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Organic Chemistry Principles and Mechanisms Joel M Karty Elon University b W W Norton & Company has been independent since its founding in 1923, when William Warder Norton and Mary D Herter Norton first published lectures delivered at the People’s Institute, the adult education division of New York City’s Cooper Union The firm soon expanded its program beyond the Institute, publishing books by celebrated academics from America and abroad By mid-century, the two major pillars of Norton’s publishing program—trade books and college texts—were firmly established In the 1950s, the Norton family transferred control of the company to its employees, and today—with a staff of four hundred and a comparable number of trade, college, and professional titles published each year—W W Norton & Company stands as the largest and oldest publishing house owned wholly by its employees Copyright © 2014 by W W Norton & Company, Inc All rights reserved Printed in the United States of America Editor: Erik Fahlgren Assistant Editor: Renee Cotton Developmental Editor: John Murdzek Manuscript Editor: Julie Henderson Project Editor: Christine D’Antonio Marketing Manager, Chemistry: Stacy Loyal Science Media Editor: Rob Bellinger Associate Media Editor: Jennifer Barnhardt Assistant Media Editor: Paula Iborra Production Manager: Eric Pier-Hocking Photo Editor: Michael Fodera Photo Researcher: Jane Miller Permissions Manager: Megan Jackson Text Design: Lisa Buckley Art Director: Hope Miller Goodell Composition: codeMantra Illustrations: Imagineering Manufacturing: Courier, Kendallville Library of Congress Cataloging-in-Publication Data has been applied for ISBN 978-0-393-91904-2 W W Norton & Company, Inc., 500 Fifth Avenue, New York, NY 10110 www.wwnorton.com W W Norton & Company Ltd., Castle House, 75/76 Wells Street, London W1T 3QT 1234567890 To Pnut, Fafa, and Jakers About the Author JOEL KARTY earned his B.S in chemistry at the University of Puget Sound and his Ph.D at Stanford University He joined the faculty of Elon University in 2001, where he currently holds the rank of associate professor He teaches primarily the organic chemistry sequence and also teaches general chemistry In the summer, Joel teaches at the Summer Medical and Dental Education Program through the Duke University medical center His research interests include investigating the roles of resonance and inductive effects in fundamental chemical systems and studying the mechanism of pattern formation in Liesegang reactions He has written a very successful student supplement, Get Ready for Organic Chemistry, Second Edition (formerly called The Nuts and Bolts of Organic Chemistry) Brief Contents Atomic and Molecular Structure Nomenclature Introduction: The Basic System for Naming Simple Organic Compounds: Alkanes, Cycloalkanes, Haloalkanes, Nitroalkanes, and Ethers 54 Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties 76 Orbital Interactions 1: Hybridization and Two-Center Molecular Orbitals 128 Nomenclature Naming Alkenes, Alkynes, and Benzene Derivatives 163 Isomerism 1: Conformational and Constitutional Isomers 176 Isomerism 2: Chirality, Enantiomers, and Diastereomers 224 Nomenclature Considerations of Stereochemistry: R and S Configurations about Tetrahedral Stereocenters and Z and E Configurations about Double Bonds 276 The Proton Transfer Reaction: An Introduction to Mechanisms, Thermodynamics, and Charge Stability 295 An Overview of the Most Common Elementary Steps 351 Interchapter Molecular Orbital Theory and Chemical Reactions 388 Nomenclature Naming Compounds with Common Functional Groups: Alcohols, Amines, Ketones, Aldehydes, Carboxylic Acids, Acid Halides, Acid Anhydrides, Nitriles, and Esters 398 An Introduction to Multistep Mechanisms: SN1 and E1 Reactions 420 11 Electrophilic Addition to Nonpolar ě Bonds 1: Addition of a Brønsted Acid 570 12 Electrophilic Addition to Nonpolar ě Bonds 2: Reactions Involving Cyclic Transition States 610 13 Organic Synthesis 1: Beginning Concepts 14 Orbital Interactions 2: Extended ě Systems, Conjugation, and Aromaticity 676 15 Structure Determination 1: Ultraviolet-Visible and Infrared Spectroscopies 715 16 Structure Determination 2: Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry 757 17 Nucleophilic Addition to Polar ě Bonds 1: Addition of Strong Nucleophiles 815 18 Nucleophilic Addition to Polar ě Bonds 2: Addition of Weak Nucleophiles and Acid and Base Catalysis 861 19 Organic Synthesis 2: Intermediate Topics of