1 00:00:00 --> 00:00:01 2 00:00:01 --> 00:00:02 The following content is provided under a Creative 3 00:00:02 --> 00:00:03 Commons license. 4 00:00:03 --> 00:00:06 Your support will help MIT OpenCourseWare continue to 5 00:00:06 --> 00:00:10 offer high-quality educational resources for free. 6 00:00:10 --> 00:00:13 To make a donation or view additional materials from 7 00:00:13 --> 00:00:15 hundreds of MIT courses, visit MIT OpenCourseWare 8 00:00:15 --> 00:00:17 at ocw.mit.edu. 9 00:00:17 --> 00:00:22 PROFESSOR: -- 10 seconds to answer the first 10 00:00:22 --> 00:00:23 clicker question. 11 00:00:23 --> 00:00:26 As you can see we're having another competition today, 12 00:00:26 --> 00:00:30 so see if you can beat out Justin's recitation and get 13 00:00:30 --> 00:00:33 the most correct today. 14 00:00:33 --> 00:00:38 So, this is a topic from Friday, which is asking us 15 00:00:38 --> 00:00:40 which of the following molecules are free 16 00:00:40 --> 00:00:42 radical species. 17 00:00:42 --> 00:00:42 OK. 18 00:00:42 --> 00:00:45 So it looks like most and you got it, that it's o h. 19 00:00:45 --> 00:00:49 Let's quickly go over why that is just in case you 20 00:00:49 --> 00:00:52 weren't one of that 74%. 21 00:00:52 --> 00:00:55 So, if we're looking at o h versus c o, what are we 22 00:00:55 --> 00:00:58 looking at to decide if we have a radical here? 23 00:00:58 --> 00:01:00 Valence electrons. 24 00:01:00 --> 00:01:06 So we have 6 plus 1 in terms of o h, that's 7, versus having 25 00:01:06 --> 00:01:10 4 plus 6, which is 10. 26 00:01:10 --> 00:01:12 Specifically, what do we look for in terms of 27 00:01:12 --> 00:01:12 valence electrons? 28 00:01:12 --> 00:01:14 Odd numbers. 29 00:01:14 --> 00:01:20 So, we have a radical here with o h, whereas we have an even 30 00:01:20 --> 00:01:24 number of valence electrons for c o, so that's not a radical, 31 00:01:24 --> 00:01:27 in terms of what we know how to think about so far, which is 32 00:01:27 --> 00:01:29 using our Lewis structures to determine that. 33 00:01:29 --> 00:01:30 All right. 34 00:01:30 --> 00:01:33 So that's a little review from Friday, let's move 35 00:01:33 --> 00:01:35 on to today's lecture. 36 00:01:35 --> 00:01:38 In terms of what we're going to be talking about today, we're 37 00:01:38 --> 00:01:42 going to finish up with talking about polar covalent bonds, and 38 00:01:42 --> 00:01:45 finish up with discussing the idea of polar molecules. 39 00:01:45 --> 00:01:48 And then we'll move on to start talking about well, we know how 40 00:01:48 --> 00:01:51 to draw our Lewis structures, how can we use those Lewis 41 00:01:51 --> 00:01:56 structures to think about the actual shapes of molecules. 42 00:01:56 --> 00:02:00 So, first thinking about our polar covalent bonds. 43 00:02:00 --> 00:02:03 What we had established is if we have a case where we have a 44 00:02:03 --> 00:02:07 covalent bond, but there's unequal sharing of electrons 45 00:02:07 --> 00:02:10 between the two atoms, namely because they have different 46 00:02:10 --> 00:02:14 electronegativities, what we see is we have a polar bond or 47 00:02:14 --> 00:02:18 a polar covalent bond where one of the atoms is going to pull 48 00:02:18 --> 00:02:21 away more of the electron density from that bond, giving 49 00:02:21 --> 00:02:24 it that partial negative charge where the other atom is going 50 00:02:24 --> 00:02:27 to end up with a partial positive charge. 51 00:02:27 --> 00:02:30 In terms of thinking about well, what do we call a 52 00:02:30 --> 00:02:33 covalent bond versus a polar covalent bond versus a purely 53 00:02:33 --> 00:02:37 ionic bond, it turns out that the distinction is a little bit 54 00:02:37 --> 00:02:41 fuzzy, it's not a clear line, but in terms of talking about 55 00:02:41 --> 00:02:43 things in general, and especially in thinking about 56 00:02:43 --> 00:02:46 problems for this course, what we're going to say is if you 57 00:02:46 --> 00:02:50 have an electronegativity or chi difference between the two 58 00:02:50 --> 00:02:53 atoms that is greater than 0 . 59 00:02:53 --> 00:02:55 4, and less than 1 . 60 00:02:55 --> 00:02:59 7, and we're talking about the Pauling electronegativity scale 61 00:02:59 --> 00:03:01 here, which is what's used throughout your book. 62 00:03:01 --> 00:03:04 If you have a difference in electronegativity anywhere in 63 00:03:04 --> 00:03:07 that range, we'll have what we call a polar covalent bond 64 00:03:07 --> 00:03:08 between those two atoms. 65 00:03:08 --> 00:03:12 So, for example, if we compare the chi values for hydrogen 66 00:03:12 --> 00:03:16 versus carbon, we see they only have a difference of 0.4 . 67 00:03:16 --> 00:03:19 0.4 is not greater than 0.4 . 68 00:03:19 --> 00:03:22 So we would not call this a c h bond, we would not say it's 69 00:03:22 --> 00:03:26 polar covalent, we would just call it covalent. 70 00:03:26 --> 00:03:30 In contrast, if we talk about a carbon oxygen bond, now we have 71 00:03:30 --> 00:03:34 a chi difference of 0.8 , so that is greater than 0.4 , so 72 00:03:34 --> 00:03:39 we'll call carbon oxygen bond polar covalent. 73 00:03:39 --> 00:03:42 We can extend this idea of talking about these polar 74 00:03:42 --> 00:03:46 covalent bonds to thinking about an entire molecule. 75 00:03:46 --> 00:03:49 I'm sure you've all heard of the term a polar molecule 76 00:03:49 --> 00:03:52 versus a non-polar molecule, and essentially, when we talk 77 00:03:52 --> 00:03:55 about having a polar molecule, what we're saying is that 78 00:03:55 --> 00:03:58 there's is a net non-zero dipole moment within 79 00:03:58 --> 00:03:59 the whole molecule. 80 00:03:59 --> 00:04:02 So essentially, what we have to do is combine all of the 81 00:04:02 --> 00:04:05 individual bonds to think about the molecule as a whole. 82 00:04:05 --> 00:04:10 So, let's take an example of carbon dioxide here, or c o 2. 83 00:04:10 --> 00:04:13 Very soon in the lecture you'll learn how to predict shapes 84 00:04:13 --> 00:04:15 based on Lewis structures. 85 00:04:15 --> 00:04:19 So you will quickly see that this is a linear molecule. 86 00:04:19 --> 00:04:21 We can think about the 2 bonds within it. 87 00:04:21 --> 00:04:25 There's two carbon oxygen bonds, and we know that there 88 00:04:25 --> 00:04:29 is a dipole in that bond, which is going toward the oxygen. 89 00:04:29 --> 00:04:31 Remember, in chemistry we always draw the arrow, we're 90 00:04:31 --> 00:04:34 always interested in what those electrons are doing. 91 00:04:34 --> 00:04:36 So if we draw in the bond dipoles, and you can draw these 92 00:04:36 --> 00:04:40 into your notes, you want to draw an arrow towards the 93 00:04:40 --> 00:04:42 oxygen in each case. 94 00:04:42 --> 00:04:45 But we're not just interested here in thinking about do we 95 00:04:45 --> 00:04:48 have a polar bond, we actually want to know in general do 96 00:04:48 --> 00:04:49 we have a polar molecule. 97 00:04:49 --> 00:04:53 So, looking at c o 2 as a whole, do you think we have 98 00:04:53 --> 00:04:56 a polar or a non-polar molecule here? 99 00:04:56 --> 00:04:56 Non-polar. 100 00:04:56 --> 00:04:59 OK, very good for all of you that said non-polar. 101 00:04:59 --> 00:05:02 The reason it's non-polar is we simply have two equal vectors 102 00:05:02 --> 00:05:05 in opposite directions, so they cancel each other out. 103 00:05:05 --> 00:05:10 The net effect is that we have a 0 dipole moment 104 00:05:10 --> 00:05:13 in the c o molecule. 105 00:05:13 --> 00:05:16 In contrast, we can look at h 2 o. 106 00:05:16 --> 00:05:19 H 2 o actually has this bent shape, and again, we'll see 107 00:05:19 --> 00:05:22 very soon and how to predict that h 2 o has a bent 108 00:05:22 --> 00:05:24 shape, that water is bent. 109 00:05:24 --> 00:05:28 And again, we do have dipole moments -- we have bonds where 110 00:05:28 --> 00:05:31 there is a polar bond and we should point our arrow toward 111 00:05:31 --> 00:05:34 the oxygen, because electron density is being pulled from 112 00:05:34 --> 00:05:37 the hydrogen atoms to the oxygen. 