Preview organic chemistry mechanistic patterns by ackroyd, nathan browning, c scott deslongchamps, ghislain dryden, neil lee, felix ogilvie, william walter sauer, effie (2017)

Preview Organic chemistry mechanistic patterns by Ackroyd, Nathan Browning, C. Scott Deslongchamps, Ghislain Dryden, Neil Lee, Felix Ogilvie, William Walter Sauer, Effie (2017) Preview Organic chemistry mechanistic patterns by Ackroyd, Nathan Browning, C. Scott Deslongchamps, Ghislain Dryden, Neil Lee, Felix Ogilvie, William Walter Sauer, Effie (2017) Preview Organic chemistry mechanistic patterns by Ackroyd, Nathan Browning, C. Scott Deslongchamps, Ghislain Dryden, Neil Lee, Felix Ogilvie, William Walter Sauer, Effie (2017) Preview Organic chemistry mechanistic patterns by Ackroyd, Nathan Browning, C. Scott Deslongchamps, Ghislain Dryden, Neil Lee, Felix Ogilvie, William Walter Sauer, Effie (2017) Preview Organic chemistry mechanistic patterns by Ackroyd, Nathan Browning, C. Scott Deslongchamps, Ghislain Dryden, Neil Lee, Felix Ogilvie, William Walter Sauer, Effie (2017)

Organic Chemistry MECHANISTIC PATTERNS Ogilvie Ackroyd Browning Deslongchamps Lee Sauer Organic Chemistry MECHANISTIC PATTERNS Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it ORGANIC CHEMWARE Organic ChemWare for use with Organic Chemistry: Mechanistic Patterns is a comprehensive ­collection of learning objects to aid in the teaching and learning of organic chemistry at the postsecondary level Designed for both individual study and classroom projection, Organic ChemWare empowers students while redefining the lecture experience It bridges the gap between the static imagery of textbooks and the dynamic world of organic chemistry Organic ChemWare includes more than 180 interactive, web-based multimedia simulations with an emphasis on: • • • • Lewis structures curved arrow notation reaction mechanisms orbital interactions • conformational analysis • stereochemistry • 1H- and 13C-NMR In the default “Study Mode,” all animations (and orbital depictions, if applicable) are accompanied by informative text vignettes, pausing the animations and describing key points and reaction details Toggling to “Presenter Mode” hides all text vignettes and zooms the animation to promote classroom focus while reducing cognitive load All animated mechanisms are depicted in dash/wedge bond line notation; the kinematic effect of bond motion helps students to perceive and understand the threedimensionality of organic structures inferred by the notation and to “think tetrahedral.” Organic ChemWare is included with every purchase of a new text Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it ORGANIC CHEMISTRY Mechanistic Patterns William Ogilvie University of Ottawa Nathan Ackroyd Mount Royal University C Scott Browning University of Toronto Ghislain Deslongchamps University of New Brunswick Felix Lee The University of Western Ontario Effie Sauer University of Toronto Scarborough Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it This is an electronic version of the print textbook Due to electronic rights restrictions, some third party content may be suppressed The publisher reserves the right to remove content from this title at any time if subsequent rights restrictions require it For valuable information on pricing, previous editions, changes to current editions, and alternate formats, please visit nelson.com to search by ISBN#, author, title, or keyword for materials in your areas of interest Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it Organic Chemistry by William Ogilvie, Nathan Ackroyd, C Scott Browning, Ghislain Deslongchamps, Felix Lee, Effie Sauer Senior Publisher, Digital and Print Content: Paul Fam Marketing Manager: Terry Fedorkiw Technical Reviewers: Philip Dutton, Barb Morra Content Development Manager: Katherine Goodes Photo and Permissions Researcher: Kristiina Paul Production Project Manager: Lila Campbell Copy Editor: Wendy Yano Cover Design: Courtney Hellam Proofreader: A Malik Basha Cover and Mechanistic Re-View Image: Yuliyan Velchev/Shutterstock.com Indexer: BIM Creatives LLC Design Director: Ken Phipps Managing Designer: Pamela Johnston Interior Design: Cathy Mayer Production Service: Cenveo Publisher Services COPYRIGHT © 2018 by Nelson Education Ltd Printed and bound in Canada 20 19 18 17 For more information contact Nelson Education Ltd., 1120 Birchmount Road, Toronto, Ontario, M1K 5G4 Or you can visit our Internet site at nelson.com Cognero and Full-Circle Assessment are registered trademarks of Madeira Station LLC ALL RIGHTS RESERVED No part of this work covered by the copyright herein may be reproduced, transcribed, or used in any form or by any means—graphic, electronic, or mechanical, including photocopying, recording, taping, Web distribution, or information storage and retrieval systems— without the written permission of the publisher For permission to use material from this text or product, submit all requests online at cengage.com/permissions Further questions about permissions can be emailed to permissionrequest@ cengage.com Every effort has been made to trace ownership of all copyrighted material and to secure permission from copyright holders In the event of any question arising as to the use of any material, we will be pleased to make the necessary corrections in future printings Organic ChemWare Icon: Ghislain Deslongchamps Art Coordinator: Suzanne Peden Illustrators: Crowle Art Group, Cenveo Publisher Services Compositor: Cenveo Publisher Services Library and Archives Canada Cataloguing in Publication Data Ogilvie, William Walter, author Organic chemistry: mechanistic patterns / William Ogilvie (University of Ottawa), Nathan Ackroyd (Mount Royal University), Felix Lee (The University of Western Ontario), Scott Browning (University of Toronto), Ghislain Deslongchamps (University of New Brunswick), Effie Sauer (University of Toronto) Includes bibliographical references and index ISBN 978-0-17-650026-9 (hardcover) 1. Chemistry, Organic— Textbooks.  