2.4 DRAWING GOOD ARROWS 27 2.11Hh6C-是N: 212G0R 2.4 DRAWING GOOD ARROWS Now that we know how to identify good arrows and bad arrows,we need to get some practice drawing arrows.We know that the tail of an arrow must come either from a bond or a lone pair and that the head of an arrow must go to form a bond or a lone pair.If we are given two resonance structures and are asked to show the arrow(s)that get us from one resonance structure to the other,it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another.Let's see this with an example. Say we have two resonance structures: 一 How would we figure out what curved arrow to draw to get us from the drawing on the left to the drawing on the right?We must look at the difference between the two structures and ask,"How should we push the electrons to get from the first structure to the second structure?Begin by looking for any double bonds or lone pairs that are disappearing.That will tell us whe re to put the tail of our arrow.In this example there are no lone pairs disappearing,but there is a double bond disappearing.So we know that we need to put the tail of our arrow on the double bond. Now,we need to know where to put the head of the arrow.We look for any lone pairs or double bonds that are appearing.We see that there is a new lone pair appearing on the oxygen.So now we know where to put the head of the arrow 货一 Notice that when we move a double bond up onto an atom to form a lone pair it creates two formal charges:a positive charge on the carbon that lost its bond and a negative charge on the oxygen that got a lone pair.This is a very important issue For nal charge strumental in drawing resonance structures.For the moment let's just fo cus on push ing arrows,and in the next section of this chapter,we will come back to focus on these formal charges
2.11 2.12 2.4 DRAWING GOOD ARROWS Now that we know how to identify good arrows and bad arrows, we need to get some practice drawing arrows. We know that the tail of an arrow must come either from a bond or a lone pair, and that the head of an arrow must go to form a bond or a lone pair. If we are given two resonance structures and are asked to show the arrow(s) that get us from one resonance structure to the other, it makes sense that we need to look for any bonds or lone pairs that are appearing or disappearing when going from one structure to another. Let’s see this with an example. Say we have two resonance structures: How would we figure out what curved arrow to draw to get us from the drawing on the left to the drawing on the right? We must look at the difference between the two structures and ask, “How should we push the electrons to get from the first structure to the second structure?” Begin by looking for any double bonds or lone pairs that are disappearing. That will tell us where to put the tail of our arrow. In this example, there are no lone pairs disappearing, but there is a double bond disappearing. So we know that we need to put the tail of our arrow on the double bond. Now, we need to know where to put the head of the arrow. We look for any lone pairs or double bonds that are appearing. We see that there is a new lone pair appearing on the oxygen. So now we know where to put the head of the arrow: Notice that when we move a double bond up onto an atom to form a lone pair, it creates two formal charges: a positive charge on the carbon that lost its bond and a negative charge on the oxygen that got a lone pair. This is a very important issue. Formal charges were introduced in the last chapter, and now they will become instrumental in drawing resonance structures. For the moment, let’s just focus on pushing arrows, and in the next section of this chapter, we will come back to focus on these formal charges. O O O O O R H3CNN 2.4 DRAWING GOOD ARROWS 27 6753_Klein_02.qxd 5/1/07 5:04 PM Page 27
28 CHAPTER 2 RESONANCE It is pretty straightforward to see how to push only one arrow that gets us from one resonance structure to another.But what about when we need to push more than one arrow to get from one resonance structure to another?Let's do an ex- ample like that. EXERCISE 2.13 For the two structures below,try to draw the curved arrows that get you from the drawing on the left to the drawing on the right: 英一文】 Answer Let's analyze the difference between these two drawings.We begin by looking for any double bonds or lone We see that the oxygen is los ng a lone C on the bo d sappearing.Thi should automatically tell us that we need two arrows.To lose a lone pair and a dou ble bond.we will need two tails. Now let's look for any double bonds or lone pairs that are appearing.We see that a C=O is appearing and a C with a negative charge is appe ing (remember that a C-means a C with a lone pair).This tells us that we need two heads.which confirms that we need two arrows. So we know we need two arrows.Let's start at the top.We lose a lone pair from the oxygen and form a C=O.Let's draw that arrow: 萸 Notice that if we stopped here,we would be violating the second commandment The central carbon atom is getting five bonds.To avoid this problem,we must im- mediately draw the second r ow The C=C dis appears (which solves our octe problem)and becomes a lone pair on the carbon.Now we can draw both arrows: Arrow pushing is much like riding a bike.If you have never done it before watching someone else will not make you an expert.You have to learn how to balance vourself.Watching someone else is a good start.but you have to get on the bike if vou want to learn.You will probably fall a few times,but that's part of the leaming pro- cess.The same is true with arrow pushing.The only way to learn is with practice. Now it's time for you to get on the arrow-pushing bike.You would never be stupid enough to try riding a bike for the first time next to a steep cliff.Do not have your first arrow-pushing experience be during your exam.Practice right now!