113 00:05:37 --> 00:05:40 And again, we can cancel some of these vectors out, but we 114 00:05:40 --> 00:05:45 still have a net dipole moment that is going to be going up if 115 00:05:45 --> 00:05:48 we think about combining these 2 vectors here. 116 00:05:48 --> 00:05:52 So, what we would say, which is what we know from everything 117 00:05:52 --> 00:05:57 we've always heard, which is that water is a polar molecule. 118 00:05:57 --> 00:05:58 So, this is a great way to think about 119 00:05:58 --> 00:06:00 molecules in general. 120 00:06:00 --> 00:06:05 It's very easy to think about if those vectors cancel each 121 00:06:05 --> 00:06:07 other out or if they're additive when we're talking 122 00:06:07 --> 00:06:13 about just single bonds, so we have two atoms or if we have 123 00:06:13 --> 00:06:17 just three atoms, as we do in these cases here. 124 00:06:17 --> 00:06:19 But it turns out, a lot of the molecules we consider are 125 00:06:19 --> 00:06:22 actually much, much, much larger than just a 126 00:06:22 --> 00:06:23 couple of atoms. 127 00:06:23 --> 00:06:27 So we can't often think about canceling all the 128 00:06:27 --> 00:06:28 different vectors out. 129 00:06:28 --> 00:06:30 And in general, when we're talking about these really 130 00:06:30 --> 00:06:33 large molecules, whether they're large organic molecules 131 00:06:33 --> 00:06:35 or we're talking about proteins, which can have 132 00:06:35 --> 00:06:39 thousands of atoms in them, instead of adding up all those 133 00:06:39 --> 00:06:43 individual polar bonds, what we do is we talk about the number 134 00:06:43 --> 00:06:45 of polar groups that it will have in the molecule. 135 00:06:45 --> 00:06:48 So we can think about a protein, for example, as having 136 00:06:48 --> 00:06:51 a lot of different polar groups or one that has not too 137 00:06:51 --> 00:06:53 many polar side groups. 138 00:06:53 --> 00:06:56 That's one way you hear people talking about proteins, and 139 00:06:56 --> 00:06:59 that has a lot to do with their solubility in water, and also 140 00:06:59 --> 00:07:02 about how the protein's going to fold. 141 00:07:02 --> 00:07:04 But let's look at a little bit about less complicated example 142 00:07:04 --> 00:07:07 than a protein with thousands of different atoms. 143 00:07:07 --> 00:07:10 Let's instead consider a couple of vitamins here, which 144 00:07:10 --> 00:07:13 instead have a few dozen different atoms in them. 145 00:07:13 --> 00:07:17 Specifically, let's look at vitamin A and vitamin B9. 146 00:07:17 --> 00:07:26 Does anyone know another name for vitamin B9? 147 00:07:26 --> 00:07:30 I don't think I hear it -- does anyone say folic acid. 148 00:07:30 --> 00:07:33 It's also sometimes called vitamin M. 149 00:07:33 --> 00:07:36 It's one of the vitamin B9 vitamins, there's actually 150 00:07:36 --> 00:07:39 other forms of vitamin B9, but this is one of the big 151 00:07:39 --> 00:07:41 ones that you hear about. 152 00:07:41 --> 00:07:43 Hopefully you're all very familiar with folic acid, 153 00:07:43 --> 00:07:46 because it's an important vitamin to take, especially if 154 00:07:46 --> 00:07:48 you're a woman, especially if there's any chance you ever 155 00:07:48 --> 00:07:51 might become pregnant in any kind of near future time, 156 00:07:51 --> 00:07:56 by accident or on purpose. 157 00:07:56 --> 00:07:59 And the reason for this is because folic acid deficiency 158 00:07:59 --> 00:08:03 in pregnant women leads to neural tube defects in babies, 159 00:08:03 --> 00:08:07 and your brain develops very, very, very early in pregnancy, 160 00:08:07 --> 00:08:10 the brain of an embryo. 161 00:08:10 --> 00:08:13 So it turns out that a lot of women don't realize they're 162 00:08:13 --> 00:08:17 pregnant while the fetus' brain is developing, and if these 163 00:08:17 --> 00:08:19 women are not getting enough folic acid, you can end up with 164 00:08:19 --> 00:08:21 spina bifida, which is an absolutely devastating and 165 00:08:21 --> 00:08:25 very preventable disease or other neural tube defects. 166 00:08:25 --> 00:08:27 So hopefully, if you're not familiar with folic acid, 167 00:08:27 --> 00:08:31 you'll become familiar at least within the next 10 or 15 years 168 00:08:31 --> 00:08:34 or so before you're thinking about maybe, on purpose, 169 00:08:34 --> 00:08:36 becoming pregnant at some time. 170 00:08:36 --> 00:08:39 Vitamin A you probably all heard about growing up, getting 171 00:08:39 --> 00:08:41 enough of your carrots so you can see at night. 172 00:08:41 --> 00:08:44 Vitamin A is important for eye health. 173 00:08:44 --> 00:08:46 But just looking at the structure of these two 174 00:08:46 --> 00:08:49 molecules, knowing what we know so far about general chemistry, 175 00:08:49 --> 00:08:52 we can already say a lot about how these are going to function 176 00:08:52 --> 00:08:55 the body or how they'll be treated in the body. 177 00:08:55 --> 00:08:58 And specifically, let's take a look at which of these two 178 00:08:58 --> 00:09:01 molecules has more polar bonds and see what that tells us. 179 00:09:01 --> 00:09:05 So take a look at your two structures and tell me which 180 00:09:05 --> 00:09:09 has, between folic acid and vitamin A, more polar 181 00:09:09 --> 00:09:19 groups within the vitamin. 182 00:09:19 --> 00:09:21 And this should go pretty quickly since you don't need to 183 00:09:21 --> 00:09:24 have an exact number, we're just looking and glancing 184 00:09:24 --> 00:09:25 and seeing which has more. 185 00:09:25 --> 00:09:39 So let's take 10 seconds on this. 186 00:09:39 --> 00:09:39 All right, great. 187 00:09:39 --> 00:09:43 So most you said folic acid or vitamin B9, so let's switch 188 00:09:43 --> 00:09:46 back to our notes and take a look at why. 189 00:09:46 --> 00:09:49 So vitamin B9 has more polar bonds. 190 00:09:49 --> 00:09:51 It's very easy to see, once I highlight, that it's not 191 00:09:51 --> 00:09:53 even a close call at all. 192 00:09:53 --> 00:09:56 We have a bunch of different polar bonds in B9 versus 193 00:09:56 --> 00:09:58 just one in vitamin A. 194 00:09:58 --> 00:10:02 Remember, the c h bond we're not calling polar covalent 195 00:10:02 --> 00:10:04 because it only has an electronegativity difference 196 00:10:04 --> 00:10:07 of 0.4 between the two atoms. 197 00:10:07 --> 00:10:12 So what we know is that vitamin B9, folic acid, is more polar. 198 00:10:12 --> 00:10:16 Remember, we also just said that water is polar, so 199 00:10:16 --> 00:10:20 would we say that B9 is water or fat soluble? 200 00:10:20 --> 00:10:23 It's going to be water soluble -- everyone knows the saying, 201 00:10:23 --> 00:10:27 "like dissolves like." B9 is going to be very water soluble. 202 00:10:27 --> 00:10:29 This actually is very important if you think 203 00:10:29 --> 00:10:31 about taking your vitamins. 204 00:10:31 --> 00:10:34 This means if it's water soluble, any vitamins that are 205 00:10:34 --> 00:10:35 water soluble, and now you should to be able to look at 206 00:10:35 --> 00:10:38 the structure of any vitamin and tell me, for example, that 207 00:10:38 --> 00:10:42 vitamin C is water soluble, that folic acid is 208 00:10:42 --> 00:10:42 water soluble. 209 00:10:42 --> 00:10:45 If you take these vitamins, what happens is that they're 210 00:10:45 --> 00:10:48 very quickly and easily excreted into your urine. 