12 CHAPTER 1 BOND-LINE DRAWINGS electrons.So we see that the oxygen actually has seven electrons,which is one more electron than it is supposed to have.Therefore,it will have a negative charge: EXERCISE 1.33 Consider the nitrogen atom in the compound below and deter- mine if it has a formal charge: H H-N-H H Answer Nitrogen is in the fifth coumn of the periodic table so it should have five electrons.Now we count how many it actually has: HW网H It only has four.So,it has one less electron than it is supposed to have.Therefore. this nitrogen atom has a positive charge: H-NPH H PROBLEMS For each of the compounds below determine if the oxygen or nitro gen atom in the molecule has a formal charge.If there is a charge,draw the charge on the structure 88芯m 8n8
electrons. So we see that the oxygen actually has seven electrons, which is one more electron than it is supposed to have. Therefore, it will have a negative charge: EXERCISE 1.33 Consider the nitrogen atom in the compound below and determine if it has a formal charge: Answer Nitrogen is in the fifth column of the periodic table so it should have five electrons. Now we count how many it actually has: It only has four. So, it has one less electron than it is supposed to have. Therefore, this nitrogen atom has a positive charge: PROBLEMS For each of the compounds below determine if the oxygen or nitrogen atom in the molecule has a formal charge. If there is a charge, draw the charge on the structure. H N H H H H N H H H H N H H H O 12 CHAPTER 1 BOND-LINE DRAWINGS O O N N 1.34 1.35 1.36 1.37 O N O N 1.38 1.39 1.40 1.41 6753_Klein_01.qxd 5/1/07 5:03 PM Page 12
1.5 IDENTIFYING FORMAL CHARGES 13 114 This brings us to the most important atom of all:carbon.We saw before tha carbon always has four bonds.This allows us to ignore the hydrogen atoms when drawing bond-line structures,because it is assumed that we know how to count to four and can figure out how many hydrogen atoms are there.When we said that,we were only talking about carbon atoms without formal charges(most carbon atoms in most structures will not hav e formal charges).But now that we have le arned what a formal charge is et's consider what happens when carbon has a formal charge. If carbon bears a formal charge,then we cannot just assume the carbon has four bonds.In fact,it will have only three.Let's see why.Let's first consider C+,and then we will move on to C= If carbon has a positive formal charge,then it has only three electrons (it is ar electro be urth col So,a carbon with a positive formal charge will have only three bonds,and you should count hydrogen atoms with this in mind: No hydrogen atoms on this C I hydrogen atom on this C* Now let's consider what happens when we have a carbon with a negative for- mal charge The reason it has a negative formal charge is because it has one more than it iss upp osed to have.Thereforeit has five l electrons form a lone pair,and the other three electrons are used to form bonds H H-C: H space called orbitals.These orbitals can overlap with orbitals from other atoms to form bonds,or the orbitals can contain two electrons(which is called a lone pair). Carbon has only four orbitals,so there is no way it could possibly form five bonds- it does not have five orbitals to use to form those bonds.This is why a carbon atom with a negative ch arge will have lone pair(if you look at the drawing above.you will count four orbitals-one for the lone pair and then three more for the bonds)
This brings us to the most important atom of all: carbon. We saw before that carbon always has four bonds. This allows us to ignore the hydrogen atoms when drawing bond-line structures, because it is assumed that we know how to count to four and can figure out how many hydrogen atoms are there. When we said that, we were only talking about carbon atoms without formal charges (most carbon atoms in most structures will not have formal charges). But now that we have learned what a formal charge is, let’s consider what happens when carbon has a formal charge. If carbon bears a formal charge, then we cannot just assume the carbon has four bonds. In fact, it will have only three. Let’s see why. Let’s first consider C, and then we will move on to C. If carbon has a positive formal charge, then it has only three electrons (it is supposed to have four electrons, because carbon is in the fourth column of the periodic table). Since it has only three electrons, it can form only three bonds. That’s it. So, a carbon with a positive formal charge will have only three bonds, and you should count hydrogen atoms with this in mind: No hydrogen atoms on this C 1 hydrogen atom on this C 2 