycloalkenes 180 Preparation of Alkenes: Elimination Reactions 181 Dehydration of Alcohols 182 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule 183 Stereoselectivity in Alcohol Dehydration 184 The Mechanism of Acid-Catalyzed Dehydration of Alcohols 185 Rearrangements in Alcohol Dehydration 187 Dehydrohalogenation of Alkyl Halides 190 Mechanism of the Dehydrohalogenation of Alkyl Halides: The E2 Mechanism 192 Anti Elimination in E2 Reactions: Stereoelectronic Effects 194 A Different Mechanism for Alkyl Halide Elimination: The E1 Mechanism 196 SUMMARY PROBLEMS 198 202 CHAPTER REACTIONS OF ALKENES: ADDITION REACTIONS 6.1 6.2 6.3 6.4 6.5 6.6 Hydrogenation of Alkenes 208 Heats of Hydrogenation 209 Stereochemistry of Alkene Hydrogenation 212 Electrophilic Addition of Hydrogen Halides to Alkenes 213 Regioselectivity of Hydrogen Halide Addition: Markovnikov’s Rule Mechanistic Basis for Markovnikov’s Rule 216 Rules, Laws, Theories, and the Scientific Method 6.7 6.8 208 214 217 Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes Free-Radical Addition of Hydrogen Bromide to Alkenes 220 219 xiv CONTENTS 6.9 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19 6.20 6.21 Addition of Sulfuric Acid to Alkenes 223 Acid-Catalyzed Hydration of Alkenes 225 Hydroboration–Oxidation of Alkenes 227 Stereochemistry of Hydroboration–Oxidation 229 Mechanism of Hydroboration–Oxidation 230 Addition of Halogens to Alkenes 233 Stereochemistry of Halogen Addition 233 Mechanism of Halogen Addition to Alkenes: Halonium Ions Conversion of Alkenes to Vicinal Halohydrins 236 Epoxidation of Alkenes 238 Ozonolysis of Alkenes 240 Introduction to Organic Chemical Synthesis 243 Reactions of Alkenes with Alkenes: Polymerization 244 234 Ethylene and Propene: The Most Important Industrial Organic Chemicals 248 6.22 SUMMARY PROBLEMS 249 252 CHAPTER STEREOCHEMISTRY 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Molecular Chirality: Enantiomers 259 The Stereogenic Center 260 Symmetry in Achiral Structures 264 Properties of Chiral Molecules: Optical Activity 265 Absolute and Relative Configuration 267 The Cahn–Ingold–Prelog R–S Notational System 268 Fischer Projections 271 Physical Properties of Enantiomers 272 Chiral Drugs 7.9 7.10 7.11 259 273 Reactions That Create a Stereogenic Center 274 Chiral Molecules with Two Stereogenic Centers 276 Achiral Molecules with Two Stereogenic Centers 279 Chirality of Disubstituted Cyclohexanes 281 7.12 7.13 7.14 7.15 7.16 Molecules with Multiple Stereogenic Centers 282 Reactions That Produce Diastereomers 284 Resolution of Enantiomers 286 Stereoregular Polymers 288 Stereogenic Centers Other Than Carbon 290 7.17 SUMMARY PROBLEMS 290 293 CHAPTER NUCLEOPHILIC SUBSTITUTION 8.1 8.2 8.3 8.4 8.5 8.6 8.7 302 Functional Group Transformation by Nucleophilic Substitution Relative Reactivity of Halide Leaving Groups 305 The SN2 Mechanism of Nucleophilic Substitution 306 Stereochemistry of SN2 Reactions 307 How SN2 Reactions Occur 308 Steric Effects in SN2 Reactions 310 Nucleophiles and Nucleophilicity 312 An Enzyme-Catalyzed Nucleophilic Substitution of an Alkyl Halide 314 302 CONTENTS 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 xv The SN1 Mechanism of Nucleophilic Substitution 315 Carbocation Stability and SN1 Reaction Rates 315 Stereochemistry of SN1 Reactions 318 Carbocation Rearrangements in SN1 Reactions 319 Effect of Solvent on the Rate of Nucleophilic Substitution 320 Substitution and Elimination as Competing Reactions 323 Sulfonate Esters as Substrates in Nucleophilic Substitution 326 Looking Back: Reactions of Alcohols with Hydrogen Halides 329 SUMMARY PROBLEMS 330 332 CHAPTER ALKYNES 339 9.1 9.2 9.3 9.4 Sources of Alkynes 339 Nomenclature 340 Physical Properties of Alkynes 341 Structure and Bonding in Alkynes: sp Hybridization Natural and “Designed” Enediyne Antibiotics 9.5 9.6 341 344 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 Acidity of Acetylene and Terminal Alkynes 344 Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes 346 Preparation of Alkynes by Elimination Reactions 348 Reactions of Alkynes 350 Hydrogenation of Alkynes 350 Metal–Ammonia Reduction of Alkynes 351 Addition of Hydrogen Halides to Alkynes 352 Hydration of Alkynes 355 Addition of Halogens to Alkynes 356 Ozonolysis of Alkynes 357 9.15 SUMMARY PROBLEMS 357 358 CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 The Allyl Group 365 Allylic Carbocations 366 Allylic Free Radicals 370 Allylic Halogenation 370 Classes of Dienes 372 Relative Stabilities of Dienes 374 Bonding in Conjugated Dienes 375 Bonding in Allenes 377 Preparation of Dienes 378 Addition of Hydrogen Halides to Conjugated Dienes Halogen Addition to Dienes 382 The Diels–Alder Reaction 382 Diene Polymers 365 379 383 10.13 The π Molecular