WEBVTT 00:02.867 --> 00:05.427 J. MICHAEL MCBRIDE: We're talking about alkenes, 00:05.433 --> 00:06.433 and in particular today, 00:06.433 --> 00:08.773 we're going to be continuing to talk about 00:08.767 --> 00:12.867 polymerization in the context of things that nature makes, 00:12.867 --> 00:14.897 which are called isoprenoids. 00:14.900 --> 00:17.770 And then we'll, talk about the properties of polymers, and 00:17.767 --> 00:18.997 then go onto acetylenes. 00:21.167 --> 00:25.127 We're talking about electrophiles being the things 00:25.133 --> 00:28.673 that attack double bonds, in particular, ones where the 00:28.667 --> 00:32.797 electrophile is carbon, like the sigma-star of something 00:32.800 --> 00:36.300 with a leaving group on it, or with an electronegative group 00:36.300 --> 00:38.800 on it, or a cation. 00:38.800 --> 00:40.400 We're talking about the synthesis of 00:40.400 --> 00:43.730 terpenes and steroids. 00:43.733 --> 00:48.533 There's a key molecule called isopentenyl pyrophosphate: 00:48.533 --> 00:52.133 pyrophosphate is this thing with two phospohorouses linked 00:52.133 --> 00:54.633 by an oxygen. 00:54.633 --> 00:57.473 It turns out you can do an allylic rearrangement I mean, 00:57.467 --> 01:02.297 when I say you, I mean you I mean the enzymes in you do it. 01:02.300 --> 01:07.370 So you can get from isopentenyl pyrophosphate to 01:07.367 --> 01:11.767 dimethylallyl pyrophosphate, which you know involves a 01:11.767 --> 01:15.997 shift of hydrogen allylically. 01:16.000 --> 01:18.970 Now, the reason you do that is to make it 01:18.967 --> 01:21.127 better for SN2 reactions. 01:21.133 --> 01:23.733 If you look at the relative rate for displacement of 01:23.733 --> 01:28.133 chloride by iodide in acetone, if you call it 1 for propyl 01:28.133 --> 01:31.603 chloride, if you make allyl chloride, so you put a double 01:31.600 --> 01:35.230 bond to make the carbon-chlorine bond allylic, 01:35.233 --> 01:39.033 it goes up by almost a 100 fold, a factor of 90. 01:39.033 --> 01:42.173 And, if it's a benzene ring instead of just a regular old 01:42.167 --> 01:47.227 double bond, it goes up another factor of 2.5. 01:47.233 --> 01:51.803 So, apparently this adjacent unsaturation, that new double 01:51.800 --> 01:58.070 bond, is especially good in stabilizing at 01:58.067 --> 01:59.267 the transition state. 01:59.267 --> 02:02.767 So at the time when chloride is leaving, and iodide is 02:02.767 --> 02:06.427 coming in, the double bond is able to interact with that 02:06.433 --> 02:09.273 orbital that's bonding simultaneously, that 02:09.267 --> 02:12.567 pentavalent carbon transition state. 02:12.567 --> 02:16.697 So both SN2 as well as SN1, where you draw a resonance 02:16.700 --> 02:20.230 structure for that kind of interaction, for the cation. 02:20.233 --> 02:22.373 It's good to have it allylic. 02:22.367 --> 02:25.797 Notice that this shift, this allylic shift from isopentenyl 02:25.800 --> 02:29.570 pyrophosphate to dimethylallyl pyrophosphate, 02:29.567 --> 02:33.297 resulted in getting the leaving group to be allylic, 02:33.300 --> 02:35.400 so it'll be easier to displace. 02:35.400 --> 02:37.600 Why do you do that? 02:37.600 --> 02:40.270 So you can take as a nucleophile then, if you have 02:40.267 --> 02:42.827 this really good leaving group, you can take as a 02:42.833 --> 02:46.403 nucleophile the isopentenyl pyrophosphate, so it's a 02:46.400 --> 02:49.700 carbon that's serving as the nucleophile. 02:49.700 --> 02:55.830 So you make a new bond, get a cation, but the cation can be 02:55.833 --> 02:58.873 lost by losing a proton next door to make a new double 02:58.867 --> 03:04.027 bond, which you'll notice is a new allylic functionality with 03:04.033 --> 03:05.373 a leaving group. 03:05.367 --> 03:08.097 So you put two of these things together, and you have the 03:08.100 --> 03:12.030 same kind of functionality you had at the beginning. 03:12.033 --> 03:15.673 That thing is called geranyl pyrophosphate, because it's 03:15.667 --> 03:19.727 related to geraniums, as well see. 03:19.733 --> 03:22.973 Notice that the starting material has five carbons, now 03:22.967 --> 03:26.897 having put two together, you have ten. 03:26.900 --> 03:32.070 So if you take geranyl pyrophosphate, and isomerize 03:32.067 --> 03:35.867 the double bond, then it's in a position for the other 03:35.867 --> 03:38.027 double bond in the same molecule to be the 03:38.033 --> 03:39.703 nucleophile. 03:39.700 --> 03:43.630 So you can do that, and get that cation. 03:43.633 --> 03:45.573 What could that cation do? 03:45.567 --> 03:47.697 Well, of course it could do the same thing it did before, 03:47.700 --> 03:53.400 and lose a proton, and that gives limonene, and I made it 03:53.400 --> 03:55.130 sort of yellow-green, you know why? 03:58.467 --> 04:00.797 To make it a little bit like lemons or limes. 04:00.800 --> 04:06.170 Lemon grass oil has limonene in it, Asian cooking has that. 04:06.167 --> 04:08.897 If you bend it up, then you can see another possibility. 04:08.900 --> 04:12.170 What other possibility do you see if I draw the conformation 04:12.167 --> 04:13.267 more like that? 04:13.267 --> 04:14.697 Anurag you got an idea? 04:14.700 --> 04:16.370 STUDENT: Bridging? 04:16.367 --> 04:20.727 PROFESSOR: Right, that double bond could add to the vacant 04:20.733 --> 04:26.103 orbital on the carbon, like that, a Markovnikov addition. 04:26.100 --> 04:27.670 Why is it not so good, Anurag? 04:27.667 --> 04:30.397 STUDENT: Steric. For steric reasons. 04:30.400 --> 04:33.370 PROFESSOR: It forms a four-membered ring, so there's 04:33.367 --> 04:35.267 going to be some ring strain there. 04:35.267 --> 04:37.597 But it does happen. 04:37.600 --> 04:40.270 If you lose the proton now, you get a compound called 04:40.267 --> 04:41.267 beta-pinene. 04:41.267 --> 04:43.167 Guess what kind of trees that comes from? 04:46.567 --> 04:49.127 Now you could also do an anti-Markovnikov addition, 04:49.133 --> 04:52.933 which doesn't form quite as strained a ring. 04:52.933 --> 04:56.403 If you do that, and then add water, and lose a proton to 04:56.400 --> 05:03.170 the carbon cation, you get after oxidation, camphor. 