Reactive Intermediates in Organic Chemistry E-Book

Maya Shankar Singh

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2014

Seitenzahl: 383

Beschreibung

Most reactions in organic chemistry do not proceed in a single step but rather take several steps to yield the desired product. In the course of these multi-step reaction sequences, short-lived intermediates can be generated that quickly convert into other intermediates, reactants, products or side products. As these intermediates are highly reactive, they cannot usually be isolated, but their existence and structure can be proved by theoretical and experimental methods. Using the information obtained, researchers can better understand the underlying reaction mechanism of a certain organic transformation and thus develop novel strategies for efficient organic synthesis. The chapters are clearly structured and are arranged according to the type of intermediate, providing information on the formation, characterization, stereochemistry, stability, and reactivity of the intermediates. Additionally, representative examples and a problem section with different levels of difficulty are included for self-testing the newly acquired knowledge. By providing a deeper understanding of the underlying concepts, this is a musthave reference for PhD and Master Students in organic chemistry, as well as a valuable source of information for chemists in academia and industry working in the field. It is also ideal as primary or supplementary reading for courses on organic chemistry, physical organic chemistry or analytical chemistry.

Leseprobe

Table of Contents

Related Titles

Title Page

Copyright

Preface

Chapter 1: Introduction

1.1 Reaction Mechanism and Reaction Arrows

1.2 Properties and Characteristics of a Reaction

1.3 Summary

Further Reading

Chapter 2: Carbocations

2.1 Introduction

2.2 History

2.3 Structures and Geometry of Carbocations

2.4 Generation of Carbocation

2.5 Carbocation Stability

2.6 Detection of Carbocations

2.7 Fate of Carbocations

2.8 Nonclassical Carbocations

2.9 Radical Cations

2.10 Summary

Further Reading

Chapter 3: Carbanions

3.1 Structure and Geometry of Carbanions

3.2 Generation of Carbanions

3.3 Stability of Carbanions

3.4 Reactions of Carbanions

3.5 Enolate Reactions with Carbonyl Groups

3.6 Rearrangements of Carbanions

3.7 Chiral Carbanions

3.8 Carbanions and Tautomerism

3.9 Summary

Further Reading

Chapter 4: Radicals

4.1 Introduction

4.2 Detection and Characterization of Radicals

4.3 Structure and Bonding of Radicals

4.4 Generation of Free Radicals

4.5 Stability of Radicals

4.6 Reactions of Free Radicals

4.7 Stereochemistry of Radical Reactions

4.8 Biradicals

4.9 Summary

Further Reading

Chapter 5: Carbenes

5.1 Structure and Geometry of Carbenes

5.2 Generation of Carbenes

5.3 Reactions of Carbenes

5.4 Carbenes and Carbene Ligands in Organometallic Chemistry

5.5 Summary

Further Reading

Chapter 6: Nitrenes

6.1 Introduction

6.2 Structure and Reactivity

6.3 Generation of Nitrenes

6.4 Reactions of Nitrenes

6.5 Summary

Further Reading

Chapter 7: Miscellaneous Intermediates

7.1 Arynes

7.2 Ketenes and Cumulenes

7.3 ortho-Quinone Methides

7.4 Zwitterions and Dipoles

7.5 Antiaromatic Systems

7.6 Tetrahedral Intermediates

7.7 Summary

Further Reading

Index

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The Author

Prof. Dr. Maya Shankar Singh

Banaras Hindu University

Faculty of Science

Department of Chemistry

Varanasi 221 005

India

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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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Print ISBN: 978-3-527-33594-7

ePDF ISBN: 978-3-527-67828-0

ePub ISBN: 978-3-527-67827-3

Mobi ISBN: 978-3-527-67826-6

oBook ISBN: 978-3-527-67825-9

Preface

Organic chemistry has always been, and continues to be, the branch of chemistry that best connects structure with properties, which attracts particular attention because of its immense importance to life and society. Organic synthesis is a creative science involving the construction and cleavage of bonds, the strategies for which represent the central theme in organic synthesis. More than any other branch of organic chemistry, synthesis has improved our understanding of the structure, dynamics, and transition of molecules. Most synthetic problems have more than one solution, and the trick is to judge which of these is likely to have the best chance of success. Even the most experienced chemists develop routes that work well on paper but fail miserably in the laboratory. However, there are some guidelines and principles that are helpful in designing a suitable route for a particular synthesis. Whether one seeks to understand nature or to create the new materials and medicines of the future, a key starting point is thus to understand structure and mechanism of a particular reaction. For synthetic chemists it is very important to understand in detail what is going on when the molecules in the starting materials react with each other and create the molecules characteristic of the product. Knowledge about mechanisms makes it possible to develop better and less expensive methods to prepare products of technical importance.

Writing a textbook of any level is always a challenging mission. This book has been designed in view of the growing importance of intermediates in the synthesis of natural and/or non-natural molecules. The ideas of functionality and stereochemistry have their origins in the second half of the nineteenth century, and the concepts of bonding and reaction mechanism undoubtedly belong to the twentieth century. The goal of this text is to incorporate basic conceptual tools and recent advances in the area of organic synthesis and particularly in the field of reactive intermediates, which are the key steps of any transformation. A systematic understanding of the mechanisms of organic reactions is necessary as without it organic chemistry is chaos, and impossible to learn.