211 00:10:48 --> 00:10:50 So, for example, some people like to take mega 212 00:10:50 --> 00:10:52 doses of vitamin C. 213 00:10:52 --> 00:10:54 What really happens is that you have a mega dose of vitamin C 214 00:10:54 --> 00:10:57 in your urine, it doesn't stick around long in your body. 215 00:10:57 --> 00:11:00 So it doesn't help and just take all of your 216 00:11:00 --> 00:11:01 vitamin C at once. 217 00:11:01 --> 00:11:03 That's why it's important to eat balanced meals throughout 218 00:11:03 --> 00:11:07 the day, you need to be getting a constant supply of these 219 00:11:07 --> 00:11:08 water soluble vitamins. 220 00:11:08 --> 00:11:11 The same is for folic acid, you can't just take it once a 221 00:11:11 --> 00:11:13 month, you need to be taking it regularly in order that you 222 00:11:13 --> 00:11:17 keep the stores up in your body, otherwise you're going to 223 00:11:17 --> 00:11:19 excrete it, you're going to get rid of it very quickly. 224 00:11:19 --> 00:11:22 In contrast, if we think about vitamin A, is this going to be 225 00:11:22 --> 00:11:25 water soluble or fat soluble? 226 00:11:25 --> 00:11:27 Yup, so this is the fat soluble vitamin. 227 00:11:27 --> 00:11:31 Vitamin E is another big one that's fat soluble that gets a 228 00:11:31 --> 00:11:36 lot of press in terms of being an important vitamin to take. 229 00:11:36 --> 00:11:39 We can think about what it means when a vitamin is 230 00:11:39 --> 00:11:41 fat soluble instead of water soluble. 231 00:11:41 --> 00:11:43 Well, now it's not going to get excreted out of our body so 232 00:11:43 --> 00:11:47 quickly, so we actually can build up amounts of vitamin A 233 00:11:47 --> 00:11:50 vitamin E, for example, but that can also pose a problem if 234 00:11:50 --> 00:11:53 you think about biologically what's happening. 235 00:11:53 --> 00:11:56 And up here I just show two different supplements that 236 00:11:56 --> 00:11:57 I found on the internet. 237 00:11:57 --> 00:12:00 This is One A Day vitamin, it has about 100% of what 238 00:12:00 --> 00:12:02 you need of everything. 239 00:12:02 --> 00:12:05 And then this vitamin is one that I found that's supposed to 240 00:12:05 --> 00:12:08 really help you with your eye health, if you have bad vision 241 00:12:08 --> 00:12:10 instead of glasses, they suggest that you try 242 00:12:10 --> 00:12:12 this vitamin here. 243 00:12:12 --> 00:12:16 And what you can see is that it has five times your daily value 244 00:12:16 --> 00:12:20 of vitamin A, and 13 times what you need of vitamin E. 245 00:12:20 --> 00:12:22 Is this a good idea? 246 00:12:22 --> 00:12:23 No. 247 00:12:23 --> 00:12:26 Basically, you are just building up more and more and 248 00:12:26 --> 00:12:29 more of these fat soluble vitamins into your body. 249 00:12:29 --> 00:12:32 And there have been studies that have come out recently 250 00:12:32 --> 00:12:34 trying to look at the health benefits of vitamin E, and in 251 00:12:34 --> 00:12:37 some of these studies they give these mega doses, and instead 252 00:12:37 --> 00:12:41 what they find is increased bleeding in these patients, an 253 00:12:41 --> 00:12:43 increase in overall different types of death that 254 00:12:43 --> 00:12:45 can be happening. 255 00:12:45 --> 00:12:48 You want to take your vitamins, it's very important, but you 256 00:12:48 --> 00:12:50 don't want it just build up, and you want to think about is 257 00:12:50 --> 00:12:54 the vitamin that I'm going and getting 800 times what I need a 258 00:12:54 --> 00:12:57 day, is that a water soluble vitamin or a fat soluble 259 00:12:57 --> 00:12:59 vitamin, and using your chemistry, you should be able 260 00:12:59 --> 00:13:02 to very quickly take a quick look at that structure 261 00:13:02 --> 00:13:04 and figure out what kind of a vitamin that is. 262 00:13:04 --> 00:13:05 All right. 263 00:13:05 --> 00:13:08 So that's just one reason we would want to be able to think 264 00:13:08 --> 00:13:10 about polarity is thinking about whether something is 265 00:13:10 --> 00:13:13 very water soluble or not. 266 00:13:13 --> 00:13:16 Let's think about some other things that have to do with 267 00:13:16 --> 00:13:19 polarity and then can tell us a lot of other information. 268 00:13:19 --> 00:13:22 So let's move on to talking about the shapes of molecules. 269 00:13:22 --> 00:13:26 And the shapes of molecules is very important for a number of 270 00:13:26 --> 00:13:28 different properties when we're thinking about chemical 271 00:13:28 --> 00:13:32 reactions and reactions that take place in the body. 272 00:13:32 --> 00:13:35 When we talk about shapes of molecules, we're talking about 273 00:13:35 --> 00:13:37 the geometry of that molecule. 274 00:13:37 --> 00:13:41 And the geometry influences all of its different properties, 275 00:13:41 --> 00:13:44 including things like melting point or boiling point, 276 00:13:44 --> 00:13:45 it's reactivity. 277 00:13:45 --> 00:13:48 We just saw when talking about polar molecules, that it 278 00:13:48 --> 00:13:52 influences whether or not the molecule itself is 279 00:13:52 --> 00:13:54 polar or apolar. 280 00:13:54 --> 00:13:57 It's also really important when we talk about biology. 281 00:13:57 --> 00:14:00 Shape is particularly important when you think about enzymes 282 00:14:00 --> 00:14:05 having an active site where a molecule needs to fit perfectly 283 00:14:05 --> 00:14:08 into the active site, it does that because of its shape. 284 00:14:08 --> 00:14:11 A quick example that we can think about is with sucrose. 285 00:14:11 --> 00:14:14 Does anyone know what sucrose is? 286 00:14:14 --> 00:14:17 It's just table sugar -- sucrose is that crystal in 287 00:14:17 --> 00:14:22 sugar that we hopefully use in most of our sugar 288 00:14:22 --> 00:14:23 intake as sweeteners. 289 00:14:23 --> 00:14:26 A lot of times we use corn syrup, which is often not 290 00:14:26 --> 00:14:30 sucrose and which tends to be not always as good for us, 291 00:14:30 --> 00:14:32 although anything in too much excess is obviously a 292 00:14:32 --> 00:14:34 non-ideal situation. 293 00:14:34 --> 00:14:38 But in order for us to use the energy from sucrose, sucrose is 294 00:14:38 --> 00:14:40 made up of two individual sugar monomers. 295 00:14:40 --> 00:14:42 It's made up of a monomer of glucose and one of fructose, a 296 00:14:42 --> 00:14:46 6 and a 5 membered ring, so in order for our body to use it, 297 00:14:46 --> 00:14:49 we need to break it into with its monomers. 298 00:14:49 --> 00:14:51 And if we that, we can do it by what's called hydrolysis -- 299 00:14:51 --> 00:14:54 sucrose and water breaks down into its two monomers, 300 00:14:54 --> 00:14:56 now our body can use it. 301 00:14:56 --> 00:14:58 The problem is this process takes on the order of 10 or 302 00:14:58 --> 00:15:01 maybe 100 years in order for us to get enough of 303 00:15:01 --> 00:15:03 the sugar broken down. 304 00:15:03 --> 00:15:06 That's why we need an enzyme molecule, and the enzyme in 305 00:15:06 --> 00:15:08 our body is called sucrase. 306 00:15:08 --> 00:15:12 Sucrase breaks down sucrose, it catalyzes that reaction, so 307 00:15:12 --> 00:15:13 it happens very quickly. 308 00:15:13 --> 00:15:17 And the key is, and here's a ball and stick shape of 309 00:15:17 --> 00:15:18 what sucrose looks like. 310 00:15:18 --> 00:15:21 It needs to fit exactly into that active site. 311 00:15:21 --> 00:15:25 When it does, it combines into the enzyme, the enzyme can then 312 00:15:25 --> 00:15:29 catalyze the hydrolysis or the breaking apart of those two 313 00:15:29 --> 00:15:32 individual monomers into its separate pieces. 314 00:15:32 --> 00:15:36 And then it let's go of the glucose and the fructose, and 315 00:15:36 --> 00:15:39 we get our enzyme back again, and something else combined 316 00:15:39 --> 00:15:40 into that active site. 317 00:15:40 --> 00:15:43 So, this is one very simple example of why molecular 318 00:15:43 --> 00:15:45 shape is very important. 