1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 17 8-H 1.50 1.51 1.52入0° Now let's look at the common situations for nitrogen atoms.When nitrogen hasno formal charge,it will have three bonds and one lone pair. 入NH2 5 the same as人NH 、是5ee的 、成 心 is the same as If nitrogen has a negative formal charge,then it must have two bonds and two lone pairs: 入8H is the same as 人 、月is the same as 、足 If nitrogen has a positive charge.then it must have four bonds and no lone pairs has no lone pairs has no lone pairs =9 has no lone pairs
Now let’s look at the common situations for nitrogen atoms. When nitrogen has no formal charge, it will have three bonds and one lone pair: If nitrogen has a negative formal charge, then it must have two bonds and two lone pairs: If nitrogen has a positive charge, then it must have four bonds and no lone pairs: N has no lone pairs N has no lone pairs N has no lone pairs N H N N is the same as is the same as is the same as NH N N N H N N is the same as is the same as is the same as NH2 N N H H H H H 1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 17 OH O O H 1.50 1.51 1.52 O 6753_Klein_01.qxd 5/1/07 5:03 PM Page 17
18 CHAPTER 1 BOND-LINE DRAWINGS EXERCISE 1.53 Draw all lone pairs in the following structure: Answer The top nitrogen has a positive formal charge and four bonds.You should nds.You should try to get to a point where you recognize that this must mean that this nitrogen has one lone pair: Until you get to the point where you can recognize this,you should be able to figure out the answer by counting.Nitrogen is supposed to have five electrons.The top ni- e.which m .This means y lone there are Since thisite s fouro electrons to form bonds.So there is no lone pair on this nitrogen atom. The bottom nitrogen atom has no formal charge,so this nitrogen atom has five electrons.It has three bonds.which means that there are two electrons left over,and they form a lone pair. PROBLEMS Review the recognize how many lone pairs there are without having to count.Then count to see if you were right. 157人158入y159 人NH2
EXERCISE 1.53 Draw all lone pairs in the following structure: Answer The top nitrogen has a positive formal charge and four bonds. You should try to get to a point where you recognize that this must mean that this nitrogen has no lone pairs. The bottom nitrogen has no formal charge and three bonds. You should try to get to a point where you recognize that this must mean that this nitrogen has one lone pair: Until you get to the point where you can recognize this, you should be able to figure out the answer by counting. Nitrogen is supposed to have five electrons. The top nitrogen atom has a positive charge, which means it is missing an electron. This means that this nitrogen atom must have 5 1 4 electrons. Now we can figure out how many lone pairs there are. Since this nitrogen has four bonds, it is using all of its electrons to form bonds. So there is no lone pair on this nitrogen atom. The bottom nitrogen atom has no formal charge, so this nitrogen atom has five electrons. It has three bonds, which means that there are two electrons left over, and they form a lone pair. PROBLEMS Review the common situations for nitrogen, and then come back to these problems. For each of the following structures, draw in all lone pairs. Try to recognize how many lone pairs there are without having to count. Then count to see if you were right. N N N N 18 CHAPTER 1 BOND-LINE DRAWINGS N N H N N N NH2 O 1.57 1.58 1.59 1.54 1.55 1.56 6753_Klein_01.qxd 5/1/07 5:03 PM Page 18
1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 19 MORE PROBLEMS For each of the following structures draw in all lone pairs member from the previous section that C+has no lone pairs and C-has one lone pair). 1.60 .61 NNO一16a义 H--NH 1.65 。H 1.66b-CN 1.670=C=N°1.68
MORE PROBLEMS For each of the following structures, draw in all lone pairs (remember from the previous section that C has no lone pairs and C has one lone pair). 1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 19 O N O H2N N OO H NH2 O C N N O H O C N 1.60 1.61 1.62 1.63 1.64 1.65 1.66 1.67 1.68 6753_Klein_01.qxd 5/1/07 5:03 PM Page 19
0 CHAPTER RESONANCE In this chapter.you will learn the tools that you need to draw resonance structures with proficiency.I cannot adequately stress the importance of this skill.Resonance is the one topic that permeates the entire subject matter from start to finish.It finds its way intoevery chapter,into every reaction.and into your nightm res if you do not master the rules of resonance.You cannot get an A in this class without master- ing resonance.So what is resonance?And why do we need it? 2.1 WHAT IS RESONANCE? In Chapter 1,we introduced one of the best ways of drawing molecules,bond-line structures.They are fast to draw and easy to read,but they have one major defi- ciency:they do not describe molecules perfectly.In facto drawing method can completely describe a molecule using only a single drawing.Here is the problem. Although our drawings are very good at showing which atoms are connected to each other,our drawings are not good at showing where all of the electrons are. because electrons aren't really solid particles that can be in one place at one time All of our drawing methods treat electrons as particles that can be placed in specific oca ations.Instead,it is best to think of ectrons as clouds of electron density.We don't mean that electrons fly around in clouds:we mean that electrons are clouds. These clouds often spread themselves across large regions of a molecule. So how do we represent molecules if we can't draw where the electrons are? The answer is resonance.We use the term resonance to describe our solution to the wings We meld thes drawings into one image in our minds.To better understand how this works,consider the following analogy. Your friend asks you to describe what a nectarine looks like,because he has never seen one.You aren't a very good artist so you say the following: Picture a peach in your mind,and now picture a plum in your mind.Well,a nectarine has features of both:the inside tastes like a peach.but the outside is smooth like a plum.So take your image of a peach together with your image of a plum and meld them together in your mind into one image.That's a nectarine. 20