I. Ackroyd, Nathan, author  II. Title QD251.3.O45 2017  547 C2016-907181-2 ISBN-13: 978-0-17-650026-9 ISBN-10: 0-17-650026-X Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it BRIEF CONTENTS About the Authors  ix Foreword  xi Preface  xii CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER Carbon and Its Compounds  Anatomy of an Organic Molecule  47 Molecules in Motion: Conformations by Rotations  86 Stereochemistry: Three-Dimensional Structure in Molecules  125 Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms  186 Acids and Bases  235 CHAPTER π Bonds as Electrophiles: Reactions of Carbonyls and Related Functional Groups  272 CHAPTER π Bonds as Nucleophiles: Reactions of Alkenes, Alkynes, Dienes, and Enols  328 CHAPTER Conjugation and Aromaticity  398 CHAPTER 10 Synthesis Using Aromatic Materials: Electrophilic Aromatic Substitution and Directed Ortho Metalation  431 CHAPTER 11 Displacement Reactions on Saturated Carbons: SN1 and SN2 Substitution Reactions  494 CHAPTER 12 Formation of π Bonds by Elimination Processes: Elimination and Oxidation Reactions  540 CHAPTER 13 Structure Determination I: Nuclear Magnetic Resonance Spectroscopy  577 CHAPTER 14 Structure Determination II: Mass Spectrometry and Infrared Spectroscopy  648 CHAPTER 15 π Bond Electrophiles Connected to Leaving Groups: Carboxylic Acid Derivatives and Their Reactions  696 CHAPTER 16 π Bonds with Hidden Leaving Groups: Reactions of Acetals and Related Compounds  764 CHAPTER 17 Carbonyl-Based Nucleophiles: Aldol, Claisen, Wittig, and Related Enolate Reactions  810 CHAPTER 18 Selectivity and Reactivity in Enolate Reactions: Control of Stereoselectivity and Regioselectivity 899 CHAPTER 19 Radicals: Halogenation, Polymerization, and Reduction Reactions  971 CHAPTER 20 Reactions Controlled by Orbital Interactions: Ring Closures, Cycloadditions, and Rearrangements 1011 Appendix A Answers to Checkpoint Problems  A-1 Appendix B Common Errors in Organic Structures and Mechanisms  A-137 Appendix C pKa Values of Selected Organic Compounds  A-141 Appendix D NMR and IR Spectroscopic Data  A-143 Appendix E Periodic Table of the Elements  A-145 Glossary  G-1 Index  I-1 NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it iii Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it CONTENTS About the Authors  ix Foreword  xi Preface  xii CHAPTER Carbon and Its Compounds  1.1 Why It Matters  1.2 Organic Molecules from the Inside Out I: The Modelling of Atoms  1.3 Organic Molecules from the Inside Out II: Bonding  1.4 Organic Molecules Represented as Lewis Structures  1.5 Covalent Bonding: Overlap of Valence Atomic Orbitals  11 1.6 The Shapes of Atoms in Organic Molecules  14 1.7 The Valence Bond Approach to Electron Sharing  19 1.8 Resonance Forms: Molecules Represented by More than One Lewis Structure  26 1.9 Molecular Orbital Approach to Electron Sharing  32 1.10 Other Representations of Organic Molecules  34 Bringing It Together  40 CHAPTER Anatomy of an Organic Molecule  47 2.1 2.2 2.3 2.4 Why It Matters  47 Structural Features of Molecules  48 Functional Groups and Intermolecular Forces  54 Relation between Intermolecular Forces, Molecular Structure, and Physical Properties  60 2.5 Naming Organic Molecules  67 Bringing It Together  80 CHAPTER CHAPTER Stereochemistry: Three-Dimensional Structure in Molecules 125 4.1 Why It Matters  125 4.2 Constitutional Isomers and Stereoisomers  127 4.3 Chirality Centres  135 4.4 Cahn-Ingold-Prelog Nomenclature  140 4.5 Drawing Enantiomers  148 4.6 Diastereomers  152 4.7 Meso Compounds  157 4.8 Double-Bond Stereoisomers  160 4.9 Physical Properties of Enantiomers and Diastereomers  163 4.10 Optical Rotation  164 4.11 Optical Purity  168 4.12 Fischer Projections  170 Bringing It Together  178 CHAPTER Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms  186 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 Why It Matters  186 Organic Reaction Mechanisms  189 Curved Arrows and Formal Charges  200 Intramolecular Reactions  203 The Stabilizing Effect of Delocalization  208 Constructing Resonance Forms  208 Evaluating Resonance Form Contributions  215 Resonance and Orbital Structure  220 Patterns in Mechanism  221 Patterns in Resonance  223 Bringing It Together  226 Molecules in Motion: Conformations by Rotations  86 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Why It Matters  86 Rotation about Single Bonds  87 Steric Strain  94 Strains in Cyclic Molecules  98 Conformations of Six-Membered Rings  102 Six-Membered Rings Flip Their Chairs  108 Six-Membered Rings with Substituents  109 Bringing It Together  116 CHAPTER Acids and Bases  235 6.1 6.2 6.3 6.4 6.5 6.6 Why It Matters  235 Electron Movements in Brønsted Acid–Base Reactions  237 Free Energy and Acid Strength  240 Qualitative Estimates of Relative Acidities  243 Relative Acidities of Positively Charged Acids  251 Quantitative Acidity Measurements  257 NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it v vi 6.7 6.8 6.9 Contents Predicting Acid–Base Equilibria  259 Lewis Acids in Organic Reactions  265 Patterns in Acids and Bases  265 Bringing It Together  266 CHAPTER π Bonds as Electrophiles: