Perspectives on Structure and Mechanism in Organic Chemistry E-Book

Felix A. Carroll

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1802

Veröffentlichungsjahr: 2011

Beschreibung

Helps to develop new perspectives and a deeper understanding of organic chemistry Instructors and students alike have praised Perspectives on Structure and Mechanism in Organic Chemistry because it motivates readers to think about organic chemistry in new and exciting ways. Based on the author's first hand classroom experience, the text uses complementary conceptual models to give new perspectives on the structures and reactions of organic compounds. The first five chapters of the text discuss the structure and bonding of stable molecules and reactive intermediates. These are followed by a chapter exploring the methods that organic chemists use to study reaction mechanisms. The remaining chapters examine different types of acid-base, substitution, addition, elimination, pericyclic, and photochemical reactions. This Second Edition has been thoroughly updated and revised to reflect the latest findings in physical organic chemistry. Moreover, this edition features: * New references to the latest primary and review literature * More study questions to help readers better understand and apply new concepts in organic chemistry * Coverage of new topics, including density functional theory, quantum theory of atoms in molecules, Marcus theory, molecular simulations, effect of solvent on organic reactions, asymmetric induction in nucleophilic additions to carbonyl compounds, and dynamic effects on reaction pathways The nearly 400 problems in the text do more than allow students to test their understanding of the concepts presented in each chapter. They also encourage readers to actively review and evaluate the chemical literature and to develop and defend their own ideas. With its emphasis on complementary models and independent problem-solving, this text is ideal for upper-level undergraduate and graduate courses in organic chemistry.