05:03.167 --> 05:05.767 Now all these things have in common that they have very 05:05.767 --> 05:08.967 characteristic odors, the lemon, the 05:08.967 --> 05:12.627 pinene, turpentine, camphor. 05:12.633 --> 05:18.003 So these things are called terpenes, essential oils. 05:18.000 --> 05:22.700 Terpene is from turpentine, that's where pinene is the 05:22.700 --> 05:24.870 major constituent of turpentine. 05:24.867 --> 05:28.497 Essential oils doesn't mean you need them, what it means 05:28.500 --> 05:32.000 is they have an essence, they have an odor, so these C10's 05:32.000 --> 05:34.800 are volatile, and your nose responds to them. 05:34.800 --> 05:40.170 These are essential oils, and notice they're C10's, 05:40.167 --> 05:41.667 terpenes are C10's. 05:41.667 --> 05:43.767 Now you can do more of the same, if you start with your 05:43.767 --> 05:47.027 geranyl pyrophosphate that you had before, you could add 05:47.033 --> 05:50.073 another isopentenyl pyrophosphate, and get a thing 05:50.067 --> 05:53.697 that's called farnesyl pyrophosphate. 05:53.700 --> 05:56.700 And things that come from that, like the compounds that 05:56.700 --> 06:00.970 came from the C10, were called terpenes, these are called 06:00.967 --> 06:01.527 sesquiterpenes. 06:01.533 --> 06:03.103 Do you know what sesqui- means? 06:03.100 --> 06:04.770 Do you know what a sesquicentennial is? 06:09.567 --> 06:11.567 Sesqui-, anybody? 06:11.567 --> 06:14.397 It means one and a half, so it's one and a 06:14.400 --> 06:16.230 half terpenes, 15. 06:16.233 --> 06:19.373 It's actually three of the isopentenyl pyrophosphates, 06:19.367 --> 06:23.727 but the name terpene came first. So, for example, 06:23.733 --> 06:27.273 caryophyllene, which is in clove, and hemp, and rosemary, 06:27.267 --> 06:30.497 is a sesquiterpene. 06:30.500 --> 06:33.170 Now, two of those things can come together, and now this 06:33.167 --> 06:36.867 reaction is different, because it's not the double bond 06:36.867 --> 06:40.627 attacking, it's two of these allylic positions coupling 06:40.633 --> 06:41.273 with one another. 06:41.267 --> 06:42.967 It's a different kind of reaction, we're not going to 06:42.967 --> 06:44.027 talk about it. 06:44.033 --> 06:46.533 But, it couples to give this, which is squalene, 06:46.533 --> 06:50.103 or shark liver oil. 06:50.100 --> 06:52.600 You see the new bond in the middle, it's symmetric about 06:52.600 --> 06:53.900 the middle. 06:53.900 --> 06:56.370 You can do interesting things with squalene. 06:56.367 --> 06:58.127 What kind of terpene would you call it 06:58.133 --> 06:59.373 incidentally, squalene? 07:03.367 --> 07:07.267 If you take two 15's, you get 30. 07:07.267 --> 07:08.497 So, what are you going to call it? 07:08.500 --> 07:14.670 Is it a monoterpene, sesquiterpene, diterpene? 07:14.667 --> 07:17.767 It's a triterpene, two one and a half's is three, so this is 07:17.767 --> 07:19.267 a triterpene. 07:19.267 --> 07:22.727 They're a lot more important triterpenes, well more 07:22.733 --> 07:24.873 important, they come from this, so this one is a 07:24.867 --> 07:26.567 starting point. 07:26.567 --> 07:28.897 There's squalene, and I'm going to twist it around into 07:28.900 --> 07:31.400 a special conformation like that. 07:31.400 --> 07:33.130 What allows me to do that? 07:33.133 --> 07:37.573 Enzymes that hold these things in particular shapes. 07:37.567 --> 07:40.427 Now, I'm going to react it with oxygen and do an 07:40.433 --> 07:43.833 epoxidation, of course it's not just O, it's something 07:43.833 --> 07:46.033 like the reagents we talked about, except it's done 07:46.033 --> 07:47.103 biologically. 07:47.100 --> 07:50.200 But, we're not interested in the particulars of the 07:50.200 --> 07:53.630 biological reagent, but it makes an expoxide. 07:53.633 --> 07:57.573 Notice that it's able to do it selectively, it gets only that 07:57.567 --> 07:59.297 double bond among all those. 08:02.000 --> 08:06.630 Now you protonate that, and what will it do? 08:06.633 --> 08:08.703 Can you see a way this can become more stable? 08:08.700 --> 08:09.970 What makes it unstable? 08:16.500 --> 08:16.800 Chris? 08:16.800 --> 08:17.770 STUDENT: Ring strain. 08:17.767 --> 08:18.897 PROFESSOR: The ring strain. 08:18.900 --> 08:21.370 How do you get rid of the ring strain? 08:21.367 --> 08:24.067 Which way are you going to break it? 08:24.067 --> 08:26.467 STUDENT: Which side? 08:26.467 --> 08:29.627 PROFESSOR: Yeah, you're either going to break this bond, or 08:29.633 --> 08:31.373 this bond, or this bond. 08:31.367 --> 08:32.427 One, two, three. 08:32.433 --> 08:33.173 STUDENT: Two. 08:33.167 --> 08:35.427 PROFESSOR: Two, why two? 08:35.433 --> 08:38.133 STUDENT: Better stability. 08:38.133 --> 08:39.033 PROFESSOR: Pardon me? 08:39.033 --> 08:39.873 STUDENT: Better stability. 08:39.867 --> 08:43.297 PROFESSOR: Why better stability? 08:43.300 --> 08:45.270 Notice that what's happened, these electrons that are 08:45.267 --> 08:47.167 shared between O and carbon are going to 08:47.167 --> 08:48.027 come onto the oxygen. 08:48.033 --> 08:50.433 STUDENT: It's a better carbocation. 08:50.433 --> 08:52.833 PROFESSOR: Right, it's a tertiary carbon cation if you 08:52.833 --> 08:53.803 break it your way. 08:53.800 --> 08:56.130 OK, so we'll do it that way. 08:56.133 --> 08:57.203 Now, what's that going to do? 08:57.200 --> 08:58.430 Anybody got an idea? 09:03.633 --> 09:05.233 Natalie, what have we been looking at 09:05.233 --> 09:08.733 attacking double bonds? 09:08.733 --> 09:11.033 We're looking at carbon cations attacking double 09:11.033 --> 09:12.733 bonds, any possibilities you see? 09:12.733 --> 09:15.333 STUDENT: It could attack the double bonds. 09:15.333 --> 09:17.073 PROFESSOR: OK. 09:17.067 --> 09:19.567 Now, you're going to form a bond between this cation and 09:19.567 --> 09:21.197 either the bottom one or the top one. 09:21.200 --> 09:24.100 Which one do you want to go to, bottom or top, if you have 09:24.100 --> 09:25.530 a cation adding to an alkene? 09:25.533 --> 09:29.073 STUDENT: Bottom. 09:29.067 --> 09:32.167 PROFESSOR: Why bottom? 09:36.667 --> 09:38.967 Who would say that's a good idea? 09:38.967 --> 09:39.767 STUDENT: Markovnikov. 09:39.767 --> 09:42.467 PROFESSOR: Markovnikov would say it's a good idea. 09:42.467 --> 09:45.697 You're right, so we do a Markovnikov addition. 