Reactions of Carbonyls and Related Functional Groups  272 7.1 Why It Matters  272 7.2 Carbonyls and Related Functional Groups Contain Electrophilic π Bonds 273 7.3 Nucleophilic Additions to Electrophilic π Bonds in Carbonyls and Other Groups  277 7.4 Over-the-Arrow Notation  284 7.5 Addition of Organometallic Compounds to Electrophilic π Bonds  288 7.6 Using Orbitals to Analyze Reactions  298 7.7 Formation of Cyanohydrins from Carbonyls  299 7.8 Leaving Groups  303 7.9 Catalysis of Addition Reactions to Electrophilic π Bonds  306 7.10 Stereochemistry of Nucleophilic Additions to π Bonds 314 7.11 Patterns in Nucleophilic Additions to π Bonds 318 Bringing It Together  320 CHAPTER π Bonds as Nucleophiles: Reactions of Alkenes, Alkynes, Dienes, and Enols  328 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Why It Matters  328 Properties of Carbon-Carbon π Bonds  330 Carbocation Formation and Function  335 Markovnikov Addition of Water to Alkenes  347 Carbocation Rearrangements  357 Addition of Halogens to Double Bonds  359 Other Types of Electrophilic Additions  364 Patterns in Alkene Addition Reactions  385 Bringing It Together  388 CHAPTER CHAPTER 10 Synthesis Using Aromatic Materials: Electrophilic Aromatic Substitution and Directed Ortho Metalation 431 10.1 10.2 10.3 10.4 Why It Matters  431 π Bonds Acting as Nucleophiles  433 Electrophilic Aromatic Substitution  434 Types of Electrophiles Used in Electrophilic Aromatic Substitution 435 10.5 Aromatic Nomenclature and Multiple Substituents  449 10.6 Directing Groups in Electrophilic Aromatic Substitution  449 10.7 Electrophilic Aromatic Substitution of Polycyclic and Heterocyclic Aromatic Compounds  466 10.8 Directed Ortho Metalation as an Alternative to Electrophilic Aromatic Substitution  472 10.9 Retrosynthetic Analysis in Aromatic Synthesis  476 10.10 Patterns in Electrophilic Aromatic Substitution Reactions  482 Bringing It Together  484 CHAPTER 11 Displacement Reactions on Saturated Carbons: SN1 and SN2 Substitution Reactions  494 11.1 Why It Matters  494 11.2 Displacement Reactions of Alkyl Halides  495 11.3 SN2 Displacements  497 11.4 SN1 Displacements  510 11.5 SN1 and SN2 as a Reactivity Continuum  520 11.6 Predicting SN1 and SN2 Reaction Mechanisms  523 11.7 Practical Considerations for Planning Displacement Reactions 524 11.8 Special Nucleophiles and Electrophiles Used in Displacement Reactions 525 11.9 Patterns in Nucleophilic Displacements on Saturated Carbons 532 Bringing It Together  534 CHAPTER 12 Conjugation and Aromaticity  398 Formation of π Bonds by Elimination Processes: Elimination and Oxidation Reactions  540 9.1 Why It Matters  398 9.2 Molecular Orbital Review: Conjugated Systems  400 9.3 Aromaticity  410 9.4 Molecular Orbital Analysis of Aromatic Rings  418 9.5 Aromatic Hydrocarbon Rings  422 Bringing It Together  426 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 Why It Matters  540 Alkene Formation by E2 Elimination Reactions  541 Alkene Formation by E1 Elimination Reactions  552 Dehydration and Dehydrohalogenation  557 Differentiation between Elimination Reactions and Nucleophilic Substitutions  559 Designing Reactions for Selectivity  561 Oxidation of Alcohols: An Elimination Reaction  563 Patterns in Eliminations and Oxidations  568 Bringing It Together  570 NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 210 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms the direction electrons flow to break the adjacent carbon-carbon p bond Positive charges are strong electron attractors, and so the electrons in the p bond will flow toward this charge, producing resonance form C Once the last p bond has broken, there are no more p electrons, and so the analysis is complete It is important to work with one bond at a time to avoid missing possible resonance structures The heteroatom is a good starting point because it has a different electronegativity than carbon This can be used to determine the proper direction to break the bond The mechanistic arrow points away from this carbon; the charge on the carbon therefore increases from to ϩ1 Ϫ O Ϫ O O ϩ ϩ A B This p bond involves two carbon atoms There is no difference in electronegativity between them and so breaking this bond produces insignificant resonance structures Organic ChemWare 5.15 Resonance: Propenal (acrolein) Organic ChemWare 5.16 Resonance: 2-Butenal (crotonaldehyde) Student Tip Not all negatively charged atoms carry lone pairs Negatively charged atoms without lone pairs not participate in resonance C The positive charge determines the direction of p bond breaking The electrons in the p bond will be attracted to the positive charge To draw resonance structures for molecules that have an existing formal charge, use the charge as the starting point for resonance analysis A positive charge attracts electrons and tends to “pull” adjacent lone pairs and p bond electrons toward it Negative charges repel electrons and “push” them into adjacent p bonds, positive charges, and atoms lacking full octets In the following example, the negative charge is a good point for beginning resonance ­analysis First determine if there are unpaired electrons associated with the negative charge If there are no unpaired electrons on a negatively charged atom, resonance involving that atom will not be possible.The negative carbon can have a negative formal charge only if the carbon has one hydrogen and one lone pair connected to it: [FC 21 (group IV) (3 bonds) (2 nonbonded electrons)] Therefore, resonance involving the charge on the negative carbon is possible p bonds beside each