In this chapter, you will learn the tools that you need to draw resonance structures with proficiency. I cannot adequately stress the importance of this skill. Resonance is the one topic that permeates the entire subject matter from start to finish. It finds its way into every chapter, into every reaction, and into your nightmares if you do not master the rules of resonance. You cannot get an A in this class without mastering resonance. So what is resonance? And why do we need it? 2.1 WHAT IS RESONANCE? In Chapter 1, we introduced one of the best ways of drawing molecules, bond-line structures. They are fast to draw and easy to read, but they have one major defi- ciency: they do not describe molecules perfectly. In fact, no drawing method can completely describe a molecule using only a single drawing. Here is the problem. Although our drawings are very good at showing which atoms are connected to each other, our drawings are not good at showing where all of the electrons are, because electrons aren’t really solid particles that can be in one place at one time. All of our drawing methods treat electrons as particles that can be placed in specific locations. Instead, it is best to think of electrons as clouds of electron density. We don’t mean that electrons fly around in clouds; we mean that electrons are clouds. These clouds often spread themselves across large regions of a molecule. So how do we represent molecules if we can’t draw where the electrons are? The answer is resonance. We use the term resonance to describe our solution to the problem: we use more than one drawing to represent a single molecule. We draw several drawings, and we call these drawings resonance structures. We meld these drawings into one image in our minds. To better understand how this works, consider the following analogy. Your friend asks you to describe what a nectarine looks like, because he has never seen one. You aren’t a very good artist so you say the following: Picture a peach in your mind, and now picture a plum in your mind. Well, a nectarine has features of both: the inside tastes like a peach, but the outside is smooth like a plum. So take your image of a peach together with your image of a plum and meld them together in your mind into one image. That’s a nectarine. It is important to realize that a nectarine does not switch back and forth every second from being a peach to being a plum. A nectarine is a nectarine all of the time. 20 CHAPTER 2 RESONANCE 6753_Klein_02.qxd 5/1/07 5:04 PM Page 20
2.2 CURVED ARROWS:THE TOOLS FOR DRAWING RESONANCE STRUCTURES 21 The image of a peach is not adequate to describe a nectarine.Neither is the image of a plum.But by gining both together at the same time.you can get a sense of wha a necta The problem with drawing molecules is similar to the problem above with the nectarine.No single drawing adequately describes the nature of the electron density spread out over the molecule.To solve this problem,we draw several drawings and then meld them together in our mind into one image.Just like the nectarine. Let's see an example ◇一◇ The compound above has two important resonance structures.Notice that we sep arate resonance structures with a straight.two-headed arrow.and we place brack ets around the structures.The arrow and brackets indicate that they are resonance ctures of one molecule.The molecule is not flipping back and forth betwe the different resonance structures.The electr molecule are not actually moving at all. Now that we know why we need resonance,we can begin to understand why resonance structures are so important.Ninety-five percent of the reactions that you will see in this course occur because one molecule has a region of low electron den- sity and the other molecule has a region of high electron density.They attract each other in sp which ca uses a re Soto predict h and wher a tw will rea with each othe we need first to pre ict where there is low electron den sity and where there is high electron density.We need to have a firm grasp of reso nance to do this.In this chapter,we will see many examples of how to predict the regions of low or high electron density by applying the rules of drawing resonance structures. 2.2 CURVED ARROWS:THE TOOLS FOR DRAWING RESONANCE STRUCTURES In the beginning of the course.vou might encounter problems like this:here is a drawing:now draw the other resonance structures.But later on in the course,it will umed and expected that you can draw all of the reson ce structures of a com pound.If you cannot actually do this,you will be in big trouble later on in the course So how do you draw all of the resonance structures of a compound?To do this,you need to learn the tools that help you:curved arrows. Here is where it can be confusing as to what is exactly going on.These arrows do not represent an actual process(such as electrons moving).This is an important point,because you will learn later abou curved arrows used in drawing reacti mechanisms.Those arrows look exactly the same,but they actually do refer to the flow of electron density.In contrast,curved arrows here are used only as tools to help
The image of a peach is not adequate to describe a nectarine. Neither is the image of a plum. But by imagining both together at the same time, you can get a sense of what a nectarine looks like. The problem with drawing molecules is similar to the problem above with the nectarine. No single drawing adequately describes the nature of the electron density spread out over the molecule. To solve this problem, we draw several drawings and then meld them together in our mind into one image. Just like the nectarine. Let’s see an example: The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. The electrons in the molecule are not actually moving at all. Now that we know why we need resonance, we can begin to understand why resonance structures are so important. Ninety-five percent of the reactions that you will see in this course occur because one molecule has a region of low electron density and the other molecule has a region of high electron density. They attract each other in space, which causes a reaction. So, to predict how and when two molecules will react with each other, we need first to predict where there is low electron density and where there is high electron density. We need to have a firm grasp of resonance to do this. In this chapter, we will see many examples of how to predict the regions of low or high electron density by applying the rules of drawing resonance structures. 2.2 CURVED ARROWS: THE TOOLS FOR DRAWING RESONANCE STRUCTURES In the beginning of the course, you might encounter problems like this: here is a drawing; now draw the other resonance structures. But later on in the course, it will be assumed and expected that you can draw all of the resonance structures of a compound. If you cannot actually do this, you will be in big trouble later on in the course. So how do you draw all of the resonance structures of a compound? To do this, you need to learn the tools that help you: curved arrows. Here is where it can be confusing as to what is exactly going on. These arrows do not represent an actual process (such as electrons moving). This is an important point, because you will learn later about curved arrows used in drawing reaction mechanisms. Those arrows look exactly the same, but they actually do refer to the flow of electron density. In contrast, curved arrows here are used only as tools to help 2.2 CURVED ARROWS: THE TOOLS FOR DRAWING RESONANCE STRUCTURES 21 6753_Klein_02.qxd 5/1/07 5:04 PM Page 21