09:45.700 --> 09:46.970 What happens next? 09:49.433 --> 09:50.503 Noelle, you got an idea? 09:50.500 --> 09:52.200 STUDENT: The same. 09:52.200 --> 09:53.830 PROFESSOR: Do the same thing again, do 09:53.833 --> 09:55.073 another Markovnikov addition. 09:58.667 --> 09:59.897 What's next? 10:06.500 --> 10:10.330 Wrong, this time it's an anti-Markovnikov addition, 10:10.333 --> 10:14.973 because of the way the molecule is held there. 10:14.967 --> 10:16.797 But, now you're set up to do another one, 10:16.800 --> 10:18.070 a Markovnikov one. 10:22.167 --> 10:24.867 Now you might think you'd use that last double bond and make 10:24.867 --> 10:28.127 another ring, since you're on a roll here. 10:28.133 --> 10:31.103 But you don't, and the reason is the way the molecule is 10:31.100 --> 10:33.700 held, so that that one's not in position to do the trick. 10:33.700 --> 10:34.970 But, something else happens. 10:37.300 --> 10:41.270 Notice that's nice and stable, it's a tertiary cation, but 10:41.267 --> 10:43.397 there's another tertiary cation available. 10:43.400 --> 10:46.400 Do you see how to get to it? 10:46.400 --> 10:47.670 Sebastian, you got an idea? 10:50.000 --> 10:53.600 How can I get from this cation to a different tertiary 10:53.600 --> 10:55.370 cation, one that would be just as stable. 10:55.367 --> 10:56.567 STUDENT: Hydride shift. 10:56.567 --> 10:58.927 PROFESSOR: Hydride shift, good. 10:58.933 --> 11:02.673 So that's tertiary, but we can do a hydride shift, and get 11:02.667 --> 11:03.897 another tertiary one. 11:07.867 --> 11:11.627 Any ideas for the next one? 11:11.633 --> 11:14.273 Lauren, you got an idea? 11:14.267 --> 11:16.127 We've got a nice stable tertiary cation. 11:16.133 --> 11:18.533 We can go back there, probably. 11:18.533 --> 11:21.203 Any other possibilities of getting it a tertiary one? 11:21.200 --> 11:22.130 STUDENT: The one to the left. 11:22.133 --> 11:24.803 PROFESSOR: The one on the left, you could do another hydride 11:24.800 --> 11:28.370 shift. So that's tertiary, too. 11:28.367 --> 11:29.997 What's next? 11:30.000 --> 11:31.070 Arvind, you got an idea? 11:31.067 --> 11:33.997 STUDENT: You might as well just do the methide. 11:34.000 --> 11:36.370 PROFESSOR: Yeah, the methide shift. 11:36.367 --> 11:38.027 OK, good. 11:38.033 --> 11:40.473 Next, Antonia? 11:40.467 --> 11:42.267 STUDENT: Another methide shift. 11:42.267 --> 11:44.827 PROFESSOR: Another methide shift, we're 11:44.833 --> 11:46.303 really going to town. 11:46.300 --> 11:46.900 Now what? 11:46.900 --> 11:49.630 STUDENT: Hydride shift. 11:49.633 --> 11:54.173 PROFESSOR: Hydride shift, obviously, right? 11:54.167 --> 11:56.527 Wrong. 11:56.533 --> 12:01.373 This time the proton just gets lost, so it goes that way, the 12:01.367 --> 12:04.727 arrow, makes a double bond and the proton goes away. 12:07.400 --> 12:12.230 Now that stuff is lanosterol and that's the source of 12:12.233 --> 12:14.503 cholesterol and all the steroid hormones. 12:14.500 --> 12:16.830 Remember these few things we showed last time that Barton 12:16.833 --> 12:19.103 was looking at, all these steroids? 12:19.100 --> 12:21.470 They have a 6-membered, 6-membered, 6-membered ring 12:21.467 --> 12:23.027 and then a 5-membered ring, and something 12:23.033 --> 12:24.533 substituted at the top. 12:24.533 --> 12:25.773 They all come from here. 12:31.467 --> 12:34.567 These are triterpenes. 12:34.567 --> 12:41.467 Now that's a cute story, but one wonders whether it's true? 12:41.467 --> 12:44.997 We'll show how you know it's true, once we get to NMR 12:45.000 --> 12:49.100 spectroscopy, the week after next, probably. 12:49.100 --> 12:51.870 But, you'll have to hold your breath until then. 12:51.867 --> 12:55.227 So, these isoprenes then, you can think about rearranging 12:55.233 --> 12:57.833 those double bonds. 12:57.833 --> 13:01.473 Notice that isopentenyl pyrophosphate has this C5 unit 13:01.467 --> 13:05.697 in, but if you do that you can make two new bonds. 13:05.700 --> 13:08.300 I'm not talking about cation/anion mechanisms, just 13:08.300 --> 13:10.770 schematic here. 13:10.767 --> 13:13.027 If you have two isoprenes you could link it together like 13:13.033 --> 13:18.933 that, and that's geraniol, rose oil, or you can have two 13:18.933 --> 13:20.573 isoprenes and put them together like 13:20.567 --> 13:24.267 that, and that's menthol. 13:24.267 --> 13:26.927 Or, you could have of four of them, and put them together 13:26.933 --> 13:31.433 like that, and that's retinal, the aldehyde over on the 13:31.433 --> 13:34.873 right, so that's the thing that's involved in vision, or 13:34.867 --> 13:38.497 that absorbs light in your eyes. So it's a tetramer. 13:38.500 --> 13:43.370 Or, you can take 30,000 isoprenes, and link them 13:43.367 --> 13:47.167 together like that, and that's latex. 13:47.167 --> 13:50.167 The Latin word latex just meant a liquid, but now it's 13:50.167 --> 13:52.797 always applied to this particular liquid, the sap 13:52.800 --> 13:56.900 that runs out of these trees, the Hevea brasiliensis. 13:56.900 --> 14:01.170 There's a real romantic story about this, and how explorers 14:01.167 --> 14:04.967 stole seeds from Brazil, which was trying to keep them, and 14:04.967 --> 14:07.627 took them to England and grew them in Kew, and then sent 14:07.633 --> 14:11.573 them to southeast Asia to make rubber plantations like this. 14:11.567 --> 14:15.997 The rubber plantations in Brazil failed because once you 14:16.000 --> 14:18.630 put the trees all close together, so that you can 14:18.633 --> 14:21.273 harvest them efficiently, rather than sending slaves out 14:21.267 --> 14:24.797 into the Amazon to find a tree here and a tree there, then it 14:24.800 --> 14:27.730 turned out they got a disease that killed all the ones down 14:27.733 --> 14:28.973 in South America. 14:28.967 --> 14:31.967 Henry Ford had a real disaster with that. 14:31.967 --> 14:35.027 Anyhow, you get that sap, and we showed you the sap last 14:35.033 --> 14:40.403 time, and we made a rubber ball with it like this. 14:40.400 --> 14:43.800 Now, that stuff was called by the Indians, caoutchouc, and 14:43.800 --> 14:48.130 in fact that's the name for rubber in French. 