other—resonance is possible O Ϫ Atoms with negative charges can only participate in resonance if they carry a lone pair, so first determine if lone pairs are present using formal charge calculation O Ϫ H The carbon must have a lone pair to carry a formal charge of Ϫ1: FC ϭ Ϫ1 ϭ (group IV) – (3 bonds) – (2 non-bonded electrons) NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.6 Constructing Resonance Forms bond must break to avoid exceeding octet on the middle carbon work one bond at a time O Ϫ O O Ϫ Ϫ A B negative charge provides a good starting point for analysis C pair of electrons adjacent to another p bond Once the possibility of resonance has been determined, start by moving the electrons associated with the negative charge toward the adjacent carbon carrying a p bond This p bond must break at the same time so that the carbon receiving the electrons from the negative carbon does not exceed its octet The electron pair from the breaking p bond flows to the left carbon in the bond (the carbon farther from the negative charge), giving resonance form B In this form, the negative charge is adjacent to the C5O p bond, so a further electron movement is possible.This movement produces resonance structure C Since electrons cannot move any farther along the molecule, the three structures shown are the only resonance forms for this molecule Student Tip Do not break single bonds when constructing resonance forms; only p bonds (in double and triple bonds) may be broken Checkpoint 5.5 You should now be able to generate a complete set of resonance forms for a given compound Solved Problem Determine which of the structures below have resonance For those that do, draw all possible resonance forms, using curved arrows to show how each structure is generated O O O II III ϩ O ϩ ϩ I IV Step 1: Expand the Lewis structure of the atoms near p bonds, heteroatoms, and charges Start by adding in any missing lone pairs that are next to p bonds or atoms with charges When first dealing with resonance structures, it is a good idea to add any implied hydrogens on or near these features, since explicitly showing these hydrogens can help you keep track of the formal charges in the resonance structures H H H H O ϩ H H I H 211 O H ϩ H H II H H O H H H H III H O ϩ H H IV Step 2: For those structures with p bonds, see if the atoms directly adjacent to the p bonds have any of the features that make resonance possible The adjacent atoms on structure II have two of the features that make resonance possible, so this structure will have resonance Structure III, however, has none of the possible features on the atoms adjacent to its p bond and therefore does not meet this criterion for resonance NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 212 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms Atoms adjacent to p bond have two of the features that make resonance possible: - charged atom with incomplete octet - lone pairs H H O H H H ϩ H H H H O H H III II Atoms adjacent to p bond have none of the features for resonance: both carbons have full octets, no charges, no non-bonding electrons, and are not part of another p bond Step 3: For those structures still under consideration, look for p bonds between atoms with different electronegativities Although structure III has a p bond, it is a C5C double bond, so the two atoms in the bond have identical electronegativities Therefore, this bond does not meet this criterion for resonance H H H H O H H III p bond made up of two carbon atoms with the same electronegativity Step 4: For those structures still under consideration, look for any non-bonding electrons next to atoms lacking a complete octet Structures I and IV have atoms with an incomplete octet: in both cases, a positively charged carbon (carbocation) In structure I, neither of the atoms next to the carbocation have any non-bonding electrons to share Therefore, structure I fails to meet this criterion for resonance Structure IV, on the other hand, has an oxygen atom with lone pairs next to its carbocation These lone pairs could share their electrons, so structure IV has resonance neighbouring atoms have no non-bonding electrons to share H H O ϩ H H I incomplete octet neighbouring oxygen atom has non-bonding electrons to share H H O ϩ H H IV incomplete octet Therefore, of the four structures, structures II and IV can participate in resonance Step 5: To generate new resonance contributors for structures with resonance, look for any charges or non-bonding electrons next to a p bond Structure II has a positively charged carbocation directly adjacent to the p bond This carbocation can receive electrons from the p bond since the carbon atom lacks a full octet A curved arrow shows the flow of electrons from the p bond to the carbocation, and generates resonance form B NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.6 Constructing Resonance Forms Structure II H H O ϩ H H H H H O ϩ H H H B A Carbocation lacks a full octet; the electrons from the adjacent p bond move over to be shared with the carbocation Resonance form B has a new carbocation with an incomplete octet The two lone pairs on the neighbouring oxygen atom can be shared with the carbocation Drawing a curved arrow from the lone pair on oxygen to the positive charge generates the third and final resonance structure, C, with a new C5O bond and a positive charge on the oxygen Structure II H H H O ϩ O H ϩ H H A O H H H B ϩ H H C Carbocation lacks a full octet There are two electrons available in the adjacent oxygen Structure IV has no p bond with adjacent charges