Orbitals of Ethylene and 1,3-Butadiene 386 10.14 A π Molecular Orbital Analysis of the Diels–Alder Reaction 388 10.15 SUMMARY PROBLEMS 390 393 xvi CONTENTS CHAPTER 11 ARENES AND AROMATICITY 11.1 11.2 398 Benzene 399 Kekulé and the Structure of Benzene 399 Benzene, Dreams, and Creative Thinking 11.3 11.4 11.5 11.6 11.7 11.8 401 A Resonance Picture of Bonding in Benzene 402 The Stability of Benzene 403 An Orbital Hybridization View of Bonding in Benzene 405 The π Molecular Orbitals of Benzene 405 Substituted Derivatives of Benzene and Their Nomenclature 406 Polycyclic Aromatic Hydrocarbons 408 Carbon Clusters, Fullerenes, and Nanotubes 11.9 11.10 11.11 11.12 11.13 11.14 11.15 11.16 11.17 11.18 11.19 11.20 11.21 11.22 410 Physical Properties of Arenes 411 Reactions of Arenes: A Preview 411 The Birch Reduction 412 Free-Radical Halogenation of Alkylbenzenes 414 Oxidation of Alkylbenzenes 416 Nucleophilic Substitution in Benzylic Halides 417 Preparation of Alkenylbenzenes 419 Addition Reactions of Alkenylbenzenes 419 Polymerization of Styrene 421 Cyclobutadiene and Cyclooctatetraene 422 Hückel’s Rule: Annulenes 423 Aromatic Ions 426 Heterocyclic Aromatic Compounds 430 Heterocyclic Aromatic Compounds and Hückel’s Rule 11.23 SUMMARY PROBLEMS 432 433 437 CHAPTER 12 REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION 443 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13 12.14 12.15 12.16 Representative Electrophilic Aromatic Substitution Reactions of Benzene 444 Mechanistic Principles of Electrophilic Aromatic Substitution 444 Nitration of Benzene 447 Sulfonation of Benzene 448 Halogenation of Benzene 448 Friedel–Crafts Alkylation of Benzene 450 Friedel–Crafts Acylation of Benzene 453 Synthesis of Alkylbenzenes by Acylation–Reduction 455 Rate and Regioselectivity in Electrophilic Aromatic Substitution 457 Rate and Regioselectivity in the Nitration of Toluene 458 Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 461 Substituent Effects in Electrophilic Aromatic Substitution: Activating Substituents 463 Substituent Effects in Electrophilic Aromatic Substitution: Strongly Deactivating Substituents 466 Substituent Effects in Electrophilic Aromatic Substitution: Halogens 469 Multiple Substituent Effects 470 Regioselective Synthesis of Disubstituted Aromatic Compounds 472 564 Iodomethylzinc iodide is known as the Simmons– Smith reagent, after Howard E Simmons and Ronald D Smith of Du Pont, who first described its use in the preparation of cyclopropanes CHAPTER FOURTEEN Organometallic Compounds ϩ Zn ICH2I Diiodomethane diethyl ether Cu Zinc ICH2ZnI Iodomethylzinc iodide What makes iodomethylzinc iodide such a useful reagent is that it reacts with alkenes to give cyclopropanes CH2CH3 CH2CH3 CH2 CH2I2, Zn(Cu) diethyl ether C CH3 CH3 2-Methyl-1-butene 1-Ethyl-1-methylcyclopropane (79%) This reaction is called the Simmons–Smith reaction and is one of the few methods available for the synthesis of cyclopropanes Mechanistically, the Simmons–Smith reaction seems to proceed by a single-step cycloaddition of a methylene (CH2) unit from iodomethylzinc iodide to the alkene: C C C I ICH2ZnI C C CH2 C ϩ ZnI2 CH2 ZnI Transition state for methylene transfer PROBLEM 14.10 What alkenes would you choose as starting materials in order to prepare each of the following cyclopropane derivatives by reaction with iodomethylzinc iodide? CH3 (a) (b) SAMPLE SOLUTION (a) In a cyclopropane synthesis using the Simmons–Smith reagent, you should remember that a CH2 unit is transferred Therefore, retrosynthetically disconnect the bonds to a CH2 group of a three-membered ring to identify the starting alkene CH3 CH2 CH3 ϩ [CH2] The complete synthesis is: CH3 CH3 CH2I2, Zn(Cu) diethyl ether 1-Methylcycloheptene 1-Methylbicyclo[5.1.0]octane (55%) Methylene transfer from iodomethylzinc iodide is stereospecific Substituents that were cis in the alkene remain cis in the cyclopropane 14.13 CH3CH2 CH2CH3 C H CH3CH2 CH2I2 Zn(Cu) ether C H CH3CH2 C H H cis-1,2-Diethylcyclopropane (34%) H C CH2CH3 H (Z)-3-Hexene Carbenes and Carbenoids CH2CH3 CH3CH2 CH2I2 Zn(Cu) ether (E)-3-Hexene H H CH2CH3 trans-1,2-Diethylcyclopropane (15%) Yields in Simmons–Smith reactions are sometimes low Nevertheless, since it often provides the only feasible route to a particular cyclopropane derivative, it is a valuable addition to the organic chemist’s store of synthetic methods 14.13 CARBENES AND CARBENOIDS Iodomethylzinc iodide is often referred to as a carbenoid, meaning that it resembles a carbene in its chemical reactions Carbenes are neutral molecules in which one of the carbon atoms has six valence electrons Such carbons are divalent; they are directly bonded to only two other atoms and have no multiple bonds Iodomethylzinc iodide reacts as if it were