14:48.133 --> 14:51.403 So people tried to find a use for it in the west, you know 14:51.400 --> 14:54.570 the natives made balls, they made galoshes, you know you 14:54.567 --> 14:56.897 put your foot down in latex and let it dry, and then 14:56.900 --> 14:59.800 you've got a rubber boot. 14:59.800 --> 15:02.900 There were lots of things, but it was difficult to work with. 15:02.900 --> 15:05.970 Thomas Hancock in England developed a thing called a 15:05.967 --> 15:09.597 "masticator" to try to chew it up to make it so you could 15:09.600 --> 15:11.400 make different things with it. 15:11.400 --> 15:17.270 Charles Macintosh in Scotland in 1823 sandwiched the rubber 15:17.267 --> 15:20.127 between cloth layers to make waterproofs, you know that's 15:20.133 --> 15:22.373 what you call a waterproof jacket in 15:22.367 --> 15:23.597 Britain is a Macintosh. 15:25.967 --> 15:29.527 The reason you had to sandwich it, was because the stuff is 15:29.533 --> 15:31.173 really sticky. 15:31.167 --> 15:33.927 I had a piece I was going to bring that I made last time, 15:33.933 --> 15:36.633 and I forgot to, but it's really sticky stuff. 15:36.633 --> 15:39.403 So, that's not so good. 15:39.400 --> 15:43.370 It gets really gooey in the heat, and it's brittle in the 15:43.367 --> 15:46.097 cold, so there's a fairly narrow temperature range where 15:46.100 --> 15:47.900 it's useful. 15:47.900 --> 15:53.900 Until Charles Goodyear in 1839 figured out the process of 15:53.900 --> 15:59.830 vulcanization, which made this stuff useful. 15:59.833 --> 16:03.573 This is his own description from his autobiography in 1855 16:03.567 --> 16:06.227 of the discovery of vulcanization. 16:06.233 --> 16:10.033 He actually, he was really nuts about rubber. 16:10.033 --> 16:14.333 Incidentally, the word rubber was coined by Priestley, the 16:14.333 --> 16:17.803 guy who discovered oxygen. 16:17.800 --> 16:18.670 You know why? 16:18.667 --> 16:21.427 Because you use it for an eraser to rub out pencil 16:21.433 --> 16:22.933 marks, so "rubber." 16:25.733 --> 16:29.503 Anyhow, Goodyear really loved rubber. 16:29.500 --> 16:32.770 In this book, he printed several copies on paper, but 16:32.767 --> 16:36.127 other copies he printed on rubber, which is now all stuck 16:36.133 --> 16:39.603 together and you can't open any of the pages. 16:39.600 --> 16:42.270 Anyhow, in describing his discovery, he said "he," that 16:42.267 --> 16:45.867 is himself, speaking in the third person here. "He was 16:45.867 --> 16:49.797 surprised to find that the specimen, being carelessly 16:49.800 --> 16:52.900 brought into contact with a hot stove, charred like 16:52.900 --> 16:55.900 leather." He was trying to figure some way to make it not 16:55.900 --> 16:58.670 be sticky, so he thought you could put a powder on it to 16:58.667 --> 16:59.867 keep it from being sticky. 16:59.867 --> 17:04.127 The powder he was trying was sulfur, but he was in Woburn, 17:04.133 --> 17:06.503 Mass, when he was doing this, and he dropped it on a hot 17:06.500 --> 17:11.630 stove, and it charred, whereas usually it melted. 17:11.633 --> 17:15.273 This surprised him, so he got his friends and relations. 17:15.267 --> 17:17.327 It says, "He endeavoured to call the attention of his 17:17.333 --> 17:20.473 brother, as well as some other individuals who were present, 17:20.467 --> 17:22.897 and who were acquainted with the manufacturer of gum 17:22.900 --> 17:26.930 elastic, to this effect, as remarkable, and unlike any 17:26.933 --> 17:29.733 before known, since gum elastic always melted when 17:29.733 --> 17:33.103 exposed to a high degree of heat. 17:33.100 --> 17:37.030 The occurrence did not at the time seem to them to be worthy 17:37.033 --> 17:40.933 of notice; it was considered as one of the frequent appeals 17:40.933 --> 17:43.633 that he was in the habit of making, in behalf of some new 17:43.633 --> 17:48.703 experiment." So, they're tired of all his inventions. 17:48.700 --> 17:52.730 But, he showed it to Benjamin Silliman at Yale. 17:52.733 --> 17:55.703 And Silliman wrote, "Having seen experiments made, and 17:55.700 --> 17:58.500 also performed them myself, with the India rubber prepared 17:58.500 --> 18:02.130 by Mr. Charles Goodyear, I can state that it does not melt, 18:02.133 --> 18:05.033 but rather chars, by heat, and that it does not stiffen by 18:05.033 --> 18:08.333 cold, but retains its flexibility with cold, even 18:08.333 --> 18:12.273 when laid between cakes of ice." So this is what 18:12.267 --> 18:15.697 Goodyear, who didn't have much money, needed in terms of a 18:15.700 --> 18:19.370 serious endorsement, to go out and try to get some capital to 18:19.367 --> 18:20.267 commercialize this. 18:20.267 --> 18:22.667 So, he got a patent, and this is the 18:22.667 --> 18:23.897 beginnings of the patent. 18:23.900 --> 18:27.470 He says, he claims "combining said gum with sulphur and with 18:27.467 --> 18:30.127 white lead, so as to form a triple compound," the lead's 18:30.133 --> 18:34.333 not used anymore, "in combination with the forgoing, 18:34.333 --> 18:37.533 the process of exposing the india-rubber fabric, to the 18:37.533 --> 18:39.973 action of a high degree of heat." He had made a whole 18:39.967 --> 18:42.797 bunch of raincoats, sort of Macintosh style, and had put 18:42.800 --> 18:46.570 them in the closet during the summer to bring out when it 18:46.567 --> 18:49.227 got rainy again, and when he went to get them, they were 18:49.233 --> 18:51.073 just all one big gooey mass. 18:51.067 --> 18:54.327 So this was really appealing to him, to patent this, so he 18:54.333 --> 18:56.133 could make fabric from it. 18:56.133 --> 18:58.533 This is 1844. 18:58.533 --> 19:02.873 Seven years later was the Crystal Palace exhibition in 19:02.867 --> 19:07.097 London, the triumph of industry in the West, in the 19:07.100 --> 19:08.630 Victorian Period. 19:08.633 --> 19:11.973 There were these industrial displays of the best products 19:11.967 --> 19:14.927 from all over the world, and here only seven years after 19:14.933 --> 19:19.173 the patent, we find Goodyear's Vulcanite Court, where he's 19:19.167 --> 19:22.367 pushing the beauties of rubber. 