22 CHAPTER 2 RESONANCE us draw all resonance structures of a molecule.The electrons are not actually mov- ing.It can be tricky because we will say things like:"this arrow shows the electrons coming from here and going to there."But we don't actually mean that the electrons are moving:they are no oving.Since each drawing treats the electrons as particle stuck in one place,we will need to"move"the electrons to get from one drawing to another.Arrows are the tools that we use to make sure that we know how to draw all resonance structures for a compound.So,let's look at the features of these important curved arrows. Every curved arrow has a head and a tail.It is essential that the head and tail of every amow be drawn in precisely the proper place. trons are coming from,and the head shows where the electrons are going (remem- ber that the electrons aren't really going anywhere,but we treat them as if they were so we can make sure to draw all resonance structures): Tail Head Therefore,there are only two things that you have to get right when drawing an arrow:the tail needs to be in the right place and the head needs to be in the right place.So we need tosee t whe you can and where you cannot draw a rows.But first we need to talk a little bit about electrons,since the arrows are de. scribing the electrons. Electrons exist in orbitals,which can hold a maximum of two electrons.So there are only three options for any orbital: .0electrons in the orbital I electron in the orbital 2 electrons in the orbital If there are no electrons in the orbital,then there's nothing to talk about (there are no electrons there).If you have one electron in the orbital,it can overlap with an- rbital (forming a bond).If two soccupy the or bital.they fl the orital).So we see that electrons c e foun in only two places:in bonds or in lone pairs.Therefore.electrons can only come from either a bond or a lone pair.Similarly,electrons can only go to form either a bond or a lone pair. Let's focus on tails of arrows first.Remember that the tail of an arrow indi- ons are from.So the tail has to a place that nance structures as an example: H H H 0=C H H、 C-H HH
22 CHAPTER 2 RESONANCE us draw all resonance structures of a molecule. The electrons are not actually moving. It can be tricky because we will say things like: “this arrow shows the electrons coming from here and going to there.” But we don’t actually mean that the electrons are moving; they are not moving. Since each drawing treats the electrons as particles stuck in one place, we will need to “move” the electrons to get from one drawing to another. Arrows are the tools that we use to make sure that we know how to draw all resonance structures for a compound. So, let’s look at the features of these important curved arrows. Every curved arrow has a head and a tail. It is essential that the head and tail of every arrow be drawn in precisely the proper place. The tail shows where the electrons are coming from, and the head shows where the electrons are going (remember that the electrons aren’t really going anywhere, but we treat them as if they were so we can make sure to draw all resonance structures): Therefore, there are only two things that you have to get right when drawing an arrow: the tail needs to be in the right place and the head needs to be in the right place. So we need to see rules about where you can and where you cannot draw arrows. But first we need to talk a little bit about electrons, since the arrows are describing the electrons. Electrons exist in orbitals, which can hold a maximum of two electrons. So there are only three options for any orbital: • 0 electrons in the orbital • 1 electron in the orbital • 2 electrons in the orbital If there are no electrons in the orbital, then there’s nothing to talk about (there are no electrons there). If you have one electron in the orbital, it can overlap with another electron in a nearby orbital (forming a bond). If two electrons occupy the orbital, they fill the orbital (called a lone pair). So we see that electrons can be found in only two places: in bonds or in lone pairs. Therefore, electrons can only come from either a bond or a lone pair. Similarly, electrons can only go to form either a bond or a lone pair. Let’s focus on tails of arrows first. Remember that the tail of an arrow indicates where the electrons are coming from. So the tail has to come from a place that has electrons: either from a bond or from a lone pair. Consider the following resonance structures as an example: C C C H H H H H C C C H H H H H Tail Head 6753_Klein_02.qxd 5/1/07 5:04 PM Page 22
2.2 CURVED ARROWS:THE TOOLS FOR DRAWING RESONANCE STRUCTURES 23 How do we get from the first structure to the second one?Notice that the electrons that make up the double bond have been oved.This is an example of electron coming froma bond.Let's see the arrow showing the eletrons comin from the bond and going to form another bond: H H c Cc-H H Now let's see what it looks like when electrons come from a lone pair: ④ H C-H H Never draw an arrow that comes from a positive charge.The tail of an arrow must come from a spot that has electrons. ple as tails.The head of an arrow shows where the head directly in between two atoms to form a bond H H H H© HH or it must point to an atom to form a lone pair. 变一 Never draw the head of an arrow going off into space:
How do we get from the first structure to the second one? Notice that the electrons that make up the double bond have been “moved.” This is an example of electrons coming from a bond. Let’s see the arrow showing the electrons coming from the bond and going to form another bond: Now let’s see what it looks like when electrons come from a lone pair: Never draw an arrow that comes from a positive charge. The tail of an arrow must come from a spot that has electrons. Heads of arrows are just as simple as tails. The head of an arrow shows where the electrons are going. So the head of an arrow must either point directly in between two atoms to form a bond, or it must point to an atom to form a lone pair. Never draw the head of an arrow going off into space: Bad arrow C O H H O O C C C H H H H H C C C H H H H H C O C H H H H H C O C H H H H H C C C H H H H H C C C H H H H H 2.2 CURVED ARROWS: THE TOOLS FOR DRAWING RESONANCE STRUCTURES 23 6753_Klein_02.qxd 5/1/07 5:04 PM Page 23
24 CHAPTER 2 RESONANCE Remember that the head of an arrow shows where the electrons are going.So the head of an arrow must point to a place where the electrons can go-either to form a bond or to form a lone pair. 2.3 THE TWO COMMANDMENTS Now we know what curved arrows are,but how do we know when to push them and where to push them?First,we need to learn where we cannot push arrows.There are two important rules that you should never violate when pushing arrows.They are the "two commandments"of drawing resonance structures: 1.Thou shall not break a single bond. 2.Thou shall not exceed an octet for second-row elements. Let's focus on one at a time. 1.Never break a single bond when drawing resonance structures.By defini- tion,resonance structures must have all the same atoms connected in the same order. 只○入X人+8 Never break a single bond There are very few exceptions to this rule,and only a trained organic chemist can b I to kn ow when it is permissible to violate this rule.Some instructors might violate this rule one or two times(about half-way through the course).If this hap pens,you should recognize that you are seeing a very rare exception.In virtually every situation that you will encounter,you cannot violate this rule.Therefore.you must get into the habit of never breaking a single bond. There is a simple way toe sure that you eve violate this rule.Just make sure that you never draw the tail of an arrow on a single bond. 2.Never exceed an octet for second-row elements.Elements in the second row (C.N.O.F)hav e only four c rbitals in thei valence shell.Eac of these four orbital can be used either to form a bond or to hold a lone pair.Each bond requires the use of one orbital,and each lone pair requires the use of one orbital.So the second-row elements can never have five or six bonds:the most is four.Similarly,they can never have four bonds and a lone pair,because this would also require five orbitals.For n th an never have three bonds and two s.The sum of oonds)+(lone pairs) or a second- can never exce ed the numb er fou Let's see some examples of arrow pushing that violate this second commandment: C-H 6N-H 96 Bad arrow Bad arrow Bad arrow
24 CHAPTER 2 RESONANCE Remember that the head of an arrow shows where the electrons are going. So the head of an arrow must point to a place where the electrons can go—either to form a bond or to form a lone pair. 2.3 THE TWO COMMANDMENTS Now we know what curved arrows are, but how do we know when to push them and where to push them? First, we need to learn where we cannot push arrows. There are two important rules that you should never violate when pushing arrows. They are the “two commandments” of drawing resonance structures: 1. Thou shall not break a single bond. 2. Thou shall not exceed an octet for second-row elements. Let’s focus on one at a time. 1. Never break a single bond when drawing resonance structures. By definition, resonance structures must have all the same atoms connected in the same order. Never break a single bond There are very few exceptions to this rule, and only a trained organic chemist can be expected to know when it is permissible to violate this rule. Some instructors might violate this rule one or two times (about half-way through the course). If this happens, you should recognize that you are seeing a very rare exception. In virtually every situation that you will encounter, you cannot violate this rule. Therefore, you must get into the habit of never breaking a single bond. There is a simple way to ensure that you never violate this rule. Just make sure that you never draw the tail of an arrow on a single bond. 2. Never exceed an octet for second-row elements. Elements in the second row (C, N, O, F) have only four orbitals in their valence shell. Each of these four orbitals can be used either to form a bond or to hold a lone pair. Each bond requires the use of one orbital, and each lone pair requires the use of one orbital. So the second-row elements can never have five or six bonds; the most is four. Similarly, they can never have four bonds and a lone pair, because this would also require five orbitals. For the same reason, they can never have three bonds and two lone pairs. The sum of (bonds) (lone pairs) for a second-row element can never exceed the number four. Let’s see some examples of arrow pushing that violate this second commandment: Bad arrow Bad arrow Bad arrow O CH H H O NH H O O H X 6753_Klein_02.qxd 5/1/07 5:04 PM Page 24