a source of the carbene H±C±H It is clear that free :CH2 is not involved in the Simmons–Smith reaction, but there is substantial evidence to indicate that carbenes are formed as intermediates in certain other reactions that convert alkenes to cyclopropanes The most studied examples of these reactions involve dichlorocarbene and dibromocarbene C C Cl Cl Dichlorocarbene Br Br Dibromocarbene Carbenes are too reactive to be isolated and stored, but have been trapped in frozen argon for spectroscopic study at very low temperatures Dihalocarbenes are formed when trihalomethanes are treated with a strong base, such as potassium tert-butoxide The trihalomethyl anion produced on proton abstraction dissociates to a dihalocarbene and a halide anion: Br3C ϩ H Tribromomethane Ϫ tert-Butoxide ion Br Br C Ϫ OC(CH3)3 ϩ H Br3C Tribromomethide ion OC(CH3)3 tert-Butyl alcohol Br Ϫ Br Tribromomethide ion C ϩ Br Ϫ Br Dibromocarbene Bromide ion When generated in the presence of an alkene, dihalocarbenes undergo cycloaddition to the double bond to give dihalocyclopropanes: 565 566 CHAPTER FOURTEEN ϩ Cyclohexene Organometallic Compounds CHBr3 Br KOC(CH3)3 (CH3)3COH Tribromomethane Br 7,7-Dibromobicyclo[4.1.0]heptane (75%) The reaction of dihalocarbenes with alkenes is stereospecific, and syn addition is observed PROBLEM 14.11 The syn stereochemistry of dibromocarbene cycloaddition was demonstrated in experiments using cis- and trans-2-butene Give the structure of the product obtained from addition of dibromocarbene to each alkene The process in which a dihalocarbene is formed from a trihalomethane corresponds to an elimination in which a proton and a halide are lost from the same carbon It is an ␣-elimination proceeding via the organometallic intermediate Kϩ [:CX3]Ϫ 14.14 TRANSITION-METAL ORGANOMETALLIC COMPOUNDS A large number of organometallic compounds are based on transition metals Examples include organic derivatives of iron, nickel, chromium, platinum, and rhodium Many important industrial processes are catalyzed by transition metals or their complexes Before we look at these processes, a few words about the structures of transition-metal complexes are in order A transition-metal complex consists of a transition-metal atom or ion bearing attached groups called ligands Essentially, anything attached to a metal is a ligand A ligand can be an element (O2, N2), a compound (NO), or an ion (CNϪ); it can be inorganic as in the examples just cited or it can be an organic ligand Ligands differ in the number of electrons that they share with the transition metal to which they are attached Carbon monoxide is a frequently encountered ligand in transition-metal complexes and Ϫ ϩ contributes two electrons; it is best thought of in terms of the Lewis structure CPO in which carbon is the reactive site An example of a carbonyl complex of a transition metal is nickel carbonyl, a very toxic substance, which was first prepared over a hundred years ago and is an intermediate in the purification of nickel It forms spontaneously when carbon monoxide is passed over elemental nickel Ni Nickel ϩ 4CO Ni(CO)4 Carbon monoxide Nickel carbonyl Many transition-metal complexes, including Ni(CO)4, obey what is called the 18electron rule, which is to transition-metal complexes as the octet rule is to main-group elements It states that for transition-metal complexes, the number of ligands that can be attached to a metal will be such that the sum of the electrons brought by the ligands plus the valence electrons of the metal equals 18 With an atomic number of 28, nickel has the electron configuration [Ar]4s23d8 (10 valence electrons) The 18-electron rule is satisfied by adding to these 10 the electrons from four carbon monoxide ligands A useful point to remember about the 18-electron rule when we discuss some reactions of transition-metal complexes is that if the number is less than 18, the metal is considered coordinatively unsaturated and can accept additional ligands PROBLEM 14.12 Like nickel, iron reacts with carbon monoxide to form a compound having the formula M(CO)n that obeys the 18-electron rule What is the value of n in the formula Fe(CO)n? 14.15 Ziegler–Natta Catalysis of Alkene Polymerization 567 Not all ligands use just two electrons to bond to transition metals Chromium has the electron configuration [Ar]4s23d (6 valence electrons) and needs 12 more to satisfy the 18-electron rule In the compound (benzene)tricarbonylchromium, of these 12 are the electrons of the benzene ring; the remaining are from the three carbonyl ligands H H H H Fe H OC Cr H CO CO (Benzene)tricarbonylchromium Ferrocene Ferrocene has an even more interesting structure A central iron is -bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich It, too, obeys the 