19:22.367 --> 19:25.997 You see, there's a desk, and that desk is made of rubber, 19:26.000 --> 19:29.700 and it still exists, it's in a museum up in Waterbury. 19:29.700 --> 19:32.400 He made everything out of rubber, you can't believe all 19:32.400 --> 19:33.530 the things he made out of rubber. 19:33.533 --> 19:37.973 He proposed making bills for currency out of rubber, so 19:37.967 --> 19:40.897 that you could wash them without destroying them. 19:40.900 --> 19:43.730 A lot of these things didn't catch on. 19:43.733 --> 19:46.273 Anyhow, latex is this polymer. 19:46.267 --> 19:48.727 What do you do when you heat it with sulfur? 19:48.733 --> 19:51.233 Well, you have several chains next to one another, and no 19:51.233 --> 19:53.133 one knows exactly how it works. 19:53.133 --> 19:56.833 But, somehow it joins adjacent chains with sulfur to make 19:56.833 --> 19:59.873 cross-links between these polymer chains. 19:59.867 --> 20:02.367 Probably there are radical addition reactions to double 20:02.367 --> 20:05.797 bonds, radical allylic substitutions, so you get 20:05.800 --> 20:08.530 maybe this allylic radical, which attacks the sulfur, and 20:08.533 --> 20:10.873 then that adds to the other double bond, 20:10.867 --> 20:11.597 something like this. 20:11.600 --> 20:14.870 But, anyhow, it makes cross-links in the chains. 20:14.867 --> 20:16.667 How does that affect the physical 20:16.667 --> 20:18.027 properties of the polymers? 20:18.033 --> 20:20.703 Remember the problem is that it gets gooey when it's hot, 20:20.700 --> 20:22.670 and brittle when it's cold, which is not what 20:22.667 --> 20:24.727 you want in a coat. 20:24.733 --> 20:27.973 We're going to study this a little bit, and we're going to 20:27.967 --> 20:32.497 follow a really important scientist who 20:32.500 --> 20:34.170 published this in 1803. 20:34.167 --> 20:37.367 "A Description of a Property of Caoutchouc, or Indian 20:37.367 --> 20:40.897 Rubber; with some Reflections on the Cause of Elasticity of 20:40.900 --> 20:43.570 this Substance." So this was done by John 20:43.567 --> 20:46.897 Gough, who lived 1757-1825. 20:46.900 --> 20:48.630 He was a very interesting person. 20:48.633 --> 20:52.703 He tutored John Dalton for example, he lived up in the 20:52.700 --> 20:54.300 Lake District in England. 20:54.300 --> 20:58.500 And Wordsworth wrote of him in this poem, "Excursion, "No 20:58.500 --> 21:01.470 floweret blooms throughout the lofty range of these rough 21:01.467 --> 21:04.197 hills, nor in the woods, that could from him conceal its 21:04.200 --> 21:08.170 birth-place; none whose figure did not live upon his touch. " 21:08.167 --> 21:11.127 "Touch" because when Gough was three years old, he had 21:11.133 --> 21:12.633 rheumatic fever and went blind. 21:15.133 --> 21:17.873 He was a great scientist, though blind. So everything he 21:17.867 --> 21:21.867 had to do by touch, or description of someone else. 21:21.867 --> 21:26.897 So all these studies of rubber, he did blind. 21:26.900 --> 21:27.970 Here's the beginning of this. 21:27.967 --> 21:31.397 He lived in Middleshaw House, near Kendal in 1802. 21:31.400 --> 21:33.800 He said, "The substance called caoutchouc, or Indian Rubber, 21:33.800 --> 21:37.230 possesses a singular property; which, I believe has never 21:37.233 --> 21:40.033 been taken notice of in print, at least by any English 21:40.033 --> 21:46.803 Writer." So, here it goes on, and he says, "I made a piece 21:46.800 --> 21:50.070 of caoutchouc a little heavier than an equal bulk of water, 21:50.067 --> 21:52.527 the temperature of which was 45 degrees: the vessel 21:52.533 --> 21:55.933 containing the resin and water was then placed on the fire, 21:55.933 --> 21:59.233 and when the contents of it were heated to 130 degrees, 21:59.233 --> 22:02.233 the caoutchouc floated on the surface." 22:02.233 --> 22:06.503 What does that say about the density of Caoutchouc, and the 22:06.500 --> 22:07.800 influence of heat on it? 22:10.833 --> 22:11.203 Mary? 22:11.200 --> 22:12.070 STUDENT: It got less dense. 22:12.067 --> 22:12.967 PROFESSOR: Pardon me? 22:12.967 --> 22:13.827 STUDENT: It got less dense. 22:13.833 --> 22:17.203 PROFESSOR: It got less dense, right? 22:17.200 --> 22:19.100 Because you know water gets less dense 22:19.100 --> 22:20.570 when you heat it up. 22:20.567 --> 22:25.027 So, when it was cold it sank, when it was hot it floated. 22:25.033 --> 22:29.773 So it must expand, when you heat it, more than water does. 22:29.767 --> 22:33.327 Heating rubber makes it expand more than water. 22:33.333 --> 22:37.103 That's the first. That's not a great surprise, that's not the 22:37.100 --> 22:37.670 important thing. 22:37.667 --> 22:40.027 Experiment one is the important thing. 22:40.033 --> 22:44.873 Here he says, "Hold one end of the slip, of the rubber, thus 22:44.867 --> 22:48.097 prepared, between the thumb and forefinger of each hand; 22:48.100 --> 22:50.730 bring the middle of the piece into slight contact with the 22:50.733 --> 22:54.673 edges of the lips"; because he has to do it by touch, "taking 22:54.667 --> 22:57.097 care to keep it straight all the time, but not to stretch 22:57.100 --> 22:59.970 it much beyond its natural length: after taking these 22:59.967 --> 23:03.597 preparatory steps, extend the slip suddenly; and you will 23:03.600 --> 23:05.270 immediately perceive"-- something. 23:05.267 --> 23:07.397 We're going to repeat his thing, but I need a balloon. 23:10.600 --> 23:13.130 Everybody take their balloon, and we'll do what Gough says 23:13.133 --> 23:15.433 to do here. 23:15.433 --> 23:17.673 Hold it between your thumb and finger, and put it against 23:17.667 --> 23:22.367 your upper lip, and stretch it a little bit. 23:22.367 --> 23:25.197 Now, having stretched it a little bit, stretch it as hard 23:25.200 --> 23:28.230 as you can while it's against your lip. 23:28.233 --> 23:31.603 Do you notice anything? 23:31.600 --> 23:34.370 Who's as sensitive as John Gough here? 23:34.367 --> 23:37.397 What happened? 23:37.400 --> 23:39.830 What's really interesting is, you stretch it pretty far and 23:39.833 --> 23:44.203 not much happens, but the last inch makes a difference. 23:44.200 --> 23:46.330 You notice that? 23:46.333 --> 23:47.333 What happened, Lauren? 