Synthesis Design, and Useful Reduction and Oxidation Reactions 919 20 Nucleophilic Addition–Elimination Reactions 1: The General Mechanism Involving Strong Nucleophiles 962 21 Nucleophilic Addition–Elimination Reactions 2: Weak Nucleophiles 1006 22 Electrophilic Aromatic Substitution 1: Substitution on Benzene; Useful Accompanying Reactions 1065 23 Electrophilic Aromatic Substitution 2: Substitution Involving Mono- and Disubstituted Benzene and Other Aromatic Rings 1104 24 The Diels–Alder Reaction and Other Pericyclic Nucleophilic Substitution and Elimination Reactions 1154 Reactions 1: Competition among SN2, SN1, E2, and E1 Reactions 466 25 Reactions Involving Free Radicals 10 Nucleophilic Substitution and Elimination Reactions 2: Reactions That Are Useful for Synthesis 524 645 1199 Interchapter Fragmentation Pathways in Mass Spectrometry 1245 26 Polymers 1255 ix Chapter Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties C arbon dioxide (CO2) and formic acid (HCO2H) are similar in their chemical makeup, but the boiling point of CO2 is 278 °C, whereas that of HCO2H is 101 °C Moreover, CO2 is only slightly soluble in water, whereas HCO2H is infinitely soluble in water Why are their physical properties so vastly different? O C O C O H OH Carbon dioxide Formic acid (Methanoic acid) Molar mass = 44 g/mol Boiling point = −78°C Water solubility = slight Molar mass = 46 g/mol Boiling point = 101°C Water solubility = infinite As we discuss here in Chapter 2, these compounds behave differently because they experience different intermolecular interactions Those intermolecular interactions are governed, in turn, by a variety of factors, including the threedimensional shapes of the molecules and the functional groups they contain We begin, therefore, with a review of the factors that determine molecular geometry and then discuss the different types of intermolecular interactions that are important in organic chemistry Geckos can climb effortlessly on almost every surface Their ability to so is attributed to ultrafine hairs on their feet, which give rise to a very large contact surface area This allows for rather strong dispersion forces, one of the intermolecular interactions we will examine in this chapter 76 These topics have a broad relevance to many aspects of CHAPTER OBJECTIVES organic chemistry Toward the end of this chapter, we explain Upon completing Chapter you should be able to: that intermolecular interactions determine how soaps and de- ● Predict both the electron and molecular geometries about an atom, given only a Lewis structure branes In Chapter 5, we explain how molecular geometry is ● Recognize molecules that possess angle strain central to the important concept of chirality—that is, whether a ● Draw accurate three-dimensional representations of molecules using dash–wedge notation, and be able to interpret the three-dimensional structures of molecules drawn in dash–wedge notation ● Determine whether a molecule is polar or nonpolar ● Explain how functional groups help determine a species’ physical properties ● Describe the origin of the various intermolecular interactions discussed and how they govern a species’ boiling point, melting point, and solubility ● Predict the relative boiling points, melting points, and solubilities of different species, given only their Lewis structures ● Distinguish a protic solvent from an aprotic solvent, and explain the role of each type of solvent in the solubility of an ionic compound ● Identify the structural features of soaps and detergents and explain how these contribute to their cleansing properties tergents function and contribute to the properties of cell mem- molecule is different from its mirror image And, in Chapter 9, we explain how intermolecular interactions can have a dramatic effect on the outcome of chemical reactions 2.1 Valence Shell Electron Pair Repulsion (VSEPR) Theory: Three-Dimensional Geometry To understand many aspects of molecular geometry, chemists routinely work with two models One, which we discuss here, is valence shell electron pair repulsion (VSEPR) theory The other, which we discuss in Chapter 3, uses the concepts of hybridization and molecular orbital (MO) theory Although hybridization and MO theory constitute a more powerful model than VSEPR theory, VSEPR theory remains extremely useful because of its simplicity: Its concepts are easier to grasp and it allows us to arrive at answers much more quickly 2.1a Basic Principles of VSEPR Theory The basic ideas of VSEPR theory are as follows: Electrons