It is pretty straightforward to see how to push only one arrow that gets us from one resonance structure to another. But what about when we need to push more than one arrow to get from one resonance structure to another? Let’s do an example like that. EXERCISE 2.13 For the two structures below, try to draw the curved arrows that get you from the drawing on the left to the drawing on the right: Answer Let’s analyze the difference between these two drawings. We begin by looking for any double bonds or lone pairs that are disappearing. We see that the oxygen is losing a lone pair, and the C¨C on the bottom is also disappearing. This should automatically tell us that we need two arrows. To lose a lone pair and a double bond, we will need two tails. Now let’s look for any double bonds or lone pairs that are appearing. We see that a C¨O is appearing and a C with a negative charge is appearing (remember that a C means a C with a lone pair). This tells us that we need two heads, which confirms that we need two arrows. So we know we need two arrows. Let’s start at the top. We lose a lone pair from the oxygen and form a C¨O. Let’s draw that arrow: Notice that if we stopped here, we would be violating the second commandment. The central carbon atom is getting five bonds. To avoid this problem, we must immediately draw the second arrow. The C¨C disappears (which solves our octet problem) and becomes a lone pair on the carbon. Now we can draw both arrows: Arrow pushing is much like riding a bike. If you have never done it before, watching someone else will not make you an expert. You have to learn how to balance yourself. Watching someone else is a good start, but you have to get on the bike if you want to learn. You will probably fall a few times, but that’s part of the learning process. The same is true with arrow pushing. The only way to learn is with practice. Now it’s time for you to get on the arrow-pushing bike. You would never be stupid enough to try riding a bike for the first time next to a steep cliff. Do not have your first arrow-pushing experience be during your exam. Practice right now! O O O O O 28 CHAPTER 2 RESONANCE 6753_Klein_02.qxd 5/1/07 5:04 PM Page 28
2.5 FORMAL CHARGES IN RESONANCE STRUCTURES 29 PROBLEMS For each drawing.try to draw the curved arrows that get you from one drawing to the next.In many cases you will need to draw more than one arrow O一⊙，义一X 16 :0 19 广人92.19 2.5 FORMAL CHARGES IN RESONANCE STRUCTURES Now we know how to draw good arrows (and how to avoid drawing bad arrows).In the last section,we were given the resonance struct ares and just had to draw in the arrows.Now we need to take this to the next level.We need to get practice drawing the resonance structures when they are not given.To ease into it,we will still show the arrows,and we will focus on drawing the resonance structures with proper for- mal charges.Consider the following example: a carbon atom.When both arrows are pushed at the same time,we are not violating either of the two commandments.So,let's focus on how to draw the resonance structure.Since we know what arrows mean,it is easy to follow the arrows.We just get rid of one lone pair on the oxygen.place a double bond between the carbon and o xygen,get rid of the carbon-carbon double bond,and place a lone pair on the carbo
PROBLEMS For each drawing, try to draw the curved arrows that get you from one drawing to the next. In many cases you will need to draw more than one arrow. 2.5 FORMAL CHARGES IN RESONANCE STRUCTURES 29 N N 2.14 2.15 2.16 2.17 O O O O N N 2.18 2.19 2.5 FORMAL CHARGES IN RESONANCE STRUCTURES Now we know how to draw good arrows (and how to avoid drawing bad arrows). In the last section, we were given the resonance structures and just had to draw in the arrows. Now we need to take this to the next level. We need to get practice drawing the resonance structures when they are not given. To ease into it, we will still show the arrows, and we will focus on drawing the resonance structures with proper formal charges. Consider the following example: In this example, we can see that one of the lone pairs on the oxygen is coming down to form a bond, and the C¨C double bond is being pushed to form a lone pair on a carbon atom. When both arrows are pushed at the same time, we are not violating either of the two commandments. So, let’s focus on how to draw the resonance structure. Since we know what arrows mean, it is easy to follow the arrows. We just get rid of one lone pair on the oxygen, place a double bond between the carbon and oxygen, get rid of the carbon–carbon double bond, and place a lone pair on the carbon: O O ? 6753_Klein_02.qxd 5/1/07 5:04 PM Page 29
30 CHAPTER 2 RESONANCE The arrows are really a language,and they tell us what to do.But here comes the tricky part:we cannot forget to put formal charges on the new drawing.If we apply the formal charges.we see that the oxygen getsap necessary to draw in the lone pairs: It is absolutely critical to draw these formal charges.Structures drawn without them are wrong.In fact,if you forget to draw the formal charges,then you are missing the whole point of resonance.Let's see why.Look at the resonance structure we just .This tells us that his ocdeniry We wld not bon atom only at the first drawing of the molecule: 可- This is why we need resonance-it shows us where there are regions of high and low electron density.If we draw resonance structures without formal charges,then what is the point in drawing the resonance structures at all? that we know how to draw them whe tle bit shaky when it comes to formal charges,go back and review formal charges in the previous chapter.But we can also see where to put formal charges without hav- ing to count each time.We saw the common situations for oxygen,nitrogen,and car bon.It is important to remember those (go back and review those if you need to). Anothe way to assign formal charges is to read the arrows properly.Let's look at our exampleagain: co- Notice what the arrows are telling us:oxygen is giving up a lone pair(two electrons entirely on the oxygen)to form a bond(two electrons being sha ne fo or the oxy gen and one for the carbon).So oxygen is losing an electron.This tells us that it must get a positive charge in the resonance structure.A similar analysis for the carbon on the bottom right shows that it will get a negative charge.Remember that the elec- trons are not really moving anywhere.Arrows are just tools that help us draw reso- nance structures.To use these tools properly,we imagine that the electrons are
