INTRODUCTION xxxix If you feel uncertain about a concept or problem in the book-or lecture-get help soon!This subject is highly cumulative,and ignored difficulties will come back to haunt you.We know that many teachers tell you that it is impossible to skip ma- terial and survive,but this time it is true.What happens in December or April depends on September,and you can't wait and wait,only to"turn it on"at the end of the semester or year.Almost no one can cram organic chemistry.Careful,atten- tive,daily work is the route to success,and getting help with a difficult concept or a vexing problem is best done immediately.Over the life of the early editions of this book,Mait interacted with many of you by e-mail,much to his pleasure.Of course, we can't begin to replace local sources of help,and we can't be relied upon in an emer- gency,as we might be out of touch with e-mail,but we can usually be reached at mj55@nyu.edu or sfleming@temple.edu.We look forward to your comments and questions
If you feel uncertain about a concept or problem in the book—or lecture—get help soon! This subject is highly cumulative, and ignored difficulties will come back to haunt you. We know that many teachers tell you that it is impossible to skip material and survive, but this time it is true. What happens in December or April depends on September, and you can’t wait and wait, only to “turn it on” at the end of the semester or year. Almost no one can cram organic chemistry. Careful, attentive, daily work is the route to success, and getting help with a difficult concept or a vexing problem is best done immediately. Over the life of the early editions of this book, Mait interacted with many of you by e-mail, much to his pleasure. Of course, we can’t begin to replace local sources of help, and we can’t be relied upon in an emergency, as we might be out of touch with e-mail, but we can usually be reached at mj55@nyu.edu or sfleming@temple.edu. We look forward to your comments and questions. INTRODUCTION xxxix
Atoms and Molecules; Orbitals and Bonding 11 Preview 1.2 Atoms and Atomic Orbitals 13 Covalent Bonds and Lewis Structures 1.4 Resonance Forms 1.5 Hydrogen (H2):Molecular Orbitals 1.6 Bond Strength 1.7 An Introduction to Reactivity: Acids and Bases 1.8 Special Topic:Quantum Mechanics and Babies 1.9 Summary 1.10 AdditionalProblems 与然 RUTHERFORD'S ATOM This photo represents Rutherford's view of electrons orbiting the nucleus.We know now that electrons are not traveling in circular paths around the nucleus,but this was the early model
Atoms and Molecules; Orbitals and Bonding 1 1.1 Preview 1.2 Atoms and Atomic Orbitals 1.3 Covalent Bonds and Lewis Structures 1.4 Resonance Forms 1.5 Hydrogen (H2): Molecular Orbitals 1.6 Bond Strength 1.7 An Introduction to Reactivity: Acids and Bases 1.8 Special Topic: Quantum Mechanics and Babies 1.9 Summary 1.10 Additional Problems 1 R U T H E R FO R D ’ S ATO M This photo represents Rutherford’s view of electrons orbiting the nucleus. We know now that electrons are not traveling in circular paths around the nucleus, but this was the early model
3 CHAPTER 1 Atoms and Molecules;Orbitals and Bonding When it comes to atoms,language can be used only as in poetry.The poet too,is not nearly so concerned with describing facts as with creating images. -NIELS BOHR TO WERNER HEISENBERG 1.1 Preview The picture of atoms that all scientists had in their collective mind's eye at the begin- ning of the last century was little different from that of the ancient Greek philoso- pher Democritus(~460-370 B.C.),who envisioned small,indivisible particles as the constituents of matter.These particles were called atoms,from the Greek word for indivisible.The British chemist John Dalton(1766-1844)had the idea that differ- ent atoms might have different characteristic masses,but he did not abandon the notion of a solid,uniform atom.That picture did not begin to change dramatically until 1897,when the English physicist J.J.Thomson(1856-1940)discovered the negatively charged elementary particle called the electron.Thomson postulated a spongelike atom with the negatively charged electrons embedded within a positive- ly charged material,rather like raisins in a pudding.Ernest Rutherford's(1871-1937) discovery,in 1909,that an atom was mostly empty space demolished the pudding picture and led to his celebrated planetary model of the atom,in which electrons were seen as orbiting a compact,positively charged nucleus,the core of positively charged protons and neutral neutrons at the center of the atom. It was Niels Bohr who made perhaps the most important modification of the planetary model.He made the brilliant and largely intuitive2 suggestion that elec- trons were required to occupy only certain orbits.Because an electron's energy depends on the distance of its orbit from the positively charged nucleus,Bohr's sug- gestion amounted to saying that electrons in atoms can have only certain energies. An electron might have energy x or energy 2x,but nothing in between.Whenever a property is restricted to certain values in this way,we say the property is quantized. Although this notion may seem strange,there are similar phenomena in the every- day world.One cannot create just any tone by blowing across the mouth of a bot- tle,for example.Say you start by blowing gently and creating a given tone.Of course the tone you hear depends on the size and shape of the particular bottle,but if you gradually increase how hard you blow,which gradually increases the energy you are supplying,the tone does not change smoothly.Instead you hear the first tone unchanged over a certain range of energy input,then a sudden change in tone when just the right"quantum"of energy has been provided. In the 1920s and 1930s,a number of mathematical descriptions emerged from the need to understand Bohr's quantum model of the atom.It became clear that one must take a probabilistic view of the subatomic world.Werner Heisenberg discov- ered that it was not possible to determine simultaneously both the position and momentum(mass times speed)of an electron.3 Thus,one can determine where an Niels Bohr(1885-1962)and Werner Heisenberg(1901-1976)were pioneers in the development of quan- tum theory,the foundation of our current understanding of chemical bonding. Some people's intuitions are better able than others'to cope with the unknown! 3This idea is extraordinarily profound-and troubling.The Heisenberg uncertainty principle(which states that the product of the uncertainty in position times the uncertainty in momentum is a constant)seems to limit fundamentally our access to knowledge.For an exquisite exposition of the human consequences of the uncertainty principle,see Jacob Bronowski,The Ascent of Man,Chapter 11(Little Brown,New York,1973)