hydrogen atoms on this C Now let’s consider what happens when we have a carbon with a negative formal charge. The reason it has a negative formal charge is because it has one more electron than it is supposed to have. Therefore, it has five electrons. Two of these electrons form a lone pair, and the other three electrons are used to form bonds: We have the lone pair, because we can’t use each of the five electrons to form a bond. Carbon can never have five bonds. Why not? Electrons exist in regions of space called orbitals. These orbitals can overlap with orbitals from other atoms to form bonds, or the orbitals can contain two electrons (which is called a lone pair). Carbon has only four orbitals, so there is no way it could possibly form five bonds— it does not have five orbitals to use to form those bonds. This is why a carbon atom with a negative charge will have a lone pair (if you look at the drawing above, you will count four orbitals—one for the lone pair and then three more for the bonds). H C H H 1.5 IDENTIFYING FORMAL CHARGES 13 N N O N 1.42 1.43 1.44 1.45 6753_Klein_01.qxd 5/1/07 5:03 PM Page 13
14 CHAPTER 1 BOND-LINE DRAWINGS Therefore,a carbon atom with a negative charge can also form only three bonds(just like a carbon with a positive charge).When you count hydrogen atoms, you should keep this in mind: 1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN From all of the cases above (oxygen,nitrogen.carbon).you can see why you have to know how many lone pairs there are to figure out the formal charge on an atom Similarly,you have to know the formal charge to figure out how many lone pairs there are on an atom.Take the case below with the nitrogen atom shown: could either be 。发 If the lone pairs were drawn.then we would be able to figure out the charge (two lone pairs would mean a negative charge and one lone pair would mean a positive charge).Similarly,if the formal charge was drawn,we would be able to figure out how many lone pairs there are(a negative charge would mean two lone pairs and a positive cha would one pair). So you can see that drawings must include either lone pairs or formal charges The convention is to always show formal charges and to leave out the lone pairs.This is much easier to draw,because you usually won't have more than one charge on a drawing (if even that).so you get to save time by not drawing every lone pair on every atom Now that we have established that formal charges must t always be drawn and that lone pairs are usually not drawn,we need to get practice in how to see the lone pairs when they are not drawn.This is not much different from training yourself to see all the hydrogen atoms in a bond-line drawing even though they are not drawn. If you know how to count,then you should be able to figure out how many lone pairs Let's ◆0 In this case,we are looking at an oxygen atom.Oxygen is in the sixth column of the periodic table,so it is supposed to have six electrons.Then,we need to take the for- extra elec can figure out how many lone pairs there are
Therefore, a carbon atom with a negative charge can also form only three bonds (just like a carbon with a positive charge). When you count hydrogen atoms, you should keep this in mind: No hydrogen atoms on this C 1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN From all of the cases above (oxygen, nitrogen, carbon), you can see why you have to know how many lone pairs there are to figure out the formal charge on an atom. Similarly, you have to know the formal charge to figure out how many lone pairs there are on an atom. Take the case below with the nitrogen atom shown: If the lone pairs were drawn, then we would be able to figure out the charge (two lone pairs would mean a negative charge and one lone pair would mean a positive charge). Similarly, if the formal charge was drawn, we would be able to figure out how many lone pairs there are (a negative charge would mean two lone pairs and a positive charge would mean one lone pair). So you can see that drawings must include either lone pairs or formal charges. The convention is to always show formal charges and to leave out the lone pairs. This is much easier to draw, because you usually won’t have more than one charge on a drawing (if even that), so you get to save time by not drawing every lone pair on every atom. Now that we have established that formal charges must always be drawn and that lone pairs are usually not drawn, we need to get practice in how to see the lone pairs when they are not drawn. This is not much different from training yourself to see all the hydrogen atoms in a bond-line drawing even though they are not drawn. If you know how to count, then you should be able to figure out how many lone pairs are on an atom where the lone pairs are not drawn. Let’s see an example to demonstrate how you do this: In this case, we are looking at an oxygen atom. Oxygen is in the sixth column of the periodic table, so it is supposed to have six electrons. Then, we need to take the formal charge into account. This oxygen atom has a negative charge, which means one extra electron. Therefore, this oxygen atom must have 6 1 7 electrons. Now we can figure out how many lone pairs there are. O N N N could either be or 14 CHAPTER 1 BOND-LINE DRAWINGS 6753_Klein_01.qxd 5/1/07 5:03 PM Page 14