The arrows are really a language, and they tell us what to do. But here comes the tricky part: we cannot forget to put formal charges on the new drawing. If we apply the rules of assigning formal charges, we see that the oxygen gets a positive charge and the carbon gets a negative charge. As long as we draw these charges, it is not necessary to draw in the lone pairs: It is absolutely critical to draw these formal charges. Structures drawn without them are wrong. In fact, if you forget to draw the formal charges, then you are missing the whole point of resonance. Let’s see why. Look at the resonance structure we just drew. Notice that there is a negative charge on a carbon atom. This tells us that this carbon atom is a site of high electron density. We would not know this by looking only at the first drawing of the molecule: This is why we need resonance—it shows us where there are regions of high and low electron density. If we draw resonance structures without formal charges, then what is the point in drawing the resonance structures at all? Now that we see that proper formal charges are essential, we should make sure that we know how to draw them when drawing resonance structures. If you are a little bit shaky when it comes to formal charges, go back and review formal charges in the previous chapter. But we can also see where to put formal charges without having to count each time. We saw the common situations for oxygen, nitrogen, and carbon. It is important to remember those (go back and review those if you need to). Another way to assign formal charges is to read the arrows properly. Let’s look at our example again: Notice what the arrows are telling us: oxygen is giving up a lone pair (two electrons entirely on the oxygen) to form a bond (two electrons being shared: one for the oxygen and one for the carbon). So oxygen is losing an electron. This tells us that it must get a positive charge in the resonance structure. A similar analysis for the carbon on the bottom right shows that it will get a negative charge. Remember that the electrons are not really moving anywhere. Arrows are just tools that help us draw resonance structures. To use these tools properly, we imagine that the electrons are moving, but they are not. Now let’s practice. O ? O O 30 CHAPTER 2 RESONANCE 6753_Klein_02.qxd 5/1/07 5:04 PM Page 30
2.5 FORMAL CHARGES IN RESONANCE STRUCTURES 31 EXERCISE 2.20 Draw the resonance structure that you get when you push the ar- rows shown below.Be sure to include formal charges. Answer We read the arrows to see what is happening.One of the lone pairs on the oxygen is coming down to form a bond,and the C=C double bond is being pushed to form a lone pair on a carbon atom.This is very similar to the example we just saw. bon and oxygen the carbon.Finally,we must put in any formal charges: There is one subtle point to mention.We said that you do not need to draw lone pairs-you only need to draw formal charges.There will be times when you will see arows being pushed nstructures that have the oe pairs drawn.When this happens,you might see an arrow coming from a negative charge 8 is the same as The drawing on the left is the common way this is drawn.Just don't forget that the electrons are really coming from a lone pair(as seen in the drawing on the right). charge on the resonance structure that you draw.This total charge should be the same as thethe structure you started with.So if the first structure has a negative charge, then the resonance structure you draw should also have a negative charge.If it doesn't,then you know you did something wrong(this is known as conservation of charge).You cannot change the total charge on a compound when drawing reso nance structures. PROBLEMS For each of the structures below,draw the resonance structure that (Hint: to take them into account even if they are not drawn-you need to train yourself to see lone pairs when they are not drawn.)
EXERCISE 2.20 Draw the resonance structure that you get when you push the arrows shown below. Be sure to include formal charges. Answer We read the arrows to see what is happening. One of the lone pairs on the oxygen is coming down to form a bond, and the C¨C double bond is being pushed to form a lone pair on a carbon atom. This is very similar to the example we just saw. We just get rid of one lone pair on the oxygen, place a double bond between the carbon and oxygen, get rid of the carbon–carbon double bond, and place a lone pair on the carbon. Finally, we must put in any formal charges: There is one subtle point to mention. We said that you do not need to draw lone pairs—you only need to draw formal charges. There will be times when you will see arrows being pushed on structures that do not have the lone pairs drawn. When this happens, you might see an arrow coming from a negative charge: The drawing on the left is the common way this is drawn. Just don’t forget that the electrons are really coming from a lone pair (as seen in the drawing on the right). One way to double check your drawing when you are done is to count the total charge on the resonance structure that you draw. This total charge should be the same as the the structure you started with. So if the first structure has a negative charge, then the resonance structure you draw should also have a negative charge. If it doesn’t, then you know you did something wrong (this is known as conservation of charge). You cannot change the total charge on a compound when drawing resonance structures. PROBLEMS For each of the structures below, draw the resonance structure that you get when you push the arrows shown. Be sure to include formal charges. (Hint: In some cases the lone pairs are drawn and in other cases they are not drawn. Be sure to take them into account even if they are not drawn—you need to train yourself to see lone pairs when they are not drawn.) O O is the same as O O ? 2.5 FORMAL CHARGES IN RESONANCE STRUCTURES 31 6753_Klein_02.qxd 5/1/07 5:04 PM Page 31