319 00:15:45 --> 00:15:48 A lot of times you're thinking about small molecules 320 00:15:48 --> 00:15:50 interacting with proteins or interacting with other 321 00:15:50 --> 00:15:52 molecules, and you want to think about the shape of that 322 00:15:52 --> 00:15:55 molecule to think about how that interaction is 323 00:15:55 --> 00:15:57 going to take place. 324 00:15:57 --> 00:16:01 So what we're going to use to think about molecular shape or 325 00:16:01 --> 00:16:05 molecular geometry is what's called valence shell electron 326 00:16:05 --> 00:16:08 repulsion or vsper theory. 327 00:16:08 --> 00:16:10 And this theory is very convenient for us to be 328 00:16:10 --> 00:16:13 thinking about because we are now masters of drawing Lewis 329 00:16:13 --> 00:16:16 structures, and vesper theory is based on Lewis structures, 330 00:16:16 --> 00:16:20 and also the principle that when we have valence electron 331 00:16:20 --> 00:16:22 pairs, they're going to repel each other. 332 00:16:22 --> 00:16:26 This make sense any time you have negatively-charged 333 00:16:26 --> 00:16:29 electrons, you want to get them as far away from each other as 334 00:16:29 --> 00:16:32 possible because they're all negatively charged. 335 00:16:32 --> 00:16:35 And the other principle is that the geometry around that 336 00:16:35 --> 00:16:38 central atom, which you've identified as the central atom 337 00:16:38 --> 00:16:40 in the Lewis structure, is going to be such that it 338 00:16:40 --> 00:16:45 minimizes the repulsion between those either bonding electrons 339 00:16:45 --> 00:16:48 or the electron pairs. 340 00:16:48 --> 00:16:50 So when we talk about vsper there's a special nomenclature 341 00:16:50 --> 00:16:57 that we do use, and in vsper we have a -- does anyone know 342 00:16:57 --> 00:16:59 what a means in vsper theory? 343 00:16:59 --> 00:17:04 It's a central atom. 344 00:17:04 --> 00:17:09 What about x? 345 00:17:09 --> 00:17:11 So this is actually the bonding, any bonding 346 00:17:11 --> 00:17:13 atom you call x. 347 00:17:13 --> 00:17:18 And then we have e, which is equal to lone pairs. 348 00:17:18 --> 00:17:21 So e is a lone pair -- e is not a lone pair electron, so if 349 00:17:21 --> 00:17:28 you have two electrons, that's one lone pair. 350 00:17:28 --> 00:17:28 All right. 351 00:17:28 --> 00:17:30 So there are some guidelines that we use when we're coming 352 00:17:30 --> 00:17:32 up with these vsper geometries. 353 00:17:32 --> 00:17:35 The first thing we talk about is the steric number, which 354 00:17:35 --> 00:17:37 we use to predict what that geometry will be. 355 00:17:37 --> 00:17:41 When we're talking about steric number, all we're talking about 356 00:17:41 --> 00:17:46 is adding together the number we have of bonded atoms, plus 357 00:17:46 --> 00:17:47 the number of lone pairs. 358 00:17:47 --> 00:17:50 So, essentially were just adding x to e and that gives 359 00:17:50 --> 00:17:52 us our steric number. 360 00:17:52 --> 00:17:55 So, for example, if we look at this molecule, which is 361 00:17:55 --> 00:17:59 a x 2 e, what is the steric number here? 362 00:17:59 --> 00:18:03 Yeah, it's 3, so we have a steric number of 3. 363 00:18:03 --> 00:18:05 Something that I want to point out is that we could have a 364 00:18:05 --> 00:18:10 different molecule, which is also a x 2 e, but in this case 365 00:18:10 --> 00:18:15 we have a double bond between the central atom and one of the 366 00:18:15 --> 00:18:18 x atoms here, and what I want to point out is in vsper theory 367 00:18:18 --> 00:18:21 we treat bonds as bonds, we don't worry about if they're 368 00:18:21 --> 00:18:23 single or they're double or they're triple. 369 00:18:23 --> 00:18:27 So again, we call this a x 2 e, this again, has 370 00:18:27 --> 00:18:28 a steric number of 3. 371 00:18:28 --> 00:18:33 So what's important in terms of vsper theory is the number of 372 00:18:33 --> 00:18:35 atoms bonded to a central atom. 373 00:18:35 --> 00:18:38 What's not important is the types of bonds that 374 00:18:38 --> 00:18:41 we're dealing with. 375 00:18:41 --> 00:18:44 A few other guidelines I want to mention is first of all, to 376 00:18:44 --> 00:18:46 think about resonance structures. 377 00:18:46 --> 00:18:49 So on Friday we were talking about the chromate anion, and 378 00:18:49 --> 00:18:52 we said that here are two of its resonance structures here, 379 00:18:52 --> 00:18:55 but that we could actually draw four more resonance structures, 380 00:18:55 --> 00:19:00 and I just want to point out when you take a molecule that 381 00:19:00 --> 00:19:03 has many different resonance structures and you want to draw 382 00:19:03 --> 00:19:08 its vsper, it's Lewis structure and its vsper geometry, you can 383 00:19:08 --> 00:19:11 take any single one of those resonance structures, it 384 00:19:11 --> 00:19:14 doesn't matter, you'll all end up with the same geometry. 385 00:19:14 --> 00:19:17 And lastly, if we're talking about a molecule that has more 386 00:19:17 --> 00:19:20 than one central atom, which is what we're very, very often 387 00:19:20 --> 00:19:22 doing, you need to deal with each one separately. 388 00:19:22 --> 00:19:27 So for example, with methanol here, we have a carbon, which 389 00:19:27 --> 00:19:30 is a central atom, and we also have an oxygen, which is a 390 00:19:30 --> 00:19:33 central atom, you need to talk about the geometry separately, 391 00:19:33 --> 00:19:35 we don't talk about the geometry of the 392 00:19:35 --> 00:19:38 entire molecule. 393 00:19:38 --> 00:19:41 So, let's go ahead and look at some of these vsper examples, 394 00:19:41 --> 00:19:45 and Professor Drennan is going to help demonstrate what some 395 00:19:45 --> 00:19:48 of these are with some models here. 396 00:19:48 --> 00:19:50 So the first case we're going to talk about is without any 397 00:19:50 --> 00:19:53 lone pairs -- this is the most straightforward case. 398 00:19:53 --> 00:19:58 And our first case that we can have is a x 3, which is going 399 00:19:58 --> 00:20:01 to have a linear shape, and that will have a bond 400 00:20:01 --> 00:20:07 angle of a 180 degrees. 401 00:20:07 --> 00:20:09 The next case that we can talk about is trigonal 402 00:20:09 --> 00:20:11 planar or a x 3. 403 00:20:11 --> 00:20:14 Now, you do need to know these geometry, you 404 00:20:14 --> 00:20:15 need to know the names. 405 00:20:15 --> 00:20:16 A lot of them are very easy to remember. 406 00:20:16 --> 00:20:20 I expect not too many of you will get linear incorrect. 407 00:20:20 --> 00:20:22 Also, trigonal planar, pretty easy. 408 00:20:22 --> 00:20:26 Trigonal, it has 3 atoms around the central atom and 409 00:20:26 --> 00:20:27 the molecule is planar. 410 00:20:27 --> 00:20:34 PROFESSOR: So, what is the bond angle for this geometry? 411 00:20:34 --> 00:20:35 120. 412 00:20:35 --> 00:20:36 PROFESSOR: All right. 413 00:20:36 --> 00:20:36 Great. 414 00:20:36 --> 00:20:41 So, next we can think about a x 4, and this is what's called 415 00:20:41 --> 00:20:43 a tetrahedral geometry. 416 00:20:43 --> 00:20:46 If you're trying to remember this name you can think of each 417 00:20:46 --> 00:20:49 of those bonding atoms as being in the corner of tetrahedron. 418 00:20:49 --> 00:20:53 One thing I want to point out is you're seeing some notation 419 00:20:53 --> 00:20:56 we haven't used before in this class where you have a wedge 420 00:20:56 --> 00:20:58 coming out at you, and you also have a dashed line to 421 00:20:58 --> 00:21:00 one of those bonds. 422 00:21:00 --> 00:21:03 Any time you see a wedge in a Lewis structure, it means that 423 00:21:03 --> 00:21:05 it's coming out at you -- it's either coming out of the screen 424 00:21:05 --> 00:21:07 or it's coming out of your paper. 