1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 15 入o°is the same as入. Let's review the process: 1.Count the number of electrons the atom should have according to the periodic table. 2.Take the formal charge into account.A negative charge means one more electron,and a positive charge means one less electron. 3.Now you know the number of electrons the atom actually has.Use this number to figure out how many lone pairs there are. that you k ho t nd commouismportant ly m more important to get to a point where you don't have to waste time counting.You need to get familiar with the common situations you will encounter.Let's go through them methodically. When oxygen has no formal charge.it will have two bonds and two lone pairs 入OH 】5he5 ame as入过H O is the same as 、处 is the same as If oxygen has a negative formal charge,then it must have one bond and three lone pairs: o 人 is the same as 入0° is the same as 入09
The oxygen has one bond, which means that it is using one of its seven electrons to form a bond. The other six must be in lone pairs. Since each lone pair is two electrons, this must mean that there are three lone pairs: Let’s review the process: 1. Count the number of electrons the atom should have according to the periodic table. 2. Take the formal charge into account. A negative charge means one more electron, and a positive charge means one less electron. 3. Now you know the number of electrons the atom actually has. Use this number to figure out how many lone pairs there are. Now we need to get used to the common examples. Although it is important that you know how to count and determine numbers of lone pairs, it is actually much more important to get to a point where you don’t have to waste time counting. You need to get familiar with the common situations you will encounter. Let’s go through them methodically. When oxygen has no formal charge, it will have two bonds and two lone pairs: If oxygen has a negative formal charge, then it must have one bond and three lone pairs: O O is the same as is the same as O O O H O O is the same as is the same as is the same as OH O O O is the same as O 1.6 FINDING LONE PAIRS THAT ARE NOT DRAWN 15 6753_Klein_01.qxd 5/1/07 5:03 PM Page 15
16 CHAPTER 1 BOND-LINE DRAWINGS If oxygen has a positive charge,then it must have three bonds and one lone pair: 人8H2 H H is the same as 心 is the same as EXERCISE 1.46 Draw in all lone pairs in the following structure: Answer The oxygen has a positive formal charge and three bonds.You should try to get to a point where you recognize that this must mean that the oxygen 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. charge, have 6-1=5 electrons.Now,we can figure out how many lone pairs there are. The oxygen has three bonds,which means that it is using three of its five elec- trons to form bonds.The other two must be in a lone pair.So there is only one lone Pair. PROBLEMS Review the common situations above,and then come back to these nize how were right
If oxygen has a positive charge, then it must have three bonds and one lone pair: EXERCISE 1.46 Draw in all lone pairs in the following structure: Answer The oxygen has a positive formal charge and three bonds. You should try to get to a point where you recognize that this must mean that the oxygen 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. Oxygen is supposed to have six electrons. This oxygen atom has a positive charge, which means it is missing an electron. Therefore, this oxygen atom must have 6 1 5 electrons. Now, we can figure out how many lone pairs there are. The oxygen has three bonds, which means that it is using three of its five electrons to form bonds. The other two must be in a lone pair. So there is only one lone pair. PROBLEMS Review the common situations above, 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. O H O H O H O O is the same as is the same as is the same as OH2 O O H H H H H 16 CHAPTER 1 BOND-LINE DRAWINGS O O O O 1.47 1.48 1.49 6753_Klein_01.qxd 5/1/07 5:03 PM Page 16