or lone pairs Step 6: If there are no charges or lone pairs next to p bonds, look for p bonds that can be broken by pushing electrons to a more electronegative atom Structure IV has no p bonds Step 7: If there are no p bonds, look for non-bonding electrons that can be shared with atoms lacking a complete octet The lone pairs on the oxygen atom of structure IV are right next to the positively charged carbocation Since this carbocation lacks a complete octet, it can accept one of the lone pairs from oxygen to make a new C5O double bond.The curved arrow starts at the lone pair and ends in the middle of the C2O bond (it could also start at the lone pair and point to the carbon).This electron flow creates a new C5O p bond and results in a neutral carbon and a positively charged oxygen lone pair on oxygen shared with carbocation H Structure IV: O ϩ H H H H ϩ O H Step 8: Double-check your work When drawing resonance structures, it is easy to involve too many bonds and electrons at once, and thus accidentally skip over a possible structure Make sure you worked only one bond at a time Also check that you considered all the nearby atoms and electrons that could possibly participate in resonance, and that you included all the formal charges on each structure NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 213 214 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms Step 9: Redraw your final answer using line drawings, omitting any unnecessary lone pairs and hydrogen atoms Structure II O ϩ O O ϩ ϩ Structure IV ϩ O ϩ O Practice Problem 5.9 Determine which of the following structures can participate in resonance For the ones that can, draw all possible resonance forms S a) ϩ Ϫ b) O c) Ϫ NH2 d) O e) Ϫ H f ) ϩ O Integrate the Skill 5.10 Each of the following structures can potentially react with positively charged molecules Identify the site(s) on each molecule that would be attracted to an atom with a positive charge, using resonance forms to justify your answer a) OCH3 O b) c) N O O NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.7 Evaluating Resonance Form Contributions 5.7 Evaluating Resonance Form Contributions When constructing resonance hybrids from resonance forms, it is important to recognize that the relative contributions of resonance forms are not always equal Forms with favourable electron distributions make larger contributions to the hybrid structure than forms with less favourable electron distributions In drawings of chemical structures and reactions, functional groups are normally represented by the “best” (highest contributing) resonance forms.The relative “quality” of resonance forms also provides a guide to the most likely sites of reactivity in a given functional group The following guidelines are used to assess the quality of individual resonance forms These guidelines are listed in order of decreasing importance In general, contributing resonance forms have the following characteristics: the most atoms with full octets the fewest number of formal charges a) negative formal charges located on the most electronegative atoms b) positive formal charges located on the most electropositive atoms a) like charges separated by the maximum distance possible b) opposite charges as close together as possible In the following example, both resonance forms of the compound have full octets on all atoms and a minimum number of formal charges These structures therefore make equal contributions to the resonance hybrid structure Ϫ H O H C C O H H O H C C H O Ϫ equal contributors In the next example, the major resonance contributor of the amide is structure A, in which all atoms have octets and no atoms are charged Structure B has two charges and an atom with less than eight valence electrons, so this form makes only a minor contribution to the overall structure Structure C makes an intermediate contribution since it has two charged atoms, but all the atoms have full octets intermediate resonance contributor (two charges) major resonance contributor Ϫ O N A Ϫ O ϩ O N N B ϩ C minor resonance contributor (carbon has incomplete octet, two charges) An amide group is therefore drawn as structure A, since this is the major resonance contributor The reactivity of this functional group can be predicted by examining the other resonance forms In this case, the reactivity of the group derives from form C (intermediate), and to a lesser extent form B (minor) (see Chapters and 15 for an explanation of this reactivity) NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 215 216 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms 5.7.1 Significant and insignificant structures Some resonance forms make no contribution to the functional group structure Such insignificant structures often have more than two formal charges on a functional group or connected system A common example of this is a carboxylate, the conjugate base of a carboxylic acid Starting with either of the two significant resonance forms, breaking the oxygen-carbon double bond produces a resonance form with three formal charges.This is an insignificant form that does not contribute to the structure or the reactivity of the group O O ϩ O Ϫ Ϫ O O Ϫ O Ϫ This form carries three charges on one functional group It is insignificant and does not contribute to the structure of the group Insignificant resonance forms also occur when electrons from a breaking p bond flow to an atom with inappropriate electronegativity In general, when drawing resonance structures, p bonds may be broken if the double bond involves two different elements, and if the