23:47.333 --> 23:48.933 STUDENT: It just got really warm. 23:48.933 --> 23:51.403 PROFESSOR: It got really warm, not in the first part of the 23:51.400 --> 23:53.870 stretching, but at the very end of the stretching. 23:53.867 --> 23:55.197 Is everybody on to that? 24:01.600 --> 24:04.700 Good, so that's what he noticed, and you can read all 24:04.700 --> 24:07.270 that he said here if you want to. 24:07.267 --> 24:10.367 Then, he tries another experiment here. 24:10.367 --> 24:14.067 He says, "If one end of a slip of Caoutchouc be fastened to a 24:14.067 --> 24:17.427 rod of metal or wood, and a weight be fixed to the other 24:17.433 --> 24:20.703 extremity, in order to keep it in a vertical position," then 24:20.700 --> 24:21.930 what will the thong do? 24:21.933 --> 24:23.473 So, let's try that one. 24:23.467 --> 24:26.697 Here's a piece of caoutchouc. 24:26.700 --> 24:28.370 We'll get it so you can see it here. 24:32.400 --> 24:33.800 There, you can see it a little bit, but 24:33.800 --> 24:35.030 we'll need some light. 24:44.333 --> 24:46.503 We'll put this on here. 24:46.500 --> 24:49.370 We're going to stretch it, that's what he says to do. 24:56.067 --> 25:00.097 I had more, when I tried this, I had more light before. 25:00.100 --> 25:02.500 Let's see if I can do better. 25:05.800 --> 25:11.730 I'll turn on this light, there we go, except that the thing 25:11.733 --> 25:13.733 is blocking it. 25:19.967 --> 25:22.197 I'm getting it into focus. 25:22.200 --> 25:25.030 So, I have lines on paper there, and you can see how 25:25.033 --> 25:26.303 much it's stretched. 25:31.067 --> 25:35.197 Now, what he's going to do is heat it. 25:35.200 --> 25:39.470 What does caoutchouc do when you heat it? 25:39.467 --> 25:41.197 What's going to happen to the weight? 25:41.200 --> 25:47.900 We see the weight just halfway down below one of those lines. 25:47.900 --> 25:50.570 What's going to happen when I heat it? 25:50.567 --> 25:53.767 I'll heat it with this thing. 25:53.767 --> 25:54.797 Which way will it move? 25:54.800 --> 25:56.930 STUDENT: It will move down. 25:56.933 --> 25:59.403 PROFESSOR: It'll expand, right. 25:59.400 --> 26:00.700 So, let's give it a try. 26:14.333 --> 26:16.603 Is it going down? 26:16.600 --> 26:17.870 Did it do anything? 26:20.600 --> 26:24.970 It may be that it's, let me slide it out a little bit 26:24.967 --> 26:27.667 here, so that it's not rubbing against the rod, there. 26:40.567 --> 26:41.597 What do you think? 26:41.600 --> 26:42.000 Derek? 26:42.000 --> 26:43.400 STUDENT: It shrivels. 26:43.400 --> 26:47.770 PROFESSOR: What's going to happen if I cool it off? 26:47.767 --> 26:49.067 STUDENT: It should go back. 26:49.067 --> 26:50.727 PROFESSOR: Let's watch. 26:57.233 --> 27:10.173 It's expanding when it cools, and it's contracting slowly 27:10.167 --> 27:11.397 when it heats. 27:14.567 --> 27:15.827 This is weird, right? 27:18.500 --> 27:22.000 It expands when it cools, and contracts when it heats. 27:25.367 --> 27:26.727 We're going to have to deal with that. 27:36.100 --> 27:38.200 Heating a tightly stretched piece of 27:38.200 --> 27:39.470 rubber makes it contract. 27:42.233 --> 27:47.033 If stretching rubber generates heat, that's what we did here. 27:47.033 --> 27:48.403 Let's think a second. 27:48.400 --> 27:50.830 What's going to happen when it contracts, if 27:50.833 --> 27:52.103 we let it go back? 27:55.000 --> 27:58.770 Well, we have several possibilities. 27:58.767 --> 28:01.667 It could be that the heat comes from friction of the 28:01.667 --> 28:04.267 molecules rubbing by one another. 28:04.267 --> 28:05.597 If that's so, what would happen 28:05.600 --> 28:08.230 when we let it contract? 28:08.233 --> 28:08.503 Karl? 28:08.500 --> 28:09.230 STUDENT: It heats either way. 28:09.233 --> 28:13.103 PROFESSOR: It will heat either way. 28:13.100 --> 28:17.100 On the other hand, if heat comes from some other cause, 28:17.100 --> 28:19.900 contraction may do the opposite and absorb heat. 28:19.900 --> 28:23.200 So, what we're going to do is stretch our rubber as far as 28:23.200 --> 28:26.230 we can and go like this to get it back to room temperature, 28:26.233 --> 28:28.873 while holding it tightly, tightly stretched because 28:28.867 --> 28:31.497 remember this phenomenon happens only at the very end. 28:31.500 --> 28:35.070 Then we're going to put it against our lip, and do that. 28:39.667 --> 28:42.667 What do you find? 28:42.667 --> 28:46.567 Which of these is it, one or two? 28:46.567 --> 28:48.197 Did it get hot when it contracted, 28:48.200 --> 28:49.470 or did it get cold? 28:51.967 --> 28:55.227 Both, did someone say? 28:55.233 --> 28:57.003 Cold, OK. 28:57.000 --> 28:59.900 That's pretty dramatic, isn't it? 28:59.900 --> 29:04.170 Gough claims to having discovered this, but in fact, 29:04.167 --> 29:05.797 I discovered it. 29:05.800 --> 29:08.470 My nephew was having a birthday, and I was blowing up 29:08.467 --> 29:10.067 balloons, and I happened to do that. 29:10.067 --> 29:11.627 I thought, wow, that's really something. 29:11.633 --> 29:12.033 [LAUGHTER] 29:12.033 --> 29:15.903 Then I found out I had been scooped by two centuries. 29:19.000 --> 29:20.000 How does this work? 29:20.000 --> 29:23.270 What the heck is going on with these polymers? 29:23.267 --> 29:26.897 Have you been to Yale Health Center? 29:26.900 --> 29:30.370 What do you see, if you go up in the Yale Health Center and 29:30.367 --> 29:31.597 look out toward the campus? 29:34.267 --> 29:40.097 This is what you see, a rather questionable view from the 29:40.100 --> 29:42.100 Yale Health Center. 29:42.100 --> 29:46.700 But, right here is something that's very interesting, and 29:46.700 --> 29:48.870 if you get up and look at it, that's the tomb of Charles 29:48.867 --> 29:52.827 Goodyear, because he was a native of New Haven, even 29:52.833 --> 29:56.133 though he discovered vulcanization in Woburn, Mass, 29:56.133 --> 29:59.403 and died in New York, a pauper incidentally. 29:59.400 --> 30:02.300 A pauper because he got involved with lawyers. People 30:02.300 --> 30:06.470 tried, everybody and his brother tried to break his 30:06.467 --> 30:09.797 patent. So he hired Daniel Webster, who was Secretary of 30:09.800 --> 30:14.670 State, and put on the most expensive defense that ever 30:14.667 --> 30:17.897 happened of a patent, and still he lost his shirt. 