425 00:21:07 --> 00:21:10 And any time you see a dashed line here, which might be 426 00:21:10 --> 00:21:13 easier to see in your notes, that means that the bond is 427 00:21:13 --> 00:21:16 actually going into the page or into the screen or 428 00:21:16 --> 00:21:17 into the blackboard. 429 00:21:17 --> 00:21:24 PROFESSOR: And what are the angles for a tetrahedral? 430 00:21:24 --> 00:21:26 PROFESSOR: All right, so they're 1 0 9 . 431 00:21:26 --> 00:21:26 5. 432 00:21:26 --> 00:21:27 Pretty close. 433 00:21:27 --> 00:21:30 So that's as far away as those bonds can get from each 434 00:21:30 --> 00:21:31 other would be 109 . 435 00:21:31 --> 00:21:33 5 degrees. 436 00:21:33 --> 00:21:37 The next case we have is a x 5. 437 00:21:37 --> 00:21:40 That is what we call trigonal bipyramidal. 438 00:21:40 --> 00:21:43 Again, we have trigonal, because we have those 3 bonds 439 00:21:43 --> 00:21:47 in the center, and if you can picture putting walls between 440 00:21:47 --> 00:21:49 all those bonds, you can see there's a pyramid on the 441 00:21:49 --> 00:21:52 top and a pyramid on the bottom, so bipyramidal. 442 00:21:52 --> 00:21:55 PROFESSOR: And there are 2 sets of angles here, what are the 443 00:21:55 --> 00:22:00 angles for equatorial atoms? 444 00:22:00 --> 00:22:01 120. 445 00:22:01 --> 00:22:02 And then for axial? 446 00:22:02 --> 00:22:02 90. 447 00:22:02 --> 00:22:07 PROFESSOR: And again, axial, those are just the ones in the 448 00:22:07 --> 00:22:12 axis, and equatorial if you put your globe around the equator. 449 00:22:12 --> 00:22:15 So the last case we have is a x 6. 450 00:22:15 --> 00:22:21 A x 6 is what we call octahedral -- you can picture 451 00:22:21 --> 00:22:24 each of those bonded atoms as a corner of an octahedron. 452 00:22:24 --> 00:22:27 PROFESSOR: And what are the angles in this, 453 00:22:27 --> 00:22:28 this one set of angles? 454 00:22:28 --> 00:22:30 90. 455 00:22:30 --> 00:22:30 PROFESSOR: Great. 456 00:22:30 --> 00:22:32 So, 90 degrees. 457 00:22:32 --> 00:22:34 All right. 458 00:22:34 --> 00:22:37 So this is all straightforward when we're just thinking about 459 00:22:37 --> 00:22:40 different molecular shapes that don't have any lone pairs in 460 00:22:40 --> 00:22:42 them, but once we start having lone pairs, we now need to 461 00:22:42 --> 00:22:44 think about how those lone pairs are going to 462 00:22:44 --> 00:22:47 affect the geometry. 463 00:22:47 --> 00:22:50 But before we do that, let's talk about some examples of 464 00:22:50 --> 00:22:52 molecules without lone pairs. 465 00:22:52 --> 00:22:56 So the first case is what we saw at the beginning of class, 466 00:22:56 --> 00:22:59 carbon dioxide, and we said that that was linear, and now 467 00:22:59 --> 00:23:04 we know why it's linear -- it has a formula of a x 2, 468 00:23:04 --> 00:23:07 and an s n number of 2. 469 00:23:07 --> 00:23:13 And that's a bond angle of 180 degrees. 470 00:23:13 --> 00:23:17 So, you can tell us about borane, borane is a x 3. 471 00:23:17 --> 00:23:20 PROFESSOR: And what is the geometry? 472 00:23:20 --> 00:23:23 And the angle? 473 00:23:23 --> 00:23:25 120. 474 00:23:25 --> 00:23:29 Feel free to yell out very loudly, we want people in 475 00:23:29 --> 00:23:34 OpenCourseWare to hear the answers coming from the room. 476 00:23:34 --> 00:23:34 PROFESSOR: All right. 477 00:23:34 --> 00:23:38 So the next case we're going to look at is c h 4 or methane, 478 00:23:38 --> 00:23:41 and let's do a clicker question here to make sure everyone is 479 00:23:41 --> 00:23:43 remembering these geometries. 480 00:23:43 --> 00:23:46 And this should be very quick, and try to not turn the page 481 00:23:46 --> 00:23:49 back one, and tell us in 10 seconds here what 482 00:23:49 --> 00:23:50 the geometry is. 483 00:23:50 --> 00:24:06 98%, that is a new record. 484 00:24:06 --> 00:24:07 Very good. 485 00:24:07 --> 00:24:07 Tetrahedral. 486 00:24:07 --> 00:24:12 PROFESSOR: All right, while I'm holding carbon dioxide in one 487 00:24:12 --> 00:24:15 hand and methane in the other hand, I just want to do an 488 00:24:15 --> 00:24:18 additional plug for the energy debate tonight. 489 00:24:18 --> 00:24:21 So there will be representatives from both 490 00:24:21 --> 00:24:24 presidential campaigns that are going to be there to 491 00:24:24 --> 00:24:25 answer questions. 492 00:24:25 --> 00:24:28 And I'm not sure of the exact format, I'm not sure whether 493 00:24:28 --> 00:24:31 all audience members are going to be able to ask 494 00:24:31 --> 00:24:33 questions or not. 495 00:24:33 --> 00:24:34 I'm not sure if I'm going to be there, I'm supposed to 496 00:24:34 --> 00:24:38 be giving a talk actually, ironically, about energy 497 00:24:38 --> 00:24:40 at the same time. 498 00:24:40 --> 00:24:43 But if I can't make it and you're allowed to ask 499 00:24:43 --> 00:24:46 questions, I'd like you to ask a question for me. 500 00:24:46 --> 00:24:50 So, my lab looks at enzymes that remove carbon monoxide 501 00:24:50 --> 00:24:52 and carbon dioxide from the environment. 502 00:24:52 --> 00:24:56 And I have an energy initiative grant to do this, because MIT 503 00:24:56 --> 00:24:59 recognizes that an important part of any energy initiative 504 00:24:59 --> 00:25:01 is thinking about lowering pollutants and 505 00:25:01 --> 00:25:03 greenhouse gases. 506 00:25:03 --> 00:25:07 So, I heard on Thursday at the debate when Governor Sarah 507 00:25:07 --> 00:25:10 Palin was asked directly if she really doesn't think there's 508 00:25:10 --> 00:25:13 any connection between global warming and any man-made 509 00:25:13 --> 00:25:16 activities, that she said, well, you know, it could be 510 00:25:16 --> 00:25:19 just regular temperature fluctuations, and she doesn't 511 00:25:19 --> 00:25:21 want to point any fingers. 512 00:25:21 --> 00:25:23 So, she's not quite sure. 513 00:25:23 --> 00:25:26 I also learned on Thursday that she will head the 514 00:25:26 --> 00:25:30 energy initiative in a McCain/Palin White House. 515 00:25:30 --> 00:25:34 So, if you go to this debate, I would like you to ask what is 516 00:25:34 --> 00:25:37 your plan for sequestering carbon dioxide and other 517 00:25:37 --> 00:25:39 greenhouse gases. 518 00:25:39 --> 00:25:41 You can ask both candidates, and please let me know what the 519 00:25:41 --> 00:25:44 answer is, because I think that's a really important 520 00:25:44 --> 00:25:47 question -- here we have carbon dioxide, here we have methane, 521 00:25:47 --> 00:25:50 we need to be thinking about greenhouse gases. 522 00:25:50 --> 00:25:52 PROFESSOR: You can also throw in the question 523 00:25:52 --> 00:25:54 that you do know this is tetrahedral and 109 . 524 00:25:54 --> 00:26:00 5 degrees -- show your chemistry knowledge here. 525 00:26:00 --> 00:26:02 All right, and let's keep showing that we know our 526 00:26:02 --> 00:26:08 geometries, let's look at p c l 5. 527 00:26:08 --> 00:26:12 PROFESSOR: And what is geometry here? 528 00:26:12 --> 00:26:12 Yup. 529 00:26:12 --> 00:26:15 And the angles? 530 00:26:15 --> 00:26:18 120 and 90. 531 00:26:18 --> 00:26:22 PROFESSOR: So, 120 equatorial, and 90 axial. 532 00:26:22 --> 00:26:28 And last we have s f 6. 533 00:26:28 --> 00:26:29 PROFESSOR: So, what is the geometry here? 534 00:26:29 --> 00:26:31 Octahedral. 535 00:26:31 --> 00:26:31 And angle? 536 00:26:31 --> 00:26:31 90. 537 00:26:31 --> 00:26:37 PROFESSOR: All right, so there's our set of examples, 538 00:26:37 --> 00:26:40 one for each for those shapes or geometries 539 00:26:40 --> 00:26:42 that have no lone pairs. 540 00:26:42 --> 00:26:44 And now let's think a little bit about once we do have 541 00:26:44 --> 00:26:46 molecules with lone pairs. 