in a Lewis structure are viewed as groups ■ A lone pair of electrons, a single bond, a double bond, and a triple bond each constitute one group of electrons (Table 2-1) TABLE 2-1 Various Types of Electron Groups in VSEPR Theory Type of Group Total Number of e2 Number of Groups Lone pair 1 Single bond 1 Double bond 1 Triple bond 2.1 Valence Shell Electron Pair Repulsion (VSEPR) Theory: Three-Dimensional Geometry / 77 The negatively charged electron groups strongly repel one another, so they tend to arrange themselves as far away from each other as possible ■ Two electron groups tend to be 180° apart (a linear configuration) ■ Three groups tend to be 120° apart (a triangular, planar configuration) ■ Four groups tend to be 109.5° apart (a tetrahedral configuration) Electron geometry describes the orientation of the electron groups about a particular atom These configurations are summarized in Table 2-2 Molecular geometry describes the arrangement of atoms about a particular atom Because atoms must be attached by bonding pairs of electrons, an atom’s molecular geometry is governed by its electron geometry H H C C N H TABLE 2-2 180° Electron geometry = linear Molecular geometry = linear (a) Ethanenitrile (acetonitrile) O Number of Electron Groups Electron Geometry Approximate Bond Angle Linear 180° Trigonal planar 120° Tetrahedral 109.5° H H C C C H H H H ~120° Electron geometry = trigonal planar Molecular geometry = trigonal planar (b) Propanone (acetone) H H H C C H H The common molecular geometries, which are summarized in Table 2-3, lead to the following conclusions: ■ H ~109.5° Electron geometry = tetrahedral Molecular geometry = tetrahedral (c) Ethane FIGURE 2-1 Compounds in which the central atom lacks lone pairs Because there are no lone pairs about the central atom, the molecular geometries of acetonitrile, acetone, and ethane are identical to their electron geometries ■ If all the electron groups are bonds (depicted in gray), then there is an atom attached to each electron group and the molecular geometry is the same as the electron geometry If one or more of the electron groups is a lone pair (depicted in yellow and red), then the molecular geometry is different than the electron geometry Some examples of molecules containing central atoms without lone pairs are shown in Figure 2-1 In acetonitrile, CH3 iC{N (Fig 2-1a), the triply bonded C has two electron groups about it: a single bond and a triple bond According to Table 2-2, its electron geometry is linear Both electron groups are bonds, moreover, so the molecular geometry about that C atom is also linear, making the CiCiN bond angle 180° In acetone, (CH3)2CwO (Fig 2-1b), the central C atom has three electron groups about it: two single bonds and a double bond As a result, both the electron and molecular geometries are trigonal planar In ethane, CH3 iCH3 (Fig 2-1c), each C atom is surrounded by four electron groups—the four single bonds—so the electron and molecular geometries are tetrahedral at each carbon 78 / CHAPTER Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties TABLE 2-3 Electron Geometry Number of Bonded Atoms/Groups Linear (180°) Trigonal Planar (120°) Tetrahedral (109.5°) Linear Bent Bent Trigonal planar Trigonal pyramidal Tetrahedral a Bonding electron groups are depicted with gray sticks; nonbonding electron groups are depicted as yellow sticks terminating in a red lone pair H2 C H 2N In 2-aminoethanol (ethanolamine), which is commonly used as feedstock for the production of a variety of industrial compounds (Fig 2-2), the N and O atoms have molecular geometries that are different than their electron geometries The electron geometry of the N atom is tetrahedral because it is surrounded by four electron groups: three single bonds and a lone pair Its molecular geometry, however, which describes only the orientation of the three single bonds, is trigonal pyramidal Likewise, the O atom of the OH group has a tetrahedral electron geometry (two single bonds and two lone pairs), but its molecular geometry is bent C H2 OH 2-Aminoethanol Electron geometry = tetrahedral Molecular geometry = bent Solved problem 2.1 Imines, which are characterized by a CwN double bond, are commonly used as intermediates in organic synthesis Use VSEPR theory to predict the electron and molecular geometries about the nitrogen atom in the acetone imine molecule below Electron geometry = tetrahedral Molecular geometry = trigonal pyramidal FIGURE 2-2 Lewis structure and VSEPR geometries about atoms in a molecule of 