Leseprobe

Contents

Preface

Acknowledgments

Introduction

1 Fundamental Concepts of Organic Chemistry

1.1 ATOMS AND MOLECULES

1.2 HEATS OF FORMATION AND REACTION

1.3 BONDING MODELS

1.4 COMPLEMENTARY DESCRIPTIONS OF THE DOUBLE BOND

1.5 CHOOSING MODELS IN ORGANIC CHEMISTRY

2 Stereochemistry

2.1 INTRODUCTION

2.2 STEREOISOMERISM

2.3 MANIFESTATIONS OF STEREOISOMERISM

2.4 STEREOTOPICITY

3 Conformational Analysis and Molecular Mechanics

3.1 MOLECULAR CONFORMATION

3.2 CONFORMATIONAL ANALYSIS

3.3 MOLECULAR MECHANICS

3.4 MOLECULAR STRAIN AND LIMITS TO MOLECULAR STABILITY

4 Applications of Molecular Orbital Theory and Valence Bond Theory

4.1 INTRODUCTION TO MOLECULAR ORBITAL THEORY

4.2 AROMATICITY

4.3 CONTEMPORARY COMPUTATIONAL METHODS

4.4 VALENCE BOND THEORY

5 Reactive Intermediates

5.1 REACTION COORDINATE DIAGRAMS

5.2 RADICALS

5.3 CARBENES

5.4 CARBOCATIONS

5.5 CARBANIONS

5.6 CHOOSING MODELS OF REACTIVE INTERMEDIATES

6 Methods of Studying Organic Reactions

6.1 MOLECULAR CHANGE AND REACTION MECHANISMS

6.2 METHODS TO DETERMINE REACTION MECHANISMS

6.3 APPLICATIONS OF KINETICS IN STUDYING REACTION MECHANISMS

6.4 ARRHENIUS THEORY AND TRANSITION-STATE THEORY

6.5 REACTION BARRIERS AND POTENTIAL ENERGY SURFACES

6.6 KINETIC ISOTOPE EFFECTS

6.7 SUBSTITUENT EFFECTS

6.8 LINEAR FREE ENERGY RELATIONSHIPS

7 Acid and Base Catalysis of Organic Reactions

7.1 ACIDITY AND BASICITY OF ORGANIC COMPOUNDS

7.2 ACID AND BASE CATALYSIS OF CHEMICAL REACTIONS

7.3 ACID AND BASE CATALYSIS OF REACTIONS OF CARBONYL COMPOUNDS AND CARBOXYLIC ACID DERIVATIVES

8 Substitution Reactions

8.1 INTRODUCTION

8.2 NUCLEOPHILIC ALIPHATIC SUBSTITUTION

8.3 ELECTROPHILIC AROMATIC SUBSTITUTION

8.4 NUCLEOPHILIC AROMATIC AND VINYLIC SUBSTITUTION

9 Addition Reactions

9.1 INTRODUCTION

9.2 ADDITION OF HALOGENS TO ALKENES

9.3 OTHER ADDITION REACTIONS

10 Elimination Reactions

10.1 INTRODUCTION

10.2 DEHYDROHALOGENATION AND RELATED 1,2-ELIMINATION REACTIONS

10.3 OTHER 1,2-ELIMINATION REACTIONS

11 Pericyclic Reactions

11.1 INTRODUCTION

11.2 ELECTROCYCLIC TRANSFORMATIONS

11.3 SIGMATROPIC REACTIONS

11.4 CYCLOADDITION REACTIONS

11.5 OTHER CONCERTED REACTIONS

11.6 A GENERAL SELECTION RULE FOR PERICYCLIC REACTIONS

11.7 ALTERNATIVE CONCEPTUAL MODELS FOR CONCERTED REACTIONS

12 Photochemistry

12.1 PHOTOPHYSICAL PROCESSES

12.2 FUNDAMENTALS OF PHOTOCHEMICAL KINETICS

12.3 PHYSICAL PROPERTIES OF EXCITED STATES

12.4 REPRESENTATIVE PHOTOCHEMICAL REACTIONS

12.5 SOME APPLICATIONS OF ORGANIC PHOTOCHEMISTRY

References for Selected Problems

Permissions

Author Index

Subject Index

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Library of Congress Cataloging-in-Publication Data:

Carroll, Felix A.

Perspectives on structure and mechanism in organic chemistry / Felix A. Carroll.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-27610-5 (cloth)

Preface

This book is the result of my experience teaching physical organic chemistry at Davidson College. During this time I felt a need for a text that not only presents concepts that are central to the understanding and practice of physical organic chemistry but that also teaches students to think about organic chemistry in new ways, particularly in terms of complementary conceptual models. Because of this approach, the first edition of Perspectives on Structure and Mechanism in Organic Chemistry attracted attention beyond the chemistry community and was even quoted in a philosophy dissertation.1

Soon after the first edition appeared, I received a telephone call from a student of the philosophy of science, who asked how I came to write a book with this emphasis. I did not have a ready answer, but as we talked I realized that this was primarily due to the influences of George Hammond and Jacob Bronowski. I was a graduate student with George Hammond. Although I cannot recall ever discussing conceptual models with him, his views were nonetheless imprinted on me—but in such a subtle way that I did not fully recognize it at the time. Jacob Bronowski’s impact was more distinct because it resulted from a single event—the film Knowledge or Certainty in a series titled The Ascent of Man. That film offers a powerful commentary on both the limits of human knowledge and the nature of science as “a tribute to what we can know although we are fallible.”2a Perhaps a hybridization of their influences led me to emphasize that familiar conceptual models are only beginning points for describing structures and reactions and that using complementary models can provide a deeper understanding of organic chemistry than can using any one model alone.

As with the first edition, the first five chapters of this book consider structure and bonding of stable molecules and reactive intermediates. There is a chapter on methods organic chemists use to study reaction mechanisms, and then acid-base reactions, substitution reactions, addition reactions, elimination reactions, pericyclic reactions, and photochemical reactions are considered in subsequent chapters. In each case I have updated the content to reflect developments since publication of the first edition.

It is essential for an advanced text to provide complete references. The literature citations in this edition range from 1851 to 2009. They direct interested readers to further information about all of the topics and also acknowledge the researchers whose efforts produced the information sum marized here. A teaching text must also provide a set of problems of varying difficulty. The nearly 400 problems in this edition do more than just allow students to test their understanding of the facts and concepts presented in a chapter. They also encourage readers to actively engage the chemical litera ture and to develop and defend their own ideas. Some problems represent straightforward applications of the information in the text, but other pro blems can best be answered by consulting the literature for background information before attempting a solution. Still other problems are open-ended, with no one “ correct” answer. I have prepared a solutions manual giving answers for problems in the first two categories as well as comments about the open-ended problems.

In Knowledge or Certainty, Bronowski shows many portraits of the same human face and observes that “we are aware that these pictures do not so much fix the face as explore it... and that each line that is added strengthens the picture but never makes it final.”2b So it is with this book. It is not a photograph but is, instead, a portrait of physical organic chemistry. As with the human face, it is not possible to fix a continually changing science—we can only explore it. I hope that the lines added in this edition will better enable readers to develop a deeper and more complete understanding of physical organic chemistry.

FELIX A. CARROLL

Davidson College

1 Weisberg, M. When Less is More: Tradeoffs and Idealization in Model Building; Ph.D. Dissertation, Stanford University, 2003. See also Weisberg, M. Philos. Sci.2004,71, 1071.

2 The quotations are from the book with the same title as the film series: Bronowski, J. The Ascent of Man; Little, Brown and Company, Boston, 1973; (a) p. 374; (b) p. 353.

Acknowledgments

I am grateful to the following colleagues for giving their time to read and to offer comments on portions of this edition.