542 00:26:46 --> 00:26:49 And the biggest point to keep in mind when we're comparing 543 00:26:49 --> 00:26:53 lone pair electrons with bonding electrons or electrons 544 00:26:53 --> 00:26:57 in bonds, is that when you have electrons in bonds they have 545 00:26:57 --> 00:27:00 less spatial distribution than lone pairs. 546 00:27:00 --> 00:27:03 So that's just a way of saying that electrons and bonds, 547 00:27:03 --> 00:27:05 they take up less space. 548 00:27:05 --> 00:27:09 So, when we think about lone pair electrons, they're taking 549 00:27:09 --> 00:27:11 up more space and this means that they're going to 550 00:27:11 --> 00:27:15 experience more repulsion in thinking about the lone pair 551 00:27:15 --> 00:27:19 electrons with either other lone pair electrons or 552 00:27:19 --> 00:27:20 with bonding electrons. 553 00:27:20 --> 00:27:23 So let's think a second about the order that this will be in. 554 00:27:23 --> 00:27:25 The biggest repulsion we would feel is if we have 555 00:27:25 --> 00:27:29 two different lone pair electrons or lone pairs. 556 00:27:29 --> 00:27:31 They're going to have the most repulsion. 557 00:27:31 --> 00:27:34 In the middle is if we're talking about a lone pair 558 00:27:34 --> 00:27:36 with a bonding pair. 559 00:27:36 --> 00:27:39 And then the least repulsion is going to be between two bonds 560 00:27:39 --> 00:27:43 or two bonding pairs of electrons. 561 00:27:43 --> 00:27:46 So, let's look at an example of where this comes up. 562 00:27:46 --> 00:27:49 So the first example I'm going to talk about is a molecule 563 00:27:49 --> 00:27:53 that has the geometry of a seesaw shape, and once we get 564 00:27:53 --> 00:27:56 to having Professor Drennan actually show you that shape, 565 00:27:56 --> 00:27:57 you will never forget. 566 00:27:57 --> 00:27:59 See-saw, that's going to be one of the easy 567 00:27:59 --> 00:28:00 geometries to remember. 568 00:28:00 --> 00:28:03 But the first thing that we need to consider in terms of 569 00:28:03 --> 00:28:08 the shape, which starts, if you picture first of all, and these 570 00:28:08 --> 00:28:11 models sometimes don't quite stay together, we could 571 00:28:11 --> 00:28:13 actually have two different possibilities. 572 00:28:13 --> 00:28:15 The first is the idea that we could have an axial lone 573 00:28:15 --> 00:28:19 pair, and the second is the possibility that we could have 574 00:28:19 --> 00:28:21 an equatorial lone pair. 575 00:28:21 --> 00:28:24 And we could consider both, and we should be able to use our 576 00:28:24 --> 00:28:26 vsper principles to think about which one is actually 577 00:28:26 --> 00:28:27 going to happen. 578 00:28:27 --> 00:28:31 So if we think about having an axial lone pair here, that 579 00:28:31 --> 00:28:34 would mean that these lone pair electrons are going to be 580 00:28:34 --> 00:28:38 within 90 degrees of three different loan pairs, and 581 00:28:38 --> 00:28:40 actually we'll see later, it will be more than 90 because 582 00:28:40 --> 00:28:42 they're actually going to push them down. 583 00:28:42 --> 00:28:45 But in terms of considering how many bonding electron pairs 584 00:28:45 --> 00:28:49 they'll repel strongly, what we care about is anything within 585 00:28:49 --> 00:28:51 90 degrees initially. 586 00:28:51 --> 00:28:54 So, what we would see with the axial lone pair is that we have 587 00:28:54 --> 00:28:59 three lone pairs that we're going to very strongly repel. 588 00:28:59 --> 00:29:01 Now would you tell me, if we think about an equatorial lone 589 00:29:01 --> 00:29:07 pair, how many different bonding electrons or a bonding 590 00:29:07 --> 00:29:10 pairs that will strongly repel if we have an 591 00:29:10 --> 00:29:11 equatorial lone pair? 592 00:29:11 --> 00:29:15 So you might need to look at your notes to actually compare 593 00:29:15 --> 00:29:31 these two before you submit your answer up here. 594 00:29:31 --> 00:29:31 All right. 595 00:29:31 --> 00:29:45 Let's take 10 seconds on that. 596 00:29:45 --> 00:29:47 OK, strong showing with the clicker questions today. 597 00:29:47 --> 00:29:51 It's going to be a tight race for clicker competition. 598 00:29:51 --> 00:29:53 It's correct that the equatorial is, in fact, 599 00:29:53 --> 00:29:58 has two, only two that it strongly repels. 600 00:29:58 --> 00:30:00 So, actually it's probably easier to look at 601 00:30:00 --> 00:30:02 Professor Drennan's model here to see that. 602 00:30:02 --> 00:30:06 So it's where the equatorial lone pair is that is the 603 00:30:06 --> 00:30:10 actual seesaw geometry. 604 00:30:10 --> 00:30:13 PROFESSOR: So, how many of you had seesaws in playgrounds 605 00:30:13 --> 00:30:15 when you were growing up? 606 00:30:15 --> 00:30:16 Oh, a lot of people. 607 00:30:16 --> 00:30:18 I know that they're not considered totally safe 608 00:30:18 --> 00:30:20 anymore, so some of them are going away. 609 00:30:20 --> 00:30:25 So this is seesaw. 610 00:30:25 --> 00:30:32 Now you should never forget it. 611 00:30:32 --> 00:30:35 PROFESSOR: All right, so seesaw, we've got seesaw. 612 00:30:35 --> 00:30:38 Just to point out a few more shapes. 613 00:30:38 --> 00:30:42 We took one out, we replaced one bond with a lone 614 00:30:42 --> 00:30:43 pair in terms of seesaw. 615 00:30:43 --> 00:30:49 If we have a x 3 e 2 now we have two equatorial lone pairs. 616 00:30:49 --> 00:30:50 This is called the T-shaped 617 00:30:50 --> 00:30:51 PROFESSOR: T-shaped. 618 00:30:51 --> 00:30:55 PROFESSOR: See, these aren't too hard to remember the 619 00:30:55 --> 00:30:58 geometries, the geometry names. 620 00:30:58 --> 00:31:02 And we also can think about if we have a x 4 e 2. 621 00:31:02 --> 00:31:06 So now in order to get our lone pairs as far away from each 622 00:31:06 --> 00:31:08 other as possible, they're going to be on the axial 623 00:31:08 --> 00:31:15 position, and this is called square planer. 624 00:31:15 --> 00:31:18 PROFESSOR: So, this is easy to remember, square, also planar. 625 00:31:18 --> 00:31:22 PROFESSOR: All right. 626 00:31:22 --> 00:31:25 So, let's talk a little bit now about what the actual angles 627 00:31:25 --> 00:31:29 are between bonds when now we have these lone pairs present. 628 00:31:29 --> 00:31:33 And remember, what we said is when we have a lone pair of 629 00:31:33 --> 00:31:36 electrons, they actually are going to have more repulsion 630 00:31:36 --> 00:31:40 than if we just have a c h bond or a bonded 631 00:31:40 --> 00:31:41 pair of a electrons. 632 00:31:41 --> 00:31:46 So essentially what that means is that in molecules that have 633 00:31:46 --> 00:31:49 lone pair electrons, so for example, if we look at n h 3 or 634 00:31:49 --> 00:31:53 ammonia versus methane here, and Professor Drennan is 635 00:31:53 --> 00:31:55 showing those models to you here. 636 00:31:55 --> 00:31:59 What you end up seeing is actually that the bonding 637 00:31:59 --> 00:32:04 angle in n h 3 between the n h bonds is smaller. 638 00:32:04 --> 00:32:08 And you can't see it with these models because these are 639 00:32:08 --> 00:32:10 not actual lone pairs. 640 00:32:10 --> 00:32:12 But if they were actually repelling each other, they 641 00:32:12 --> 00:32:16 would be pushing those bonds as far away as possible, 642 00:32:16 --> 00:32:18 and instead of being 109 . 643 00:32:18 --> 00:32:21 5, you'd see that the angle is now 106 . 644 00:32:21 --> 00:32:22 7. 645 00:32:22 --> 00:32:25 So it's a smaller angle between bonds, because you have more 646 00:32:25 --> 00:32:29 repulsion from those lone pairs pushing those bonds down. 647 00:32:29 --> 00:32:32 We can also think about the influence of atomic size 648 00:32:32 --> 00:32:34 in terms of this effect. 649 00:32:34 --> 00:32:37 So first of all, what happens to atomic size as you go 650 00:32:37 --> 00:32:39 down the periodic table? 651 00:32:39 --> 00:32:41 Good, it increases. 