2 CHAPTER 1 Atoms and Molecules; Orbitals and Bonding When it comes to atoms, language can be used only as in poetry. The poet too, is not nearly so concerned with describing facts as with creating images. —NIELS BOHR TO WERNER HEISENBERG 1 1.1 Preview The picture of atoms that all scientists had in their collective mind’s eye at the beginning of the last century was little different from that of the ancient Greek philosopher Democritus ( 460–370 B.C.), who envisioned small, indivisible particles as the constituents of matter. These particles were called atoms, from the Greek word for indivisible. The British chemist John Dalton (1766–1844) had the idea that different atoms might have different characteristic masses, but he did not abandon the notion of a solid, uniform atom. That picture did not begin to change dramatically until 1897, when the English physicist J. J. Thomson (1856–1940) discovered the negatively charged elementary particle called the electron. Thomson postulated a spongelike atom with the negatively charged electrons embedded within a positively charged material, rather like raisins in a pudding. Ernest Rutherford’s (1871–1937) discovery, in 1909, that an atom was mostly empty space demolished the pudding picture and led to his celebrated planetary model of the atom, in which electrons were seen as orbiting a compact, positively charged nucleus, the core of positively charged protons and neutral neutrons at the center of the atom. It was Niels Bohr who made perhaps the most important modification of the planetary model. He made the brilliant and largely intuitive 2 suggestion that electrons were required to occupy only certain orbits. Because an electron’s energy depends on the distance of its orbit from the positively charged nucleus, Bohr’s suggestion amounted to saying that electrons in atoms can have only certain energies. An electron might have energy x or energy 2 x, but nothing in between. Whenever a property is restricted to certain values in this way, we say the property is quantized. Although this notion may seem strange, there are similar phenomena in the everyday world. One cannot create just any tone by blowing across the mouth of a bottle, for example. Say you start by blowing gently and creating a given tone. Of course the tone you hear depends on the size and shape of the particular bottle, but if you gradually increase how hard you blow, which gradually increases the energy you are supplying, the tone does not change smoothly. Instead you hear the first tone unchanged over a certain range of energy input, then a sudden change in tone when just the right “quantum” of energy has been provided. In the 1920s and 1930s, a number of mathematical descriptions emerged from the need to understand Bohr’s quantum model of the atom. It became clear that one must take a probabilistic view of the subatomic world. Werner Heisenberg discovered that it was not possible to determine simultaneously both the position and momentum (mass times speed) of an electron. 3 Thus, one can determine where an ' 1 Niels Bohr (1885–1962) and Werner Heisenberg (1901–1976) were pioneers in the development of quantum theory, the foundation of our current understanding of chemical bonding. 2 Some people’s intuitions are better able than others’ to cope with the unknown! 3 This idea is extraordinarily profound—and troubling. The Heisenberg uncertainty principle (which states that the product of the uncertainty in position times the uncertainty in momentum is a constant) seems to limit fundamentally our access to knowledge. For an exquisite exposition of the human consequences of the uncertainty principle, see Jacob Bronowski, The Ascent of Man, Chapter 11 (Little Brown, New York, 1973)
1.1 Preview electron is at any given time only in terms of probability.One can say,for example, that there is a 90%probability of finding the electron in a certain volume of space, but one cannot say that at a given instant the electron is at a particular point in space. The further elaboration of this picture of the atom has given us the conceptual basis for all modern chemistry:the idea of the orbital.Loosely speaking,an orbital describes the region of space surrounding an atomic nucleus that may be occupied by either an electron or a pair of electrons of a certain energy.Both the combining of atoms to form molecules and the diverse chemical reactions these molecules undergo involve,at a fundamental level,the interactions ofelectrons in orbitals.This notion will appear throughout this book;it is the most important unifying princi- ple of organic chemistry.In atoms,we deal with atomic orbitals,and in molecules, we deal with molecular orbitals. Various graphic conventions are used in this book to represent atoms and molecules-letters for atoms,dots for electrons not involved in bonding,and lines for electrons in bonds-but it is important to keep in mind from the outset that the model that most closely approximates our current understanding of reality at the atomic and molecular level is the cloudy,indeterminate-one might even say poetic-image of