2-aminoethanol NH C H 3C CH3 Think How many electron groups surround the N atom? Are any of them lone pairs? The electron geometries about the NH2 nitrogen and the OH oxygen atoms in 2-aminoethanol are both tetrahedral, but they have different molecular geometries because N has one lone pair and O has two 2.1 Valence Shell Electron Pair Repulsion (VSEPR) Theory: Three-Dimensional Geometry / 79 Electron geometry = trigonal planar Molecular geometry = trigonal planar H C H H + C H H Solve There are three groups of electrons around the nitrogen atom: one double bond, one single bond, and one lone pair According to VSEPR theory, therefore, its electron geometry is trigonal planar, and its molecular geometry is bent – ~120° H Electron geometry = trigonal planar Molecular geometry = bent (a) (b) problem 2.2 Electron geometry = tetrahedral Molecular geometry = trigonal pyramidal FIGURE 2-3 Lewis structures and Prop-2-yn-1-ol (propargyl alcohol) is used as an intermediate in organic synthesis and can be polymerized to make poly(propargyl alcohol) Use VSEPR theory to predict the electron and molecular geometries about each nonhydrogen atom in the molecule three-dimensional geometries of the methyl cation and methyl anion The central C atom in CH31 is surrounded by three single bonds only (i.e., no lone pairs), so its electron and molecular geometries are the same The central C atom in CH32 , on the other hand, is surrounded by three single bonds and one lone pair, so its electron and molecular geometries are different Your Turn 2.1 OH Prop-2-yn-1-ol (Propargyl alcohol) The rules of VSEPR theory apply equally well to ions Figure 2-3a shows, for example, that the methyl cation, CH31, has a trigonal planar electron geometry, consistent with a carbon atom that is surrounded by three groups of electrons (i.e., three single bonds) The methyl anion (Fig 2-3b), on the other hand, is surrounded by four groups of electrons (i.e., three single bonds and a lone pair) Its electron geometry therefore, is tetrahedral, and its molecular geometry is trigonal pyramidal Circle each electron group in the Lewis structures of CH31 and CH2 in Figure 2-3 Answers to Your Turns are in the back of the book 2.1b Angle Strain Geometric constraints can force an atom to deviate significantly from its ideal bond angle—that is, the bond angle predicted by VSEPR theory Most commonly this happens in ring structures For example, the carbon atoms in a molecule of cyclopropane (Fig 2-4a) should have an ideal bond angle of 109.5°, given that each carbon is surrounded by four groups of electrons (four single bonds) To form the ring, however, the CiCiC bond angles must be 60° Similarly, each carbon atom of cyclobutadiene (Fig 2-4b) has an ideal bond angle of 120°, but geometric constraints force the angles in the ring to be 90° H2 C H2C FIGURE 2-4 Examples of angle strain In cyclopropane (a), the ideal CiCiC bond angle is 109.5°, but the actual angle is 60° In cyclobutadiene (b), the ideal CiCiC bond angle is 120°, but the actual angle is 90° CH2 HC CH HC CH Cyclopropane Cyclobutadiene Ideal bond angle = 109.5° Real bond angle = 60° (a) Ideal bond angle = 120° Real bond angle = 90° (b) 80 / CHAPTER Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties The deviation of a bond angle from its ideal angle results in an increase in energy, called angle strain Angle strain weakens bonds and makes a species more reactive In some cases, excessive angle strain can preclude the existence of a molecule altogether 2.2 Dash–Wedge Notation Although molecules are three-dimensional, representing them on paper is confined to the two dimensions of the page To work around this problem, we introduce dash–wedge notation, which provides a means to represent atoms both in front of and behind the plane of the paper Dash–wedge notation has three components: Rules for dash–wedge notation A straight line (i) represents a bond that is in the plane of the paper ■ Atoms at either end of the bond are also in the plane of the paper A wedge ( ) represents a bond that comes out of the plane of the paper and points toward you ■ In general, the atom at the thinner end of the wedge is in the plane of the paper, whereas the atom bonded at the thicker end is in front of the page A dash ( ) represents a bond that is pointed away from you ■ In general, you may assume in this book that the atom bonded at the thicker end of this dash is behind the plane of the paper (You may see different conventions in other books.) Using the dash–wedge notation, there are two