652 00:32:41 --> 00:32:45 So we have size increasing as we go down the periodic table. 653 00:32:45 --> 00:32:49 Phosphorous is right underneath nitrogen on the periodic table, 654 00:32:49 --> 00:32:51 so phosphorous is going to be bigger than nitrogen. 655 00:32:51 --> 00:32:54 In terms of picturing what happens with those lone pairs, 656 00:32:54 --> 00:32:57 when we have a larger atom, the orbitals are also going to 657 00:32:57 --> 00:33:00 be larger, they can take up more space. 658 00:33:00 --> 00:33:04 That means these electrons, this lone pair electrons are 659 00:33:04 --> 00:33:06 going to take up more space. 660 00:33:06 --> 00:33:09 This means they're going to push away those bonding 661 00:33:09 --> 00:33:11 electrons even more. 662 00:33:11 --> 00:33:16 So would you expect the bond to be larger or smaller for p h 3? 663 00:33:16 --> 00:33:17 STUDENT: [INAUDIBLE] 664 00:33:17 --> 00:33:18 PROFESSOR: It's going to be smaller. 665 00:33:18 --> 00:33:22 So, what we see is that angles between bonded atoms actually 666 00:33:22 --> 00:33:25 decrease as you go down a column on the periodic table. 667 00:33:25 --> 00:33:29 So the actual angles for p h 3 are now going to be really 668 00:33:29 --> 00:33:32 quite a bit smaller, they're 93 . 669 00:33:32 --> 00:33:38 3 degrees. 670 00:33:38 --> 00:33:38 All right. 671 00:33:38 --> 00:33:42 So let's create a list for ourselves in terms of all the 672 00:33:42 --> 00:33:45 geometries that we can have now that we're dealing 673 00:33:45 --> 00:33:46 with lone pairs. 674 00:33:46 --> 00:33:49 One thing I want to point out in terms of remembering the 675 00:33:49 --> 00:33:52 names of the different geometries, when you're naming 676 00:33:52 --> 00:33:55 a geometry, the geometry name, for example, when we looked at 677 00:33:55 --> 00:34:00 square planar, we're only actually naming where the bonds 678 00:34:00 --> 00:34:02 are, where actual atoms are. 679 00:34:02 --> 00:34:05 The name doesn't really depend on the lone pairs. 680 00:34:05 --> 00:34:07 And also, when you draw a geometry, you don't always have 681 00:34:07 --> 00:34:11 to draw the lone pairs in, but you have to remember that the 682 00:34:11 --> 00:34:14 lone pairs are very much affecting the angles within 683 00:34:14 --> 00:34:17 your molecule and also the actual shape. 684 00:34:17 --> 00:34:20 So, for example, let's start talking about different types 685 00:34:20 --> 00:34:22 that have lone pairs in it. 686 00:34:22 --> 00:34:26 First of all, thinking about a x 2 e, so we have one lone 687 00:34:26 --> 00:34:31 pair, and the geometry here is bent, and I want you, thinking 688 00:34:31 --> 00:34:34 about lone pair repulsion, to tell us what you think 689 00:34:34 --> 00:34:44 the angle between these bonds are going to be? 690 00:34:44 --> 00:34:59 And let's take 10 seconds on that. 691 00:34:59 --> 00:34:59 OK, good. 692 00:34:59 --> 00:35:01 75%, that's not bad. 693 00:35:01 --> 00:35:07 Let's think about why. 694 00:35:07 --> 00:35:11 It's going to be less than 120 degrees, because we know that 695 00:35:11 --> 00:35:16 normally in a trigonal planar situation, we have an 696 00:35:16 --> 00:35:17 angle of 120 degrees. 697 00:35:17 --> 00:35:21 And since lone pairs are going to cause more repulsion, we're 698 00:35:21 --> 00:35:26 actually pushing down these two bonds here closer together, so 699 00:35:26 --> 00:35:28 what we end up seeing is that they're going to be 700 00:35:28 --> 00:35:30 less than 120 degrees. 701 00:35:30 --> 00:35:35 And it depends on the actual molecule what the exact angles 702 00:35:35 --> 00:35:37 are going to be, so you never have to learn the exact angles 703 00:35:37 --> 00:35:39 in terms of lone pair electrons, you just need to be 704 00:35:39 --> 00:35:41 able to tell us if the bond angle is going to be 705 00:35:41 --> 00:35:44 less than 120 degrees. 706 00:35:44 --> 00:35:48 So let's look at another example here, a x 3 707 00:35:48 --> 00:35:50 with one lone pair, e. 708 00:35:50 --> 00:35:53 This is called trigonal pyramidal. 709 00:35:53 --> 00:35:56 Again, you have trigonal because there's three atoms 710 00:35:56 --> 00:36:02 bonded to the central atom, and this looks like a pyramid here. 711 00:36:02 --> 00:36:03 PROFESSOR: So, what would the angle of this be? 712 00:36:03 --> 00:36:04 STUDENT: [INAUDIBLE] 713 00:36:04 --> 00:36:09 PROFESSOR: I think I heard it. 714 00:36:09 --> 00:36:10 PROFESSOR: I think so, too. 715 00:36:10 --> 00:36:13 Less than 109 . 716 00:36:13 --> 00:36:14 5. 717 00:36:14 --> 00:36:18 Let's take another example, a x 2 e 2. 718 00:36:18 --> 00:36:19 This is also bent. 719 00:36:19 --> 00:36:22 Tell us what the geometry -- we told you the geometry, tell us 720 00:36:22 --> 00:36:25 what the bond angle is going to be between these bonds. 721 00:36:25 --> 00:36:30 So, let's take 10 seconds on this. 722 00:36:30 --> 00:36:38 Let's re-poll F 4. 723 00:36:38 --> 00:36:52 So 10 seconds again. 724 00:36:52 --> 00:36:53 All right. 725 00:36:53 --> 00:36:57 In about 10 seconds, Darcy, in 10 seconds we'll 726 00:36:57 --> 00:36:59 hit the next side. 727 00:36:59 --> 00:37:09 OK, so it's less than 109 . 728 00:37:09 --> 00:37:10 5 degrees now. 729 00:37:10 --> 00:37:15 So some of you wrote that it was less than 120 degrees, and 730 00:37:15 --> 00:37:18 we can think about if we switch back to the class notes 731 00:37:18 --> 00:37:20 the difference here. 732 00:37:20 --> 00:37:24 So even though, so this is now going to be less than 109 . 733 00:37:24 --> 00:37:28 5, if we looked at the case we had bent in the first case, 734 00:37:28 --> 00:37:31 that started with 120 and one of those bonds was replaced 735 00:37:31 --> 00:37:35 with a lone pair, but now we have two lone pairs here, so 736 00:37:35 --> 00:37:38 now what we're going to see is that the two bents are 737 00:37:38 --> 00:37:39 not, in fact, equal. 738 00:37:39 --> 00:37:42 The bent where you started with the tetrahedral shape 739 00:37:42 --> 00:37:44 is actually going to be less than 109 . 740 00:37:44 --> 00:37:48 5, and Professor Drennan can maybe show us that 741 00:37:48 --> 00:37:49 with the models here. 742 00:37:49 --> 00:37:53 PROFESSOR: It's a little had to see, actually, between the two, 743 00:37:53 --> 00:37:59 but I think it's easier to see on this, that if you consider 744 00:37:59 --> 00:38:04 the sort of starting place for the two, if you look at the 745 00:38:04 --> 00:38:08 bottom set and you put orbitals, so it also depends on 746 00:38:08 --> 00:38:09 your starting geometry. 747 00:38:09 --> 00:38:15 It can be bent if take off these bonds on the top, but 748 00:38:15 --> 00:38:18 it depends on what the starting geometries were. 749 00:38:18 --> 00:38:20 And so you will go with your starting geometry, 750 00:38:20 --> 00:38:22 if it was 109 . 751 00:38:22 --> 00:38:23 5, it'll be less than that. 752 00:38:23 --> 00:38:26 If it's 120, then it's less than that. 753 00:38:26 --> 00:38:28 PROFESSOR: So, this is a good case of where, even when you 754 00:38:28 --> 00:38:30 have the geometry the same, you need to think about how many 755 00:38:30 --> 00:38:32 lone pairs you're dealing with in your molecule, and think 756 00:38:32 --> 00:38:35 about what would the geometry be if I just pictured those 757 00:38:35 --> 00:38:38 lone pairs in there as bonds and then push them even closer 758 00:38:38 --> 00:38:41 together so that it's less than that actual angle. 759 00:38:41 --> 00:38:47 OK, let's talk about a x 4 e, and that is what we 760 00:38:47 --> 00:38:49 call the seesaw shape. 761 00:38:49 --> 00:38:54 PROFESSOR: And so, what difference in angles 762 00:38:54 --> 00:38:55 are you going to have? 763 00:38:55 --> 00:38:58 STUDENT: [INAUDIBLE] 764 00:38:58 --> 00:38:58 PROFESSOR: Yup. 765 00:38:58 --> 00:39:02 PROFESSOR 2: So, two sets again, one is less than 766 00:39:02 --> 00:39:05 120, and one is going to be less than 90. 