electrons transfer to the more electronegative element The resulting structure will have adjacent negative and positive charges, with the negative charge located on the more electronegative atom of the pair In the following diagram, for example, the carbonyl group has two significant resonance forms: one with a C5O double bond, and one where the p bond is broken to put a negative formal charge on the more electronegative oxygen atom (blue arrows) Breaking the p bond in the other direction pushes the electrons onto the less electronegative carbon atom, resulting in an insignificant resonance form Ϫ O ϩ significant form (no charges) significant form (negative charge on oxygen) O ϩ O Ϫ insignificant form (negative charge on carbon) Breaking an alkene C5C bond to form differently charged carbon atoms is not a significant possibility The two carbon atoms share the bond electrons equally because the two carbons are equally electronegative, and so the alkene does not exhibit resonance This can be shown in drawing the two possible forms by breaking the p bond in both directions Combining the resonance forms would give a resonance hybrid that is identical to the starting alkene In effect, the resonance forms “cancel” each other when combined to make a hybrid structure NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.7 Evaluating Resonance Form Contributions 217 ϩ Ϫ These two forms not contribute to resonance They “cancel out” in a resonance hybird alkene Ϫ ϩ significant form (no charges) insignificant form breaking the p bond places charges on atom of the same type Checkpoint 5.6 You should now be able to rank the quality of forms contributing to a resonance hybrid and identify any insignificant resonance forms Organic ChemWare 5.17 Resonance: Carbonyl group Solved Problem Examine the proposed set of resonance contributors below Identify any insignificant resonance structures, and explain why they should not be included For the remaining structures, rank them according to their contribution to the overall resonance hybrid (1 contributes the most) ϩ O O O Ϫ O Ϫ Ϫ O O ϩ O O A B C D O O O ϩ E Ϫ Organic ChemWare 5.18 Resonance: Carbonyl group (protonated) ϩ Ϫ ϩ O ϩ Ϫ F Step 1: Look for any structures with more than two formal charges Structure F has four formal charges, whereas all the others have either two or none.Therefore, structure F can be considered an insignificant resonance structure O Ϫ ϩ O ϩ Ϫ too many formal charges; insignificant contribution F Step 2: Look for any structures that are the result of breaking a p bond and moving the electrons away from the more electronegative atom Without the curved arrows showing the generation of each resonance structure, it can be difficult to spot these types of insignificant structures You can either draw all the missing curved arrows (good practice!) or NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 218 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms look for any adjacent positive and negative charges where the positive charge is on the more electronegative atom Structure A has a positive charge on an oxygen atom next to a negative charge on a carbon atom Check to see how this structure is generated: from resonance form B, structure A is made by breaking the C5O p bond and giving the electrons to the less electronegative carbon atom instead of the oxygen atom.Therefore, structure A is insignificant p bond breaks in wrong direction (toward less electronegative atom) ϩ O O O Ϫ O B A insignificant resonance structure Step 3: Look for any structures that result from breaking a p bond made of two atoms with identical electronegativities Again, this can be difficult to spot without the curved arrows Instead, you can look for structures that have positive and negative charges on adjacent atoms of the same element (e.g., two side-by-side carbon atoms, one with a positive charge and one with a negative charge) None of the resonance contributors shown have this feature Step 4: Rank the significant structures based on—and in this order of decreasing contribution—the presence of complete octets, the number of formal charges, the placement of charges on appropriate atoms, and the separation of charges Of the six structures provided, only B, C, D, and E are significant structures Structure C is the only one that has any atoms with an incomplete octet (its carbocation), so structure C is the lowest ranked of the structures Structure B is the only contributor with no formal charges, so it will contribute the most to the overall resonance hybrid Finally, a comparison of structures E and D shows their only difference is the placement of the negative charge Since oxygen is more electronegative than carbon, structure D, with the negative charge on the oxygen atom, is better than E.Therefore, structure D is ranked as the second most-contributing structure negative charge is more stable on oxygen than carbon; structure D contributes more than structure E O O Ϫ O ϩ E O Ϫ O Ϫ O ϩ O O B C D no formal charges; contributes the most ϩ carbon has an incomplete octet; contributes the least The final ranking is as follows: O O Ϫ O Ϫ O ϩ O O E B C D O ϩ Ranking: Ϫ O ϩ NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.7 Evaluating Resonance Form Contributions Practice Problem 5.11 Rank the resonance structures of each set in order of decreasing quality Some structures might have equal quality Justify your answers H H3C C C N a) Ϫ Ϫ H3C C C N H Ϫ Ϫ O O ϩ b) O ϩ O Ϫ O c) O O Ϫ O O Ϫ d) ϩ NH2 NH2 O NH2 NH2 NH2 ϩ Br e) ϩ NH2 ϩ O ϩ O Br Br ϩ Integrate the Skill 5.12 The