Igor V. Alabugin, Florida State University

John E. Baldwin, Syracuse University

Christopher M. Hadad, Ohio State University

Richard P. Johnson, University of New Hampshire

Jeffrey I. Seeman, University of Richmond

Benjamin T. King, University of Nevada, Reno

Nancy S. Mills, Trinity University

Sason S. Shaik, Hebrew University, Jerusalem

Richard G. Weiss, Georgetown University

Frank H. Quina, University of Sao Paulo

I am also grateful to readers of the first edition who pointed out errors and made suggestions. In particular, I acknowledge Professor Robert G. Bergman of the University of California, Berkeley and his students for their helpful comments.

Sean Ohlinger of Wavefunction, Inc. helped to generate the cover image for this edition, and Kay Filar of Davidson College assisted in the preparation of the indices. I also thank Davidson students Chris Boswell, Will Crossland, Jon Huggins, Josh Knight, Jon Maner, Anna Nam, and Stephanie Scott for their thoughtful comments on an early draft of the book.

Finally, I thank the staff of John Wiley & Sons for bringing the manuscript into print, especially Senior Acquisitions Editor Anita Lekhwani, Editorial Program Coordinator Rebekah Amos, Senior Production Editor Rosalyn Farkas. I also thank Christina Della Bartolomea for copyediting the manuscript.

F. A. C

Introduction

Every organic chemist instantly recognizes the drawing in Figure 1 as benzene, or at least one of the Kekulé structures of benzene. Yet, it is not benzene. It is a geometric figure consisting of a regular hexagon enclosing three extra lines, prepared by marking white paper with black ink. When we look at the drawing, however, we see benzene. That is, we visualize a colorless liquid, and we recall a pattern of physical properties and chemical reactivity associated with benzene and with the concept of aromaticity. The drawing in Figure 1 is therefore only a macroscopic representation of a presumed submicroscopic entity. Even more, the drawing symbolizes the concept of benzene, particularly its structural features and patterns of reactivity.1

FIGURE 1.1 A familiar drawing.

That all organic chemists instantly recognize the drawing in Figure 1 as benzene is confirmation that they have been initiated into the chemical fraternity. The tie that binds the members of this fraternity is more than a collective interest. It is also a common way of viewing problems and their solutions. The educational process that initiates members into this fraternity, like other initiations, can lead to considerable conformity of thinking and of behavior.2 Such conformity facilitates communication among members of the group, but it can limit independent behavior and action.

This common way of looking at problems was explored by T. S. Kuhn in The Structure of Scientific Revolutions.3 Kuhn described processes fundamental to all of the sciences, and he discussed two related meanings of the term paradigm:

On the one hand, it stands for the entire constellation of beliefs, values, techniques, and so on shared by the members of a given community. On the other it denotes one sort of element in that constellation, the concrete puzzle solutions which, employed as models or examples, can replace explicit rules as a basis for the solution of the remaining puzzles of normal science. 3a,4

The parallel with a fraternity is more closely drawn by Kuhn’s observation

… one of the things a scientific community acquires with a paradigm is a criterion for choosing problems that, while the paradigm is taken for granted, can be assumed to have solutions. To a great extent these are the only problems that the community will admit as scientific or encourage its members to undertake. Other problems... are rejected as metaphysical, as the concern of another discipline, or sometimes as just too problematic to be worth the time. A paradigm can, for that matter, even insulate the community from those socially important problems that are not reducible to the puzzle form, because they cannot be stated in terms of the conceptual and instrumental tools the paradigm supplies.3b,5,6

The history of phlogiston illustrates how paradigms can dictate chemical thought. Phlogiston was said to be the “principle” of combustibility—a substance thought to be given off by burning matter.7 The phlogiston theory was widely accepted and was taught to students as established fact.8 As is the case with the ideas we accept, the phlogiston theory could rationalize observable phenomena (combustion) and could account for new observations (such as the death of animals confined in air-tight containers).9 As is also the case with contemporary theories, the phlogiston model could be modified to account for results that did not agree with its predictions. For example, experiments showed that some substances actually gained weight when they burned, rather than losing weight as might have been expected if a real substance had been lost by burning. Rather than abandoning the phlogiston theory, however, some of its advocates rationalized the results by proposing that phlogiston had negative weight.

As this example teaches us, once we have become accustomed to thinking about a problem in a certain way, it becomes quite difficult to think about it differently. Paradigms in science are therefore like the operating system of a computer: they dictate the input and output of information and control the operation of logical processes. Chamberlin stated the same idea with a human metaphor:

The moment one has offered an original explanation for a phenomenon which seems satisfactory, that moment affection for his intellectual child springs into existence.... From an unduly favored child, it readily becomes master, and leads its author whithersoever it will.10

Recognizing that contemporary chemistry is based on widely (if perhaps not universally) accepted paradigms does not mean that we should resist using them. This point was made in 1929 in an address by Irving Langmuir, who was at that time president of the American Chemical Society.