18electron rule Each cyclopentadienyl ligand contributes electrons for a total of 10 and iron, with an electron configuration of [Ar]4s 23d contributes Alternatively, ferrocene can be viewed as being derived from Fe2ϩ (6 valence electrons) and two aromatic cyclopentadienide rings (6 electrons each) Indeed, ferrocene was first prepared by adding iron(II) chloride to cyclopentadienylsodium Instead of the expected -bonded species shown in the equation, ferrocene was formed H Ϫ Naϩ Cyclopentadienylsodium ϩ FeCl2 Iron(II) chloride H Fe ϩ 2NaCl (Not formed) After ferrocene, a large number of related molecules have been prepared—even some in which uranium is the metal There is now an entire subset of transition-metal organometallic complexes known as metallocenes based on cyclopentadienide ligands These compounds are not only structurally interesting, but many of them have useful applications as catalysts for industrial processes Naturally occurring compounds with carbon–metal bonds are very rare The best example of such an organometallic compound is coenzyme B12, which has a carbon–cobalt bond (Figure 14.3) Pernicious anemia results from a coenzyme B12 deficiency and can be treated by adding sources of cobalt to the diet One source of cobalt is vitamin B12, a compound structurally related to, but not identical with, coenzyme B12 14.15 ZIEGLER–NATTA CATALYSIS OF ALKENE POLYMERIZATION In Section 6.21 we listed three main methods for polymerizing alkenes: cationic, freeradical, and coordination polymerization In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregular polymer with superior physical properties Because the catalysts responsible for coordination polymerization are organometallic compounds, we are now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works The first page of this chapter displayed an electrostatic potential map of ferrocene You may wish to view a molecular model of it on Learning By Modeling Cyclopentadienylsodium is ionic Its anion is the cyclopentadienide ion, which contains six electrons 568 CHAPTER FOURTEEN Organometallic Compounds AN ORGANOMETALLIC COMPOUND THAT OCCURS NATURALLY: COENZYME B12 P ernicious anemia is a disease characterized, as are all anemias, by a deficiency of red blood cells Unlike ordinary anemia, pernicious anemia does not respond to treatment with sources of iron, and before effective treatments were developed, was often fatal Injection of liver extracts was one such treatment, and in 1948 chemists succeeded in isolating the “antipernicious anemia factor” from beef liver as a red crystalline compound, which they called vitamin B12 This compound had the formula C63H88CoN14O14P Its complexity precluded structure determination by classical degradation techniques, and spectroscopic methods were too primitive to be of much help The structure was solved by Dorothy Crowfoot Hodgkin of Oxford University in 1955 using X-ray diffraction techniques and is shown in Figure 14.3a Structure determination by X-ray crystallography can be superficially considered as taking a photograph of a molecule with X-rays It is a demanding task and earned Hodgkin the 1964 Nobel Prize in chemistry Modern structural studies by X-ray crystal- H2N O O CH3 H2N R CH3 CH3 O N ϩ NH2 O N N CH3 CH3 H3C HN O OϪ O R ϭϪCPN (b) O CH3 HO NH2 OH Rϭ CH2 O N N N N P O H3C N (a) O N O CH3 NH2 CH3 Co H2N lography use computers to collect and analyze the diffraction data and take only a fraction of the time required years ago to solve the vitamin B12 structure The structure of vitamin B12 is interesting in that it contains a central cobalt atom that is surrounded by six atoms in an octahedral geometry One substituent, the cyano (±CN) group, is what is known as an “artifact.” It appears to be introduced into the molecule during the isolation process and leads to the synonym cyanocobalamin for vitamin B12 This material is used to treat pernicious anemia, but this is not the form in which it exerts its activity The biologically active material is called coenzyme B12 and differs from vitamin B12 in the substituent attached to cobalt (Figure 14.3b) Coenzyme B12 is the only known naturally occurring substance that has a carbon–metal bond Moreover, coenzyme B12 was discovered before any compound containing an alkyl group -bonded to cobalt had ever been isolated in the laboratory! HO N CH3 O HOCH2 FIGURE 14.3 The structures of (a) vitamin B12 and (b) coenzyme B12 H2N 14.15 Ziegler–Natta Catalysis of Alkene Polymerization In the early 1950s, Karl Ziegler, then at the Max Planck Institute for Coal Research in Germany, was studying the use of aluminum compounds as catalysts for the oligomerization of ethylene nH2C CH2 Al(CH2CH3)3 CH3CH2(CH2CH2)nϪ2CH Ethylene CH2 Ethylene oligomers Ziegler found that adding