following diagram shows the first step in a multi-step reaction The structure formed after the first step is a hybrid of the resonance structures shown Add curved arrows to show the mechanism for the first step of the reaction (from the starting materials to the first resonance structure drawn) and for the formation of each resonance form Then rank the resonance structures according to their contribution to the resonance hybrid (1 contributes the most) Note that formal charges are missing from the resonance structures and need to be added OCH3 OCH3 OCH3 OCH3 OCH3 Br Br Br Br ϩ Br –Br ϩ Br DID YOU KNOW? Many organic compounds exhibit a special kind of resonance called aromaticity In these structures, there is a continuous band of p electrons that circulates in a ring structure that creates a very strong connection between the atoms that make up the ring The bonding in these systems can be described by drawing resonance forms that involve the movement of p electrons in a circular fashion Following the guidelines for the construction of mechanistic arrows, it is possible to construct resonance forms all the way around these rings, and arrive back at the starting structure These compounds are described in detail in Chapter Continued NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 219 220 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms The name aromaticity is derived from the observation that many aromatic hydrocarbons have strong smells, whereas aliphatic hydrocarbons not In the mid-1800s, the term aromatic was used to differentiate between hydrocarbons that had a smell from those that did not Later it was discovered that smell has nothing to with whether a compound is aromatic or not, but by then the name was in common use and has been maintained to this day 5.8 Resonance and Orbital Structure The actual structure of a group undergoing resonance can be represented by drawing all the p orbitals involved in the resonance The electrons involved in resonance then form a single orbital structure in which each atom of the functional group contributes one orbital The result is a network of orbitals that describes a region in space in which the resonating electrons may be found For resonance to be possible, the functional group should have the following conditions: a p bond made up of atoms with different electronegativities a p bond directly beside at least one of the following features: a) paired or unpaired electrons b) atoms with incomplete octets c) other p bonds d) charged atoms lacking octets or carrying lone pairs an atom with an incomplete octet adjacent to an atom with a pair of non-bonding electrons All of these conditions create a situation in which atoms with p orbitals (or those that can hybridize with p orbitals) can align with the p orbitals of other atoms or with p bonds For example, the resonance of an isolated ester group involves three atoms These three atoms form a single p orbital system with four electrons spread over all of the atoms To this, the p orbitals which contribute to the p system must all align to allow them to overlap Ϫ O O Ϫ O O ϩ O ϩ O O C O This requires that each atom involved in the resonance has the correct hybridization Because resonance involves p bonds, the atoms involved must be either sp2 or sp hybridized (to provide a p orbital that can participate in p bonding) To determine the hybridization involved, consider the hybridization of an atom in all the forms Although the actual hybridization is a “blend,” it is usually considered to be the one with the most s character (highest contribution from an s orbital) NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.9 Patterns in Mechanism CHEMISTRY: EVERYTHING AND EVERYWHERE Delocalization Is Responsible for the Colour of Many Organic Molecules Coloured organic molecules always have extended networks of p bonds that, because of resonance, function as a single, extended functional group If these p systems involve charged atoms or atoms with different electronegativities, electrons get “shuttled” from one side of the molecule to the other, which allows the molecule to interact strongly with visible light The most expensive spice in the world is saffron, which is made of the stigmas of the saffron crocus flower Each flower produces only three stigmas, and harvesting them is very labour intensive O HO OH O In addition to its flavour, saffron is highly valued for the golden yellow it imparts to food This colour is produced by a pigment called crocin The crocin molecule has an extensive network of p bonds, arranged one beside the other This allows for a great many resonance structures, which contribute to the stability of the molecule and to its ability to interact with visible light 5.9 Patterns in Mechanism Organic reactions are systematic and follow patterns These patterns can be depicted with mechanistic arrows that indicate the movement of electrons during reactions Electronegativity and formal charges can help in determining the direction of electron flow Line structures highlight functional groups and facilitate mechanistic analysis Organic compounds can be considered collections of functional groups held together by a scaffold of carbon atoms ammonium amide aromatic ring H3CO ether ϩ NH3 O thioaminal H N O lactam S N Ϫ CO2 carboxylate amoxicillin Bonds form when one atom shares electrons with another atom Some atoms have an available pair of electrons and donate them to form bonds These sites can often be identified by the presence of a negative charge (− or d−) Other atoms accept electrons to form