the orbital.4 There is great conceptual overlap between the concept of an orbital and the notion you probably encountered in general chemistry of shells of electrons surround- ing the atomic nucleus.For example,you are accustomed to thinking of the noble gas elements as having filled shells of electrons,two electrons for helium in the first shell,two electrons in the first shell and eight in the second shell for neon,and so on.In the noble gases,the outermost,or valence shells are filled.We will speak of those valence shells as valence orbitals.We shall say much more about orbitals in a moment,especially about their shapes,but the point to "get"here is the move from the old word sbell to the new word orbital. ESSENTIAL SKILLS AND DETAILS The following list of Essential Skills and Details,a version of which will appear in every chapter,is designed to alert you to the important parts of the chapter and,especially,to aid you in reviewing.After you finish the chapter,or before an examination,it is a good idea to return to this list and make sure you are clear on all the Essential Skills and Details. 1.Writing correct Lewis dot structures for atoms,ions(charged atoms and molecules), and neutral molecules is an absolutely critical skill that will be essential throughout this book. 2.Take charge!It is necessary to be able to determine the formal charge of an atom, especially an atom in a molecule. 3.You have to be able to write the resonance forms(different electronic structures)that, taken together,give a more accurate picture of molecules than does any single structure. 4.Learn how to use the curved arrow formalism to "push"pairs of electrons in writing resonance forms and in sketching electron flow in chemical reactions. 5.Remember the sign convention for exothermic(AH is negative)and endothermic (△H°is positive)reactions.. In the wonderful quote that opens this chapter,Niels Bohr points out that once we transcend the visible world,all that is possible is modeling or image-making.To us,what is even more marvelous about the quote is the simple word o:It was obvious to Bohr that scientists speak in images,and he was pointing out to Heisenberg that there was another group of people out there in the world who did the same thingpoets
1.1 Preview 3 electron is at any given time only in terms of probability. One can say, for example, that there is a 90% probability of finding the electron in a certain volume of space, but one cannot say that at a given instant the electron is at a particular point in space. The further elaboration of this picture of the atom has given us the conceptual basis for all modern chemistry: the idea of the orbital. Loosely speaking, an orbital describes the region of space surrounding an atomic nucleus that may be occupied by either an electron or a pair of electrons of a certain energy. Both the combining of atoms to form molecules and the diverse chemical reactions these molecules undergo involve, at a fundamental level, the interactions of electrons in orbitals.This notion will appear throughout this book; it is the most important unifying principle of organic chemistry. In atoms, we deal with atomic orbitals, and in molecules, we deal with molecular orbitals. Various graphic conventions are used in this book to represent atoms and molecules—letters for atoms, dots for electrons not involved in bonding, and lines for electrons in bonds—but it is important to keep in mind from the outset that the model that most closely approximates our current understanding of reality at the atomic and molecular level is the cloudy, indeterminate—one might even say poetic—image of the orbital.4 There is great conceptual overlap between the concept of an orbital and the notion you probably encountered in general chemistry of shells of electrons surrounding the atomic nucleus. For example, you are accustomed to thinking of the noble gas elements as having filled shells of electrons, two electrons for helium in the first shell, two electrons in the first shell and eight in the second shell for neon, and so on. In the noble gases, the outermost, or valence shells are filled. We will speak of those valence shells as valence orbitals. We shall say much more about orbitals in a moment, especially about their shapes, but the point to “get” here is the move from the old word shell to the new word orbital. ESSENTIAL SKILLS AND DETAILS The following list of Essential Skills and Details, a version of which will appear in every chapter, is designed to alert you to the important parts of the chapter and, especially, to aid you in reviewing. After you finish the chapter, or before an examination, it is a good idea to return to this list and make sure you are clear on all the Essential Skills and Details. 1. Writing correct Lewis dot structures for atoms, ions (charged atoms and molecules), and neutral molecules is an absolutely critical skill that will be essential throughout this book. 2. Take charge! It is necessary to be able to determine the formal charge of an atom, especially an atom in a molecule. 3. You have to be able to write the resonance forms (different electronic structures) that, taken together, give a more accurate picture of molecules than does any single structure. 4. Learn how to use the curved arrow formalism to “push” pairs of electrons in writing resonance forms and in sketching electron flow in chemical reactions. 5. Remember the sign convention for exothermic ( is negative) and endothermic ( is positive) reactions. ¢H ° ¢H ° 4 In the wonderful quote that opens this chapter, Niels Bohr points out that once we transcend the visible world, all that is possible is modeling or image-making. To us, what is even more marvelous about the quote is the simple word too: It was obvious to Bohr that scientists speak in images, and he was pointing out to Heisenberg that there was another group of people out there in the world who did the same thing—poets