2.3 THE TWO COMMANDMENTS 25 In each of these drawings,the central atom cannot form another bond because it does rawings.it can be more dif ficult to see the violation because we cannot see the hydrogen atoms (and,very often,we cannot see the lone pairs either;for now,we will continue to draw lone pairs to ease you into it).You have to train yourself to see the hydrogen atoms and to recognize when you are exceeding an octet: 人6：isthe same as HHH At first it is difficult to see that the arrow on the left structure violates the second commandment.But when we count the hydrogen atoms.we can see that the arrow above would give a carbon atom with five bonds. From no on,we will re er to the second commandment as"the octet rule. But be careful-for purposes of drawing resonance structures,it is only a violation if we exceed an octet for a second-row element.However,there is no problem at all with a second-row element having fewer than an octet of electrons.For example: ⊙ This carbon atom does not have an octet. Our two commandments (never break a single bond,and never violate "the octet rule")reflect the two parts of a curved arrow (the head and the tail).A bad tail violates the first commandment,and a bad head violates the second commandment. EXERCISE 2.1 For the compound below,look at the arrow drawn on the struc- ture and determine whether it violates either of the two commandments for drawing onance structures: 。 Answer First we need to ask if the first commandment has been violated:did we break a single bond?To determine this,we look at the tail of the arrow.If the tail of the arrow is coming from a single bond,then that means we are breaking that single bond.If the tail is coming from a double bond,then we have not violated the first
In each of these drawings, the central atom cannot form another bond because it does not have a fifth orbital that can be used. This is impossible. Don’t ever do this. The examples above are clear, but with bond-line drawings, it can be more dif- ficult to see the violation because we cannot see the hydrogen atoms (and, very often, we cannot see the lone pairs either; for now, we will continue to draw lone pairs to ease you into it). You have to train yourself to see the hydrogen atoms and to recognize when you are exceeding an octet: At first it is difficult to see that the arrow on the left structure violates the second commandment. But when we count the hydrogen atoms, we can see that the arrow above would give a carbon atom with five bonds. From now on, we will refer to the second commandment as “the octet rule.” But be careful—for purposes of drawing resonance structures, it is only a violation if we exceed an octet for a second-row element. However, there is no problem at all with a second-row element having fewer than an octet of electrons. For example: This drawing is perfectly acceptable, even though the central carbon atom has only six electrons surrounding it. For our purposes, we will only consider the “octet rule” to be violated if we exceed an octet. Our two commandments (never break a single bond, and never violate “the octet rule”) reflect the two parts of a curved arrow (the head and the tail). A bad tail violates the first commandment, and a bad head violates the second commandment. EXERCISE 2.1 For the compound below, look at the arrow drawn on the structure and determine whether it violates either of the two commandments for drawing resonance structures: Answer First we need to ask if the first commandment has been violated: did we break a single bond? To determine this, we look at the tail of the arrow. If the tail of the arrow is coming from a single bond, then that means we are breaking that single bond. If the tail is coming from a double bond, then we have not violated the first O O This carbon atom does not have an octet. O C C O H H H H H is the same as 2.3 THE TWO COMMANDMENTS 25 6753_Klein_02.qxd 5/1/07 5:04 PM Page 25
26 CHAPTER 2 RESONANCE commandment.In this example,the tail is on a double bond,so we did not violate the first commandment. Now we need to ask if the second commandment has been violated:did we vi- olate the octet rule?To determine this,we look at the head of the arrow.Are we forming a fifth bond?Remember that C+only has three bonds,not four.When we count the hydrogen atoms attached to this carbon,we see that there is only one hy- drogen atom,not two,to give that carbon a total of three bonds.When we move the arrow shown above,the carbon will now get four bonds,and the second command- ment has not been violated. The arrow above is valid,because the two commandments were not violated PROBLEMS For each of the problems below,determine which arrows violate either one of the two commandments,and explain why.(Don't forget to count all hydrogen atoms and all lone pairs.You must do this to solve these problems.) 2.2 H 2.3 2.4 2.5 2.6 2.8 2.9 2.10
26 CHAPTER 2 RESONANCE commandment. In this example, the tail is on a double bond, so we did not violate the first commandment. Now we need to ask if the second commandment has been violated: did we violate the octet rule? To determine this, we look at the head of the arrow. Are we forming a fifth bond? Remember that C only has three bonds, not four. When we count the hydrogen atoms attached to this carbon, we see that there is only one hydrogen atom, not two, to give that carbon a total of three bonds. When we move the arrow shown above, the carbon will now get four bonds, and the second commandment has not been violated. The arrow above is valid, because the two commandments were not violated. PROBLEMS For each of the problems below, determine which arrows violate either one of the two commandments, and explain why. (Don’t forget to count all hydrogen atoms and all lone pairs. You must do this to solve these problems.) 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 C N O O H O H O O N H H 6753_Klein_02.qxd 5/1/07 5:04 PM Page 26