common ways of representing a tetrahedral carbon atom like that in CH4 They are illustrated in Figure 2-5a and 2-5b Both illustrations represent the same molecule with the same 109.5° bond angles The only difference is the vantage point from which you view the molecule The vantage point giving rise to the depiction of CH4 in Figure 2-5b is the basis for a shorthand representation of tetrahedral carbon atoms, called the Fischer projection, which we introduce in Chapter C H bond pointed away from you C H bond in the plane of the paper H H C H H C H bond pointed away from you H C H H H C H bond pointed toward you C H bond pointed toward you (a) (b) FIGURE 2-5 Representations of CH4 using dash–wedge notation The two different depictions imply views of the molecule from different vantage points 2.2 Dash–Wedge Notation / 81 This V is in the plane of the paper The two Vs open in opposite directions H H This V is perpendicular to the plane of the paper H H C H H H C H C FIGURE 2-6 Tetrahedral geometry viewed as two perpendicular Vs A tetrahedral atom can be viewed as the fusing together of two V shapes that are in perpendicular planes—one in the plane of the paper and one perpendicular to the plane of the paper The two Vs must “open” in opposite directions Your Turn 2.2 Ball-and-stick representations of NH41 from two different vantage points are provided Next to each one, draw the corresponding cation using dash–wedge notation Notice in Figure 2-5 that the four bonds of a tetrahedral atom define two perpendicular Vs It is helpful to think of this whenever drawing the dash–wedge notation in Figure 2-5a ■ ■ Your Turn 2.3 One V is in the plane of the paper, whereas the other is perpendicular to the plane of the paper The Vs must open in opposite directions! This is explicitly shown in Figure 2-6 In the Lewis structure at the right of Figure 2-6, trace the V that is in the plane of the page and draw an arrow in the direction in which it opens Do the same to the V that is perpendicular to the page Dash–wedge notation can be combined with line structures to illustrate the threedimensional geometry of more complex molecules, such as butan-2-ol: OH Butan-2-ol 82 / CHAPTER Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties In the box provided, draw the line structure of butan-2-ol using dash–wedge notation Note that the CiO bond points away from you 2.4 Your Turn Cl Cl OH Construct model problem 2.3 Draw line structures of each of the following molecules using dash–wedge notation Assume that no atoms have formal charges (Note: Black carbon, white hydrogen, green-yellow chlorine, and blue nitrogen.) (a) (b) (c) problem 2.4 The following is a common mistake made with dash–wedge notation Explain what is incorrect about it and then fix it Rotate model Br 180° Br 2.3 Strategies for Success: The Molecular Modeling Kit Much of organic chemistry requires us to manipulate molecules in three dimensions Unfortunately, we are limited to two dimensions when we represent a molecule on paper, even when we use dash–wedge notation Molecular modeling kits can help Instead of having to rotate a three-dimensional image mentally, you can construct real models and rotate them in your hands For example, let’s use a modeling kit to determine what the following cyclopentane derivative looks like after it has been flipped over vertically Redraw OH Cl Flip 180° Cl ? Cl (2-1) Cl OH You may develop your own process for these kinds of manipulations, but for now carry out the following steps, which are depicted in Figure 2-7: Construct the molecular model exactly as indicated in the accurate dash–wedge notation FIGURE 2-7 Model kits and 3D manipulations To draw the molecule in Equation 2-1 after it is flipped 180Û, (1) construct the molecule with a model kit, (2) flip the molecule over 180Û, and (3) use the model as a guide to redraw the molecule in dash–wedge notation 2.3 Strategies for Success: The Molecular Modeling Kit / 83 ... Organic Chemistry Principles and Mechanisms Joel M Karty Elon University b W W Norton & Company has been independent since its founding in 1923, when William Warder Norton and Mary D... own Preface for the Student Organic Chemistry and You You are taking organic chemistry for a reason—you might be pursuing a career in which an understanding of organic chemistry is crucial, or the... stratospheric ozone layer (Fig P.3b), and global warming (Fig P.3c) Organic chemistry, for example, is helping (b) (a) FIGURE P.3 Organic chemistry and the environment Organic chemistry continues to play