767 00:39:05 --> 00:39:09 So, let's look at a x 3 e 2, you'll notice there's a lot 768 00:39:09 --> 00:39:11 more combinations we can get to once we're talking 769 00:39:11 --> 00:39:13 about lone pairs. 770 00:39:13 --> 00:39:17 This is called T-shaped, and we had briefly discussed that but 771 00:39:17 --> 00:39:21 not mention what the angles would be. 772 00:39:21 --> 00:39:22 PROFESSOR: So, what angle would you have here? 773 00:39:22 --> 00:39:25 Yup. 774 00:39:25 --> 00:39:26 PROFESSOR: All right, great. 775 00:39:26 --> 00:39:28 Less than 90 degrees. 776 00:39:28 --> 00:39:34 So, we'll start a new page here and talk about a x 2 e 3, 777 00:39:34 --> 00:39:41 and we'll let you tell us what this geometry is. 778 00:39:41 --> 00:39:44 Yup, so I hear a lot of linear out there. 779 00:39:44 --> 00:39:47 And you need to keep in mind we give the names based on where 780 00:39:47 --> 00:39:50 the actual bonds are, not where the lone pairs are, even though 781 00:39:50 --> 00:39:52 the lone pairs, of course, affect the structure 782 00:39:52 --> 00:39:54 in the geometry. 783 00:39:54 --> 00:39:56 PROFESSOR: The angle? 784 00:39:56 --> 00:39:58 STUDENT: 180. 785 00:39:58 --> 00:39:58 PROFESSOR: Yup, 180. 786 00:39:58 --> 00:40:02 PROFESSOR: So, it's exactly 180 here, not less than. 787 00:40:02 --> 00:40:08 PROFESSOR: So, for a x 5 e, what we have here is 788 00:40:08 --> 00:40:09 called square pyramidal. 789 00:40:09 --> 00:40:13 This is square because of the four square at the bottom of 790 00:40:13 --> 00:40:16 your pyramid, then you can picture the pyramid as 791 00:40:16 --> 00:40:17 you go to the top. 792 00:40:17 --> 00:40:21 PROFESSOR: And so what angles would you have here? 793 00:40:21 --> 00:40:22 Yup, less than 90. 794 00:40:22 --> 00:40:26 PROFESSOR: So, less than 90, great. 795 00:40:26 --> 00:40:32 All right, there are more. 796 00:40:32 --> 00:40:35 So, our next combination that we can think about is if we 797 00:40:35 --> 00:40:40 have four bonded atoms and two lone pairs, a x 4 e 2, that's 798 00:40:40 --> 00:40:42 going to be called square planar. 799 00:40:42 --> 00:40:47 PROFESSOR: So, again, we have square and it's planar. 800 00:40:47 --> 00:40:49 And so what are your angles here? 801 00:40:49 --> 00:40:51 Yup, exactly 90. 802 00:40:51 --> 00:40:58 PROFESSOR: So next let's look at a x 3 e 3. 803 00:40:58 --> 00:41:01 This is what we call T-shape, this is also T-shape. 804 00:41:01 --> 00:41:08 PROFESSOR: And angle here? 805 00:41:08 --> 00:41:12 PROFESSOR: Yup, I think we heard less than, it's 806 00:41:12 --> 00:41:14 less than 90 degrees. 807 00:41:14 --> 00:41:20 And let's look at a x 2 e 4, and again, you guys can tell 808 00:41:20 --> 00:41:24 us what this geometry is, first of all. 809 00:41:24 --> 00:41:25 Yup, it's linear. 810 00:41:25 --> 00:41:28 So, we have lots of different ways we can get to a linear 811 00:41:28 --> 00:41:29 molecule, some of which have lots of lone pairs, some of 812 00:41:29 --> 00:41:31 which have no lone pairs at all. 813 00:41:31 --> 00:41:37 This is linear and a 180 degrees. 814 00:41:37 --> 00:41:37 All right. 815 00:41:37 --> 00:41:41 So we'll do just a few examples to show you some actual 816 00:41:41 --> 00:41:42 molecules that have these geometries. 817 00:41:42 --> 00:41:45 We won't go through all of them, because as we saw, 818 00:41:45 --> 00:41:47 there's lots of different combinations we can have 819 00:41:47 --> 00:41:49 once we start getting into lone pairs. 820 00:41:49 --> 00:41:53 The first we'll look at is water here, and water is a good 821 00:41:53 --> 00:41:57 one to look at because we had seen that at the beginning of 822 00:41:57 --> 00:42:00 class here when we were talking about all polar molecules. 823 00:42:00 --> 00:42:03 When we talk about the formula type of water, what when you 824 00:42:03 --> 00:42:07 say the formula type is? 825 00:42:07 --> 00:42:09 STUDENT: [INAUDIBLE] 826 00:42:09 --> 00:42:11 PROFESSOR: A x 2 e 2, that's correct. 827 00:42:11 --> 00:42:14 And what is the geometry of water? 828 00:42:14 --> 00:42:16 It's bent. 829 00:42:16 --> 00:42:19 So what we saw at the beginning of class is water is bent and 830 00:42:19 --> 00:42:21 now you can see why and should be able to predict 831 00:42:21 --> 00:42:22 that yourself. 832 00:42:22 --> 00:42:25 I just want to mention that if you look, and depending on the 833 00:42:25 --> 00:42:28 edition of the book you have, in one edition it's called 834 00:42:28 --> 00:42:30 bent, in the other edition it's called angular. 835 00:42:30 --> 00:42:34 We'll call it bent, but just remember that bent and angular, 836 00:42:34 --> 00:42:37 they're the same geometry. 837 00:42:37 --> 00:42:41 Let's look at another example, s f 4. 838 00:42:41 --> 00:42:43 And here it is drawn here. 839 00:42:43 --> 00:42:49 What is the formula type for s f 4? 840 00:42:49 --> 00:42:52 I think I heard it, a x 4 e. 841 00:42:52 --> 00:42:55 And the geometry? 842 00:42:55 --> 00:42:56 Seesaw, good. 843 00:42:56 --> 00:42:59 All right, we never get seesaw wrong, that's 844 00:42:59 --> 00:43:02 the easy give-away one. 845 00:43:02 --> 00:43:05 All right, what about b r f 3? 846 00:43:05 --> 00:43:08 This is the shape here. 847 00:43:08 --> 00:43:13 What is the geometry for b f 3 -- the original geometry, 848 00:43:13 --> 00:43:17 not the new geometry here? 849 00:43:17 --> 00:43:18 What was that? 850 00:43:18 --> 00:43:19 STUDENT: T. 851 00:43:19 --> 00:43:21 PROFESSOR: T-shaped, that's right. 852 00:43:21 --> 00:43:30 B f 3 is T-shaped. 853 00:43:30 --> 00:43:35 All right, so let's look at xenon f 2. 854 00:43:35 --> 00:43:39 That's going to have a formula type of a x 2 e 3, if 855 00:43:39 --> 00:43:43 we actually look at the Lewis structure here. 856 00:43:43 --> 00:43:47 So if you think about what the geometry is, and you should 857 00:43:47 --> 00:43:49 just be able to look at this and see, what would you say 858 00:43:49 --> 00:43:52 the geometry of xenon f 2 is? 859 00:43:52 --> 00:43:53 STUDENT: Linear. 860 00:43:53 --> 00:43:55 PROFESSOR: Linear, good. 861 00:43:55 --> 00:43:55 All right. 862 00:43:55 --> 00:44:01 And let's try one more here, which is a x 4 e 2, or a 863 00:44:01 --> 00:44:06 xenon f 4 -- very explosive, but a good example. 864 00:44:06 --> 00:44:08 STUDENT: Square planar. 865 00:44:08 --> 00:44:10 PROFESSOR: Square planar, all right, great. 866 00:44:10 --> 00:44:14 So, in general, when we think about vsper theory, even though 867 00:44:14 --> 00:44:18 it doesn't talk about or tell us anything about the energies 868 00:44:18 --> 00:44:21 of these different shapes, it's very useful in making a first 869 00:44:21 --> 00:44:24 approximation and coming very close to thinking about what 870 00:44:24 --> 00:44:26 the actual shapes of molecules are. 871 00:44:26 --> 00:44:28 So one thing I want to point out in terms of what you're 872 00:44:28 --> 00:44:32 responsible for is you should be able to fill out one of 873 00:44:32 --> 00:44:37 these charts with only seeing the labels up here -- tell us 874 00:44:37 --> 00:44:40 the formula type, tell us the steric number, tell us the 875 00:44:40 --> 00:44:43 geometry, you should be able to draw the Lewis structure, you 876 00:44:43 --> 00:44:47 already know how to do that, and also talk about the angles. 877 00:44:47 --> 00:44:50 So, as you finish up your problem-set, you can, I 878 00:44:50 --> 00:44:53 think now, get through just about all of it. 879 00:44:53 --> 00:44:55 You can do now this part with the geometry. 880 00:44:55 --> 00:44:57 So, we'll see you on Wednesday -- we're going to get out 881 00:44:57 --> 00:44:59 a little bit early today.