certain metals or their compounds to the reaction mixture led to the formation of ethylene oligomers with 6–18 carbons, but others promoted the formation of very long carbon chains giving polyethylene Both were major discoveries The 6–18 carbon ethylene oligomers constitute a class of industrial organic chemicals known as linear ␣ olefins that are produced at a rate of 109 pounds/year in the United States The Ziegler route to polyethylene is even more important because it occurs at modest temperatures and pressures and gives high-density polyethylene, which has properties superior to the low-density material formed by free-radical polymerization described in Section 6.21 A typical Ziegler catalyst is a combination of titanium tetrachloride (TiCl4) and diethylaluminum chloride [(CH3CH2)2AlCl], but other combinations such as TiCl3/(CH3CH2)3Al also work as catalysts based on metallocenes Although still in question, a plausible mechanism for the polymerization of ethylene in the presence of such catalysts has been offered and is outlined in Figure 14.4 Step 1: A titanium halide and an ethylaluminum compound combine to place an ethyl group on titanium, giving the active catalyst Titanium has one or more vacant coordination sites, shown here as an empty orbital CH3CH2 W ClnTi Step 2: Ethylene reacts with the active form of the catalyst The π orbital of ethylene with its two electrons overlaps with the vacant titanium orbital to bind ethylene as a ligand to titanium CH3CH2 W ClnTi ϩ H2CœCH2 CH3CH2 W ClnTi CH2 X CH2 Step 3: The flow of electrons from ethylene to titanium increases the electron density at titanium and weakens the TiQCH2CH3 bond The ethyl group migrates from titanium to one of the carbons of ethylene CH3CH2 W ClnTi CH3CH2 CH2 X CH2 CH2 W ClnTi±CH2 Step 4: The catalyst now has a butyl ligand on titanium instead of an ethyl group Repeating steps and converts the butyl group to a hexyl group, then an octyl group, and so on After thousands of repetitions, polyethylene is formed FIGURE 14.4 A proposed mechanism for the polymerization of ethylene in the presence of a Ziegler–Natta catalyst 569 570 CHAPTER FOURTEEN Organometallic Compounds Ziegler had a working relationship with the Italian chemical company Montecatini, for which Giulio Natta of the Milan Polytechnic Institute was a consultant When Natta used Ziegler’s catalyst to polymerize propene, he discovered that the catalyst was not only effective but that it gave mainly isotactic polypropylene (Recall from Section 7.15 that free-radical polymerization of propene gives atactic polypropylene.) Isotactic polypropylene has a higher melting point than the atactic form and can be drawn into fibers or molded into hard, durable materials Before coordination polymerization was discovered by Ziegler and applied to propene by Natta, there was no polypropylene industry Now, more than 1010 pounds of it are prepared each year in the United States Ziegler and Natta shared the 1963 Nobel Prize in chemistry: Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular polymerization 14.16 SUMMARY Section 14.1 Section 14.2 Organometallic compounds contain a carbon–metal bond They are named as alkyl (or aryl) derivatives of metals CH3CH2CH2CH2Li C6H5MgBr Butyllithium Phenylmagnesium bromide Carbon is more electronegative than metals and carbon–metal bonds are polarized so that carbon bears a partial to complete negative charge and the metal bears a partial to complete positive charge H H ␦Ϫ C Li␦ϩ HC Ϫ C Naϩ H Methyllithium has a polar covalent carbon–lithium bond Sodium acetylide has an ionic bond between carbon and sodium Section 14.3 See Table 14.4 Section 14.4 See Table 14.4 Section 14.5 Organolithium compounds and Grignard reagents are strong bases and react instantly with compounds that have ±OH groups R M ϩH O RЈ R H ϩ Mϩ Ϫ O RЈ These organometallic compounds cannot therefore be formed or used in solvents such as water and ethanol The most commonly employed solvents are diethyl ether and tetrahydrofuran Section 14.6 See Tables 14.3 and 14.5 Section 14.7 See Table 14.5 Section 14.8 See Table 14.5 Section 14.9 When planning the synthesis of a compound using an organometallic reagent, or indeed any synthesis, the best approach is to reason backward from the product This method is called retrosynthetic analysis Retrosynthetic analysis of 1-methylcyclohexanol suggests it can be prepared by the reaction of methylmagnesium bromide and cyclohexanone 14.16 TABLE 14.4 Summary 571 Preparation of Organometallic Reagents Used in Synthesis General equation for preparation and specific example Type of organometallic reagent (section) and comments Organolithium reagents (Section 14.3) Lithium metal reacts with organic halides to produce organolithium compounds The organic