bonds and can be identified by the presence of a positive charge (1 or d1) In reaction mechanisms, electrons flow from an area of high electron density (lone pair or bond) to an area of low electron density (atom lacking octet or positively charged atom) NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 221 222 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms when bonds break, the electrons move onto the atom that is the best electron attractor (most electronegative or positively charged) some atoms donate electrons some atoms accept electrons O Br Ϫ O O Ϫ Br ϩ MgBr O ϩ MgBr When a bond is broken, the bonding electrons tend to move toward the atom in the bond that is the strongest electron attractor Often this is the more electronegative atom, but positive charges also attract electrons very strongly When a bond involving a positive charge breaks, its electrons generally flow toward a positive charge when a bond breaks, the bonding electrons move to the more electronegative element Ϫ Br O Ϫ O Br p bonds involving positive charges break toward the positively charged atom ϩ H O ϩ OH Ϫ O Ϫ O O ϩ N F H O OH Ϫ ϩ O O N Ϫ F O If a double-barb arrow points away from an atom, the formal charge integer on the atom increases by one unit Similarly, if the arrow points toward an atom, the formal charge integer on the atom decreases by If an atom has one arrow pointing toward it and another arrow pointing away from it, the charge on that atom does not change NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 5.10 Patterns in Resonance source of electrons in a reaction increases its charge integer H NH2 O ϩ N H Ϫ O destination of electrons in a reaction reduces its charge integer In each step of a reaction mechanism, the number of electrons, the number of atoms, and the total net charge are conserved 5.10 Patterns in Resonance Of all the factors that stabilize organic molecules, delocalization (described by resonance) is the most profound The overall patterns of electron movement are systematic and predictable; the same patterns apply to both neutral molecules and charged species For example, electrons in a p bond are attracted to the atom that attracts electrons most strongly, usually either a positively charged atom or the most electronegative atom nearby isolated double bonds: Ϫ O resonance is possible if the atoms are different types resonance is not possible if the atoms are the same O ϩ ϩ OH positive charge on an sp2 atom will lead to resonance OH ϩ When a p bond is adjacent to a charge, a lone pair, another p bond, or an atom with an incomplete octet, it will be possible to construct resonance forms Look for these features in molecules, and work systematically with mechanistic arrows to fully analyze resonance structures.Working with one bond at a time is an effective technique for drawing all possible resonance forms double bonds beside charges and non-bonded electrons ϩ ϩ positive charge lacking octet adjacent to a p bond will lead to resonace Ϫ Ϫ negative charge with lone pair adjacent to a p bond will lead to resonace OH Ϫ OH ϩ lone pair adjacent to a p bond will lead to resonace NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it 223 224 Chapter   Organic Reaction Mechanism: Using Curved Arrows to Analyze Reaction Mechanisms Checkpoint 5.7 Organic ChemWare 5.19 Resonance: Carbonyl group You should now be able to identify resonance patterns in a wide variety of organic compounds Solved Problem Here are four compounds in which resonance is possible Organic ChemWare 5.20 Resonance: Carbonyl group (protonated) ϩ O N H N A Organic ChemWare 5.21 Resonance: Allyl cation O Ϫ O B C D a)  Group the molecules together by the type of resonance pattern they exhibit b) Draw the resonance structures for each molecule Use curved arrows to show the similarities between the resonance patterns in the grouped structures Step 1: Expand the Lewis structure of the atoms near p bonds, heteroatoms, and charges Organic ChemWare 5.22 Resonance: Allyl anion ϩ O Organic ChemWare 5.24 Resonance: 1,3-Butadiene N H N A Organic ChemWare 5.23 Resonance: Enol O Ϫ O B C D Step 2: Determine the structural features of each molecule that give rise to resonance Compounds A and D both have an isolated p bond that is not adjacent to any non-bonding electrons, charges, or atoms with incomplete octets The p bonds are, however, made up of two different elements with different electronegativities and can therefore participate in resonance Compounds B and C each have a p bond with an adjacent heteroatom bearing lone pairs These lone pairs can participate in resonance with the p bond O Ϫ O ϩ O N H N A B C D Step 3: Redraw the structures so that the matching electron systems are drawn in the same orientation ϩ O A O D Ϫ N H N B O C NEL Copyright 2018 Nelson Education Ltd All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s) Nelson Education reserves the right to remove additional content at any time if subsequent rights restrictions require it ... rights restrictions require it Organic Chemistry by William Ogilvie, Nathan Ackroyd, C Scott Browning, Ghislain Deslongchamps, Felix Lee, Effie Sauer Senior Publisher, Digital and Print Content:... ORGANIC CHEMWARE Organic ChemWare for use with Organic Chemistry: Mechanistic Patterns is a comprehensive ­collection of learning objects to aid in the teaching and learning of organic chemistry. .. Lewis structure of the following Identify any dipoles that may be present a) (CH3)3CCl b) CH 3C( O)CH3 c) CH3CH2CH2CHOHCH3 d) HOCH2CH2CH2CH2CHO 1.6 The Shapes of Atoms in Organic Molecules The