30:21.067 --> 30:26.367 He didn't know how it worked, but maybe he can find out now, 30:26.367 --> 30:30.367 because there he is, and if you go to that obelisk there, 30:30.367 --> 30:33.097 and then down just a little bit farther, you get to this 30:33.100 --> 30:37.330 guy. And that guy could tell him, probably, and 30:37.333 --> 30:40.833 conceivably, did talk to him, because this is J. Willard 30:40.833 --> 30:45.703 Gibbs who got his a Bachelor's degree at Yale in 1858, so two 30:45.700 --> 30:47.670 years before Goodyear died. 30:47.667 --> 30:50.997 So they could have met, but I don't think they did. 30:51.000 --> 30:53.000 Gibbs could have given him some idea about what was going 30:53.000 --> 30:53.930 on with rubber. 30:53.933 --> 30:55.973 But, the persons who really could have done 30:55.967 --> 30:58.497 it, are over here. 30:58.500 --> 31:03.070 Here are two gravestones that have different designs, 31:03.067 --> 31:05.427 they're both people from this Chemistry Department. 31:05.433 --> 31:08.773 One is John Gamble Kirkwood, who was a physical chemist and 31:08.767 --> 31:11.797 a theorist about polymers. 31:11.800 --> 31:15.970 His gravestone tells everything he did, not 31:15.967 --> 31:18.227 everything, but quite a few things, the awards 31:18.233 --> 31:20.273 he got and so on. 31:20.267 --> 31:24.297 Next to his, is the tomb of Lars Onsager, his friend and 31:24.300 --> 31:31.530 colleague, and his tomb says, Nobel Laureate. 31:31.533 --> 31:35.673 His wife died 15 years later, and the stone was recarved to 31:35.667 --> 31:38.667 put her name on it as well, and also to put on a footnote. 31:41.400 --> 31:44.670 [LAUGHTER] 31:44.667 --> 31:47.367 Perhaps they're communing down there with Goodyear now, and 31:47.367 --> 31:49.367 telling him what's going on with this. 31:49.367 --> 31:51.927 It has to do with statistical mechanics, you remember, 31:51.933 --> 31:55.373 that's what Gibbs did, because statistics is what makes 31:55.367 --> 31:58.567 rubber contract after you've stretched it. 31:58.567 --> 32:01.367 If you have a chain all stretched out like this, 32:01.367 --> 32:03.167 there's only one way to arrange it 32:03.167 --> 32:05.797 between the two ends. 32:05.800 --> 32:11.770 But, if you contract it, like that, you can have it that 32:11.767 --> 32:16.027 way, or that way, or that way, or that way, or that way. 32:16.033 --> 32:18.933 There's a zillion ways to have it if it's contracted, so 32:18.933 --> 32:21.933 statistics, entropy says, it wants to have a shorter 32:21.933 --> 32:25.103 distance between the ends. 32:25.100 --> 32:28.630 Even though it might be most stable, in terms of heat, that 32:28.633 --> 32:32.133 is, the best conformation when it's stretched out, entropy 32:32.133 --> 32:34.203 will contract it again. 32:34.200 --> 32:36.500 If you have a whole bunch of these polymer chains, then you 32:36.500 --> 32:40.400 start to stretch them out, they're all tangled around, 32:40.400 --> 32:44.500 and as you stretch, they get more and more parallel to one 32:44.500 --> 32:51.130 another, and finally, they'll get so near, some of them will 32:51.133 --> 32:55.473 get near one another enough, to crystallize, just locally. 32:55.467 --> 32:57.327 I mean, these aren't like diamonds, right. 32:57.333 --> 33:00.273 But, little local units of the thing are together, and when 33:00.267 --> 33:03.727 they come together like that, they're lower in energy, so 33:03.733 --> 33:05.333 they give off heat. 33:05.333 --> 33:08.203 So, when you stretch it, at first it doesn't make any 33:08.200 --> 33:11.800 difference, but the very last bit of the stretching causes 33:11.800 --> 33:15.970 this crystallization, and heat comes out. 33:15.967 --> 33:20.867 There's local crystallization which contributes rigidity and 33:20.867 --> 33:23.697 releases heat. 33:23.700 --> 33:29.400 On the other hand, if you put heat in, it will melt these 33:29.400 --> 33:32.130 crystals, and entropy will cause it to contract. 33:36.567 --> 33:39.327 So, then it will allow statistics to make the 33:39.333 --> 33:43.303 material contract like this, and you'll go backwards. 33:43.300 --> 33:46.730 So it's actually entropy that brings rubber back to its 33:46.733 --> 33:47.903 original shape. 33:51.000 --> 33:55.170 Fixed, irregular cross-links between adjacent chains 33:55.167 --> 33:59.367 prevents crystallization, so it doesn't become brittle when 33:59.367 --> 34:01.127 it's cold, because it doesn't crystallize. 34:05.100 --> 34:07.770 It also prevents it from being gooey when it's hot. 34:07.767 --> 34:08.767 How does that happen? 34:08.767 --> 34:13.067 The gooeyness has to do with the flowing of the molecules, 34:13.067 --> 34:15.467 but if you have something that's all tangled up like 34:15.467 --> 34:20.427 this, how can you move a molecule, if it's got a really 34:20.433 --> 34:22.073 complicated shape? 34:22.067 --> 34:23.897 The way it happens is very interesting. 34:23.900 --> 34:28.570 It's a process called reptation, like a reptile. 34:28.567 --> 34:31.897 This was an idea of a French physicist who got the Nobel 34:31.900 --> 34:35.000 Prize for this kind of thing. 34:35.000 --> 34:40.370 So, suppose you try to move the chain lengthwise, what 34:40.367 --> 34:43.767 will happen is, like a carpet, when you try to move a carpet, 34:43.767 --> 34:47.097 you make a little wiggle in it, a little hump. 34:47.100 --> 34:50.970 Then you could move the hump along, without moving anything 34:50.967 --> 34:54.567 else, and it would go on and on all along the chain, and 34:54.567 --> 34:57.227 finally get to the other end, where it would be like that, 34:57.233 --> 35:01.003 and finally the other end could move. 35:01.000 --> 35:05.970 So, it sort of crawls, or goes like a snake, through all this 35:05.967 --> 35:09.627 tangle of other polymers. 