halide may be alkyl, alkenyl, or aryl Iodides react most and fluorides least readily; bromides are used most often Suitable solvents include hexane, diethyl ether, and tetrahydrofuran ϩ RX Alkyl halide RLi Lithium Alkyllithium Li diethyl ether CH3CH2CH2Br Grignard reagents (Section 14.4) Grignard reagents are prepared in a manner similar to that used for organolithium compounds Diethyl ether and tetrahydrofuran are appropriate solvents Mg RMgX Magnesium Alkylmagnesium halide (Grignard reagent) Mg diethyl ether Benzyl chloride Alkyllithium R2CuLi Copper(I) halide Lithium dialkylcuprate Methyllithium 1-Methylcyclohexanol diethyl ether CuI Copper(I) iodide ϩ Zn CH2I2 Diiodomethane O ϩ Cyclohexanone diethyl ether Cu Zinc CH3MgBr Section 14.12 See Tables 14.4 and 14.5 Section 14.13 Carbenes are species that contain a divalent carbon; that is, a carbon with only two bonds One of the characteristic reactions of carbenes is with alkenes to give cyclopropane derivatives CH3 ϩ C CH3 2-Methylpropene CH3 Cl (CH3)2CuLi Iodomethylzinc iodide Section 14.11 See Tables 14.4 and 14.5 KOC(CH3)3 CHCl3 (CH ) COH 3 Lithium halide ICH2ZnI Methylmagnesium bromide CH3 LiX Lithium dimethylcuprate Section 14.10 See Table 14.5 H2C ϩ CuX ϩ 2CH3Li C6H5CH2MgCl Benzylmagnesium chloride (93%) ϩ 2RLi OH CH3CH2CH2Li ϩ C6H5CH2Cl CH3 Lithium halide Propyllithium (78%) Alkyl halide Iodomethylzinc iodide (Section 14.12) This is the Simmons–Smith reagent It is prepared by the reaction of zinc (usually in the presence of copper) with diiodomethane LiX Propyl bromide RX Lithium dialkylcuprates (Section 14.11) These reagents contain a negatively charged copper atom and are formed by the reaction of a copper(I) salt with two equivalents of an organolithium reagent ϩ 2Li Cl 1,1-Dichloro-2,2-dimethylcyclopropane (65%) ϩ LiI Lithium iodide 572 TABLE 14.5 CHAPTER FOURTEEN Organometallic Compounds Carbon–Carbon Bond-Forming Reactions of Organometallic Reagents Reaction (section) and comments Alcohol synthesis via the reaction of Grignard reagents with carbonyl compounds (Section 14.6) This is one of the most useful reactions in synthetic organic chemistry Grignard reagents react with formaldehyde to yield primary alcohols, with aldehydes to give secondary alcohols, and with ketones to form tertiary alcohols General equation and specific example O X RЈCRЉ RMgX ϩ Grignard reagent Aldehyde or ketone Alcohol O X ϩ CH3CH2CH2CH CH3MgI Methylmagnesium iodide Reaction of Grignard reagents with esters (Section 14.10) Tertiary alcohols in which two of the substituents on the hydroxyl carbon are the same may be prepared by the reaction of an ester with two equivalents of a Grignard reagent Grignard reagent ϩ Alkyllithium RЈ W RCOH W R Tertiary alcohol O X ϩ C6H5COCH2CH3 Phenylmagnesium bromide O X ϩ CH3CC(CH3)3 3,3-Dimethyl2-butanone (C6H5)3COH Triphenylmethanol (89–93%) diethyl ether H3Oϩ Aldehyde or ketone Cyclopropyllithium diethyl ether H3Oϩ Ethyl benzoate O X RЈCRЉ Li CH3CH2CH2CHCH3 W OH 2-Pentanol (82%) diethyl ether H3Oϩ Ester RLi diethyl ether H3Oϩ Butanal O X 2RMgX ϩ RЈCORЉ 2C6H5MgBr Synthesis of alcohols using organolithium reagents (Section 14.7) Organolithium reagents react with aldehydes and ketones in a manner similar to that of Grignard reagents to produce alcohols RЈ W RCOH W RЉ diethyl ether H3Oϩ RЈ W RCOH W RЉ Alcohol diethyl ether H3Oϩ OH W CC(CH3)3 W CH3 2-Cyclopropyl3,3-dimethyl2-butanol (71%) (Continued) Certain organometallic compounds resemble carbenes in their reactions and are referred to as carbenoids Iodomethylzinc iodide (Section 14.12) is an example Section 14.14 Transition-metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a bond or through its system Metallocenes are transition-metal complexes in which one or more of the ligands is a Problems TABLE 14.5 573 Carbon–Carbon Bond-Forming Reactions of Organometallic Reagents (Continued) Reaction (section) and comments Synthesis of acetylenic alcohols (Section 14.8) Sodium acetylide and acetylenic Grignard reagents react with aldehydes and ketones to give alcohols of the type CPC±COH General equation and specific example O X RCRЈ NaCPCH ϩ Sodium acetylide NH3, Ϫ33°C H3Oϩ Aldehyde or ketone Alcohol O X NaCPCH ϩ CH3CCH2CH3 Sodium acetylide Preparation of alkanes using lithium dialkylcuprates (Section 14.11) Two alkyl groups may be coupled together to form an alkane by the reaction of an alkyl halide with a lithium dialkylcuprate Both alkyl groups must be primary (or methyl) Aryl and vinyl halides may be used in place of alkyl halides The Simmons-Smith reaction (Section 14.12) Methylene transfer from iodomethylzinc iodide converts alkenes to cyclopropanes The reaction is a stereospecific syn addition of a CH2 group to the double bond OH W HCPCCRЈ W R 2-Butanone ϩ OH W