Skepticism in regard to an absolute meaning of words, concepts, models or mathematical theories should not prevent us from using all these abstractions in describing natural phenomena. The progress of physical chemistry was probably set back many years by the failure of the chemists to take full advantage of the atomic theory in describing the phenomena that they observed. The rejection of the atomic theory for this purpose was, I believe, based primarily upon a mistaken attempt to describe nature in some absolute manner. That is, it was thought that such concepts as energy, entropy, temperature, chemical potential, etc., represented something far more nearly absolute in character than the concept of atoms and molecules, so that nature should preferably be described in terms of the former rather than the latter. We must now recognize, however, that all of these concepts are human inventions and have no absolute independent existence in nature. Our choice, therefore, cannot lie between fact and hypothesis, but only between two concepts (or between two models) which enable us to give a better or worse description of natural phenomena.11

Langmuir’s conclusion is correct but, I think, incomplete. Saying that we often choose between two models does not mean that we must, from the time of that choice forward, use only the model that we accept. Instead, we must continually make selections, consciously or subconsciously, among many complementary models.12 Our choice of models is usually shaped by the need to solve the problems at hand. For example, Lewis electron dot structures and resonance theory provide adequate descriptions of the structures and reactions of organic compounds for some purposes, but in other cases we need to use molecular orbital theory or valence bond theory. Frequently, therefore, we find ourselves alternating between these models. Furthermore, consciously using complementary models to think about organic chemistry reminds us that our models are only human constructs and are not windows into reality.

In each of the chapters of this text, we will explore the use of different models to explain and predict the structures and reactions of organic compounds. For example, we will consider alternative explanations for the hybridization of orbitals, the σ,π description of the carbon–carbon double bond, the effect of branching on the stability of alkanes, the electronic nature of substitution reactions, the acid–base properties of organic compounds, and the nature of concerted reactions. The complementary models presented in these discussions will give new perspectives on the structures and reactions of organic compounds.

1 For a discussion of “Representation in Chemistry,” including the nature of drawings of benzene rings, see Hoffmann, R.; Laszlo, P. Angew. Chem. Int. Ed. Engl.1991,30,1. For a discussion of the iconic nature of some chemical drawings, see Whitlock, H. W. J. Org. Chem.1991,56, 7297.

2 Moreover, the interaction of these scientists with those who do not share their interests can be inhibited through what might be called a “sociological hydrophobic effect.”

3 Kuhn, T. S. The Structure of Scientific Revolutions, 2nd ed.; The University of Chicago Press: Chicago, 1970; (a) p. 175; (b) p. 37.

4 The paradigm that we may think of chemistry only through paradigms may be an appropriate description of Western science only. For an interesting discussion of “Sushi Science and Hamburger Science,” see Motokawa, T. Perspect. Biol. Med.1989,32, 489.

5 See also the discussion of Sternberg, R. J. Science1985,230, 1111.

6 The peer review process for grant proposals can be one way a scientific community limits the problems its members are allowed to undertake.

7 White, J. H. The History of the Phlogiston Theory; Edward Arnold & Co.: London, 1932.

8 Conant, J. B. Science and Common Sense; Yale University Press: New Haven, 1951; pp. 170–171.

9 Note the defense of phlogiston by Priestly cited by Pimentel, G. Chem. Eng. News1989 (May 1), p. 53.

10 Chamberlin, T. C. Science1965,148, 754; reprinted from Science (old series) 1890,15, 92. For further discussion of this view, see Bunnett, J. F. in Lewis, E. S., Ed. Investigation of Rates and Mechanisms of Reactions, 3rd ed., Part I; Wiley-Interscience: Hoboken, NJ, 1975; p. 478–479.

11 Langmuir, I. J. Am. Chem. Soc.1929,51, 2847.

12 For other discussions of the role of models in chemistry, see (a) Hammond, G. S.; Osteryoung, J.; Crawford, T. H.; Gray, H. B. Models in Chemical Science: An Introduction to General Chemistry; W. A. Benjamin, Inc.: New York, 1971; pp. 2–7; (b) Sunko, D. E. Pure Appl. Chem.1983,55, 375; (c) Bent, H. A. J. Chem. Educ.1984,61, 774; (d) Goodfriend, P. L. J. Chem. Educ.1976,53, 74; (e) Morwick, J. J. J. Chem. Educ.1978,55,662; (f) Matsen, F. A. J. Chem. Educ.1985,62,365; (g) Dewar, M. J. S. J. Phys. Chem.1985,89, 2145.