Theory, mechanism, synthesis, structure, and stereochemistry are discussed throughout the book in a qualitative to semiquantitative fashion. During the writing of this book I have always tried to anticipate the questions of a student and to challenge them to think about the subject, motivating them to understand and to realize why, rather than just memorizing material. Chemists present chemistry in terms of structural diagrams and for this reason all reactions have been drawn using curly arrows; the handwriting of chemistry. Curved arrows and chemical reactions introduce students to the notational systems employed in all of the mechanistic discussions in the text. Such a course is frequently offered as a course material in organic chemistry at the undergraduate and beginning graduate level. I guess one will enjoy many fruitful hours of insight in the course of studying this book and I welcome your constructive comments on its content and approach. In attempting to accomplish these objectives, my approach is substantially different from currently available titles.

I have tried to put equal weight to the three basic fundamental aspects of the study of reactive intermediates, that is, reactions, mechanisms, and stereochemistry. The organization is based on these concepts, so that students can understand the large number of organic reactions based on relatively few principles. Accordingly, this book is divided into seven chapters. The first gives a brief introduction dealing with some basic, very frequently used terms, concepts of steric and electronic effects, and sites of chemical reactivity. The student is also told why such information will be important in the study of a particular reaction mechanism. Chapters 2–6 cover specific reactive intermediates in detail regarding their structure, geometry, generation, stability, and reactions. Chapter 7 gives a brief survey of the miscellaneous intermediates. End-of-chapter summaries review the major concepts of the chapter in a concise narrative format to help readers to understand the key points. The problems at the end of each chapter represent the application of concepts, rather than a review of material explicitly presented in the text. They are designed so that students can test themselves on the material just covered before they go on to the next section. I hope the level of difficulty will present a considerable challenge to students. These problems allow students to practice and test their mastery of core principles within each chapter. A concerted effort was made to make none of the problems so difficult that the student loses confidence.

I would greatly appreciate comments and suggestions from users that will improve the text or correct errors. I can only conclude by expressing my wish that others will enjoy using this text as much as I have enjoyed writing it. In particular, I want to thank the many wonderful and talented students I have had over the years, who taught me how to be a teacher and researcher. I also want to thank the dedicated people at Wiley-VCH, Germany, Dr. Anne Brennführer (Commissioning Editor), Lesley Belfit (Project Editor), and Claudia Nussbeck (Editorial Assistant), for their truly superior editorial ability and for keeping me happy and on track.

Finally, I am grateful to my wife Meera and my son Keshav whose contributions to the project are beyond measure, and so I thank them for their understanding, love, encouragement, and assistance during the lengthy process of writing this book.

Chapter 1

Introduction

Chemistry is an old science that influences every aspect of life on earth (from toothpaste to life-saving medicines) because just about everything that we can touch and feel is made of chemicals, which is why it is known as the mother science or central science. The chemical cornucopia (a hollow basket filled with various kinds of festive materials) is truly impressive. While chemistry is, indeed, an old subject (∼1000 BC), modern chemistry (Antoine-Laurent de Lavoisier (1743–1794), the “Father of modern chemistry”) is ∼230 years old, while organic synthesis is only about 150 years old. The essential feature of this central science is synthesis. The chemist who designs and completes an original and aesthetically pleasing synthesis is like the composer, artist, or poet, who with great individuality fashions new forms of beauty from the interplay of mind and spirit.

Chemistry occupies a unique middle position between physics and mathematics on the one side and biology, ecology, sociology, and economics on the other. It is said that chemistry is reducible into physics and finally mathematics. On the one hand, it deals with biology and provides explanations for how molecules determine the processes of life. On the other hand, it mingles with physics as well as mathematics, and finds explanations for chemical phenomena in the fundamental processes and particles of the universe:

“The greatest scientific advance of the last 50 years is the way biology is becoming a molecular science (chemistry)…”

Chemistry is playing a vital role in every area of our increasingly technological society that links the familiar with the fundamental.

Like all sciences, chemistry has a unique place in our pattern of understanding of the universe. It is the science of molecules, but organic chemistry is something more, that is, a tentative attempt to understand the chemistry of life. The task of the organic chemist is to make tools (molecules), that is, the art and science of constructing the molecules of nature available for various uses. Essentially all chemical reactions that take place in living systems, including in our own bodies, are organic reactions because the molecules of life – proteins, enzymes, vitamins, lipids, carbohydrates, and nucleic acids – all are organic compounds. All things originating from living things are organic but anything containing carbon is also organic. The food we eat, the wood to make our homes, the clothing we wear (whether natural cotton or polyester), gasoline, rubber, plastics, medicines, pesticides, herbicides, all are made from organic compounds. We can thank organic chemistry for making our life easier in the modern age, and furthermore a responsibility lies on the shoulders of synthetic organic chemists to make life even better.

Chemistry is a vibrant subject filled with light, colors, fragrance, flavors, action, and excitement; a subject that begs to be taught by the points of inquiry method. When you picked up this book, your muscles were performing chemical reactions on sugars to give you the energy you required. As you go through this book, your eyes are using an organic compound (11--retinal) to switch visible light into nerve impulses. Gaps between your brain cells are being bridged by simple organic molecules (neurotransmitter amines) so that nerve impulses can be passed around your brain. Organic chemistry often studies life by making new molecules that give information not available from the molecules actually present in living things. Whether one seeks to understand nature or to create the new materials and medicines of the future, a key starting point is thus to understand structure and mechanism. Organic chemistry has always been, and continues to be, the branch of chemistry that best connects structure with properties.