HCPCCCH2CH3 W CH3 NH3, Ϫ33°C H3Oϩ 3-Methyl-1-pentyn-3-ol (72%) RЈCH2X RCH2RЈ Lithium dialkylcuprate Primary alkyl halide Alkane (CH3)2CuLi ϩ C6H5CH2Cl diethyl ether R2CuLi Lithium dimethylcuprate C6H5CH2CH3 Benzyl chloride Ethylbenzene (80%) R R2CœCR2 ϩ Alkene ICH2ZnI R Iodomethylzinc iodide Bicyclo[3.1.0]hexane (53%) cyclopentadienyl ring Ferrocene was the first metallocene synthesized; its structure is shown on the opening page of this chapter Section 14.15 Coordination polymerization of ethylene and propene has the biggest eco- nomic impact of any organic chemical process Ziegler–Natta polymerization is carried out in the presence of catalysts derived from transition metals such as titanium -Bonded and -bonded organometallic compounds are intermediates in coordination polymerization Problems 14.13 Write structural formulas for each of the following compounds Specify which compounds qualify as organometallic compounds (a) Cyclopentyllithium (d) Lithium divinylcuprate (b) Ethoxymagnesium chloride (e) Sodium carbonate (c) 2-Phenylethylmagnesium iodide (f) Benzylpotassium R Cyclopropane derivative CH2I2, Zn(Cu) diethyl ether Cyclopentene R diethyl ether ϩ ZnI2 Zinc iodide 574 CHAPTER FOURTEEN Organometallic Compounds 14.14 Dibal is an informal name given to the organometallic compound [(CH3)2CHCH2]2AlH, used as a reducing agent in certain reactions Can you figure out the systematic name from which “dibal” is derived? 14.15 Suggest appropriate methods for preparing each of the following compounds from the starting material of your choice (a) CH3CH2CH2CH2CH2MgI (c) CH3CH2CH2CH2CH2Li (b) CH3CH2CPCMgI (d) (CH3CH2CH2CH2CH2)2CuLi 14.16 Which compound in each of the following pairs would you expect to have the more polar carbon–metal bond? Compare the models on Learning By Modeling with respect to the charge on the carbon bonded to the metal (a) CH3CH2Li or (CH3CH2)3Al (c) CH3CH2MgBr or HCPCMgBr (b) (CH3)2Zn or (CH3)2Mg 14.17 Write the structure of the principal organic product of each of the following reactions: (a) 1-Bromopropane with lithium in diethyl ether (b) 1-Bromopropane with magnesium in diethyl ether (c) 2-Iodopropane with lithium in diethyl ether (d) 2-Iodopropane with magnesium in diethyl ether (e) Product of part (a) with copper(I) iodide (f) Product of part (e) with 1-bromobutane (g) Product of part (e) with iodobenzene (h) Product of part (b) with D2O and DCl (i) Product of part (c) with D2O and DCl (j) Product of part (a) with formaldehyde in ether, followed by dilute acid (k) Product of part (b) with benzaldehyde in ether, followed by dilute acid (l) Product of part (c) with cycloheptanone in ether, followed by dilute acid O X (m) Product of part (d) with CH3CCH2CH3 in ether, followed by dilute acid O X (n) Product of part (b) with C6H5COCH3 (2 mol) in ether, followed by dilute acid (o) 1-Octene with diiodomethane and zinc–copper couple in ether (p) (E)-2-Decene with diiodomethane and zinc–copper couple in ether (q) (Z )-3-Decene with diiodomethane and zinc–copper couple in ether (r) 1-Pentene with tribromomethane and potassium tert-butoxide in tert-butyl alcohol 14.18 Using 1-bromobutane and any necessary organic or inorganic reagents, suggest efficient syntheses of each of the following alcohols: (a) 1-Pentanol (d) 3-Methyl-3-heptanol (b) 2-Hexanol (e) 1-Butylcyclobutanol (c) 1-Phenyl-1-pentanol 14.19 Using bromobenzene and any necessary organic or inorganic reagents, suggest efficient syntheses of each of the following: (a) Benzyl alcohol (b) 1-Phenyl-1-hexanol Problems (c) Bromodiphenylmethane (e) 1-Phenylcyclooctanol (d) 4-Phenyl-4-heptanol (f) trans-2-Phenylcyclooctanol 14.20 Analyze the following structures so as to determine all the practical combinations of Grignard reagent and carbonyl compound that will give rise to each: (a) CH3CH2CHCH2CH(CH3)2 (d) 6-Methyl-5-hepten-2-ol OH (b) CH (e) OCH3 OH OH (c) (CH3)3CCH2OH 14.21 A number of drugs are prepared by reactions of the type described in this chapter Indicate what you believe would be a reasonable last step in the synthesis of each of the following: OH (a) CH3CH2CC CH Meparfynol, a mild hypnotic or sleep-inducing agent CH3 CH3 (b) (C6H5)2CCH N Diphepanol, an antitussive (cough suppressant) OH OH CH3 C CH Mestranol, an estrogenic component of oral contraceptive drugs (c) CH3O 14.22 Predict the principal organic product of each of the following reactions: O (a) ϩ NaC C ϩ CH3CH2Li (b) CH diethyl ether H3Oϩ O (c) Br Mg, THF O X HCH H3Oϩ CH2CH (d) liquid ammonia H3Oϩ CH2 CH2I2 Zn(Cu) diethyl ether 575 576 CHAPTER FOURTEEN Organometallic Compounds H (e) CH3 C CH2 C H CH2I2 Zn(Cu) ether I ϩ LiCu(CH3)2 (f) CH3O O CH2OS (g) CH3 ϩ LiCu(CH2CH2CH2CH3)2 O O 14.23 Addition of phenylmagnesium bromide to 4-tert-butylcyclohexanone gives two isomeric ter- tiary alcohols as products Both alcohols yield the same alkene when subjected to acid-catalyzed dehydration Suggest reasonable structures for these two alcohols O C(CH3)3 4-tert-Butylcyclohexanone 14.24 (a) Unlike other esters, which react with Grignard reagents to give tertiary alcohols, ethyl O X formate (HCOCH2CH3) yields a different class of alcohols on treatment with Grignard reagents What kind of alcohol is formed in this case and why? O X (b) Diethyl carbonate (CH3CH2OCOCH2CH3) reacts with excess Grignard reagent to yield alcohols of a particular type What is the structural feature that characterizes alcohols prepared in this way? 14.25 Reaction of lithium diphenylcuprate with optically active 2-bromobutane yields 2-phenylbutane, with high net inversion of configuration When the 2-bromobutane used has the stereostructure shown, will the 2-phenylbutane formed have the R or the S configuration? CH3CH2 CH3 C H Br 14.26 Suggest reasonable structures for compounds A, B, and C in the following reactions: OTs LiCu(CH3)2 (CH3)3C compound A ϩ compound B (C11H22) (C10H18) OTs LiCu(CH3)2 (CH3)3C compound B ϩ compound C (C11H22) Compound C is more stable than compound A OTs stands for toluenesulfonate Problems 14.27 The following conversion has been reported in the chemical literature It was carried out in two steps, the first of which involved formation of a p-toluenesulfonate ester Indicate the reagents for this step, and show how you could convert the p-toluenesulfonate to the desired product two steps O OH O 14.28 Sometimes the strongly basic properties of Grignard reagents can be turned to synthetic advantage A chemist needed samples of butane specifically labeled with deuterium, the mass isotope of hydrogen, as shown: (a) CH3CH2CH2CH2D (b) CH3CHDCH2CH3 Suggest methods for the preparation of each of these using heavy water (D2O) as the source of deuterium, butanols of your choice, and any necessary organic or inorganic reagents 14.29 Diphenylmethane is significantly more acidic than benzene, and triphenylmethane is more acidic than either Identify the most acidic proton in each compound, and suggest a reason for the trend in acidity C6H6 (C6H5)2CH2 (C6H5)3CH Benzene Ka Ϸ 10Ϫ45 Diphenylmethane Ka Ϸ 10Ϫ34 Triphenylmethane Ka Ϸ 10Ϫ32 14.30 The 18-electron rule is a general, but not universal, guide for assessing whether a certain transition-metal complex is stable or not Both of the following are stable compounds, but only one obeys the 18-electron rule Which one? H Cl Ti H H OC Cl Fe H CO CO 14.31 One of the main uses of the “linear ␣-olefins” prepared by oligomerization of ethylene is in the preparation of linear low-density polyethylene Linear low-density polyethylene is a copolymer produced when ethylene is polymerized in the presence of a “linear ␣-olefin” such as 1-decene [CH2œCH(CH2)7CH3] 1-Decene replaces ethylene at random points in the growing polymer chain Can you deduce how the structure of linear low-density polyethylene differs from a linear chain of CH2 units? 14.32 Make a molecular model of 7,7-dimethylbicyclo[2.2.1]heptan-2-one Two diastereomeric alcohols may be formed when it reacts with methylmagnesium bromide Which one is formed in greater amounts? H3C CH3 O 7,7-Dimethylbicyclo[2.2.1]heptan-2-one 577 578 CHAPTER FOURTEEN Organometallic Compounds 14.33 Make molecular models of the product of addition of dichlorocarbene to: (a) trans-2-Butene (b) cis-2-Butene Which product is achiral? Which one is formed as a racemic mixture? 14.34 Examine the molecular model of ferrocene on Learning By Modeling Does ferrocene have a dipole moment? Would you expect the cyclopentadienyl rings of ferrocene to be more reactive toward nucleophiles or electrophiles? Where is the region of highest electrostatic potential? 14.35 Inspect the electrostatic potential surface of the benzyl anion structure given on Learning By Modeling What is the hybridization state of the benzylic carbon? Does the region of highest electrostatic potential lie in the plane of the molecule or perpendicular to it? Which ring carbons bear the greatest share of negative charge? ... Nanotubes 11 .9 11 .10 11 .11 11 .12 11 .13 11 .14 11 .15 11 .16 11 .17 11 .18 11 .19 11 .20 11 . 21 11. 22 410 Physical Properties of Arenes 411 Reactions of Arenes: A Preview 411 The Birch Reduction 412 Free-Radical... Rule 11 .23 SUMMARY PROBLEMS 432 433 437 CHAPTER 12 REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION 443 12 .1 12.2 12 .3 12 .4 12 .5 12 .6 12 .7 12 .8 12 .9 12 .10 12 .11 12 .12 12 .13 12 .14 12 .15 12 .16 ... Ethers and Epoxides 6 21 Physical Properties of Ethers 622 Crown Ethers 622 Polyether Antibiotics 16 .5 16 .6 16 .7 16 .8 16 .9 16 .10 16 .11 16 .12 16 .13 16 .14 16 .15 16 .16 16 .17 16 .18 619 624 Preparation