35:09.633 --> 35:13.833 What does vulcanization have to do with that? 35:13.833 --> 35:16.833 If you have cross-links between the chains, then they 35:16.833 --> 35:21.533 can't slide by one another, so you can't do reptation, so the 35:21.533 --> 35:26.103 stuff doesn't flow and become gooey when it's hot. 35:26.100 --> 35:28.900 That's what vulcanization does, so there's no flow when 35:28.900 --> 35:32.100 hot, and it inhibits crystallization, so it's not 35:32.100 --> 35:34.130 brittle when it's cold. 35:34.133 --> 35:37.503 There's also vulcanization that happens in the home. 35:37.500 --> 35:41.000 As I look around, I'm not sure that any of you have practiced 35:41.000 --> 35:43.970 this, but you've heard of it, I'm sure. 35:43.967 --> 35:46.727 Here's Robin Kelly. 35:46.733 --> 35:49.873 I got permission to use this by talking to Robin Kelly's 35:49.867 --> 35:52.367 mother, and she tells me that Robin Kelly is now a student 35:52.367 --> 35:55.597 at Harvard Law School, this was from some years ago. 35:55.600 --> 35:58.300 But, Robin Kelly did Irish dancing, and she wanted her 35:58.300 --> 36:01.600 hair to look like this, for Irish dancing. 36:01.600 --> 36:05.770 So, she vulcanized it. 36:05.767 --> 36:09.867 Hair has sulfide groups coming out, and they link to other 36:09.867 --> 36:13.967 chains, so it's cross-linked, it's vulcanized to begin with, 36:13.967 --> 36:17.667 that holds the chains next to one another in shape. 36:17.667 --> 36:20.667 So what she did first, was reduce the disulfide 36:20.667 --> 36:27.567 cross-links by excess basic thiol. 36:27.567 --> 36:30.727 These are the two thiols that are commonly used for that, 36:30.733 --> 36:35.003 and notice that the SH is much more acidic than hydroxide. 36:35.000 --> 36:38.100 So if you have basic RSH, you have RS-. 36:41.100 --> 36:45.370 So, that could do an SN2 reaction on sulfur and break 36:45.367 --> 36:50.067 that bond, and that one can attack another thiol, take its 36:50.067 --> 36:56.797 hydrogen, and then that thiolate anion can do another 36:56.800 --> 36:57.970 displacement. 36:57.967 --> 37:07.067 So what's happened, is that we've reduced the disulfide 37:07.067 --> 37:11.067 links to make it two RSH groups. 37:11.067 --> 37:15.297 You can do the same thing again, and again, and again, 37:15.300 --> 37:18.200 and now the chains are pliable, and can be moved past 37:18.200 --> 37:19.270 one another. 37:19.267 --> 37:22.667 So now she puts the soft spikes in, and curls her hair 37:22.667 --> 37:26.297 up tight, and changes it shape. 37:26.300 --> 37:29.170 Now she's got to make new cross-links again, to 37:29.167 --> 37:33.267 vulcanize it, so she's got it like this. 37:33.267 --> 37:36.567 Now she oxidizes the thiols back to disulfide with 37:36.567 --> 37:38.297 hydrogen peroxide. 37:38.300 --> 37:40.870 Now look at the bond dissociation energies. 37:40.867 --> 37:46.767 The OH-OH, remember, the O-O is a very weak bond, but the S-S 37:46.767 --> 37:52.597 bond is not so weak, and S-H bond is weak, compared to O-H. 37:52.600 --> 37:55.400 So, it's much favorable, to start on the left 37:55.400 --> 37:56.300 and go to the right. 37:56.300 --> 37:59.330 It's favorable by 30 kilocalories per mole. 37:59.333 --> 38:05.703 So, if she puts in peroxide, with thiol, she'll get out the 38:05.700 --> 38:09.170 new cross-links and alcohol. 38:09.167 --> 38:12.367 So you get the new cross-links holding the chains together in 38:12.367 --> 38:16.167 the shape you want them to be. So now her hair is vulcanized, 38:16.167 --> 38:19.467 and everything's fine, and she can go Irish dancing. 38:22.067 --> 38:24.697 Another way to hold the chains together with cross-links is 38:24.700 --> 38:27.000 to have ionic groups on them. 38:27.000 --> 38:29.900 These ions tend to get together with counterions in 38:29.900 --> 38:33.300 clusters, so that serves as a cross-link, but it's not a 38:33.300 --> 38:37.330 permanent cross-link the way covalent bonds are. 38:37.333 --> 38:40.673 It's possible, if you warm it, to break those, and make 38:40.667 --> 38:41.427 things flow. 38:41.433 --> 38:44.473 This is thermoplastic. You heat the polymer, and you can 38:44.467 --> 38:46.197 reshape it, then you cool it, and then 38:46.200 --> 38:48.770 it's got the new shape. 38:48.767 --> 38:51.567 So there's after warming, and they got malleable 38:51.567 --> 38:52.767 cross-links. 38:52.767 --> 38:57.797 This is Father Nieuwland at Notre Dame University, who 38:57.800 --> 39:01.600 figured out how to polymerize not the thing that had methyl 39:01.600 --> 39:04.770 up here, which was the biological isoprene unit, but 39:04.767 --> 39:08.367 the one that has chloro, so that gave a new compound 39:08.367 --> 39:11.167 called neoprene, which had different properties. 39:11.167 --> 39:15.267 They're all these different polymers that have been made, 39:15.267 --> 39:19.167 and this list of according to how much they cost, they get 39:19.167 --> 39:22.267 very expensive for fluorosilicone, how heat 39:22.267 --> 39:27.827 resistant they are, at low temperature, and the maximum 39:27.833 --> 39:29.603 temperature you can use them. 39:29.600 --> 39:34.100 Also, how they last, oxidation, whether they're 39:34.100 --> 39:37.500 subject to weathering by ozone, how strong they are. 39:37.500 --> 39:41.830 So, natural rubber is nice and strong, it resists abrasion 39:41.833 --> 39:46.333 and tear, it's very resilient, this is after cross-linking 39:46.333 --> 39:49.233 it, after vulcanization, but it's only fair on 39:49.233 --> 39:50.603 oxidation or ozone. 39:50.600 --> 39:52.930 You've probably seen rubber that's been around awhile, or 39:52.933 --> 39:55.903 old telephone lines, the rubber is falling off because 39:55.900 --> 39:58.330 ozone has gotten to it. 39:58.333 --> 40:01.973 They're others that have a variety of properties, and 40:01.967 --> 40:05.067 also an important one that's not listed here, is how they 40:05.067 --> 40:08.127 interact with oil or something like that. 40:08.133 --> 40:11.473 Natural rubber is very bad with oil, but some of these 40:11.467 --> 40:15.827 others, like neoprene is much better. 40:15.833 --> 40:20.303 Designing a tire for your car is a very complex thing, to 40:20.300 --> 40:23.100 mix different polymers to get exactly the properties in 40:23.100 --> 40:26.230 different parts of the tire that are necessary. 40:26.233 --> 40:29.533 There are other ways of doing polymerization as well, for 40:29.533 --> 40:32.533 example, styrene, you could imagine these double bonds 40:32.533 --> 40:34.873 going together with a radical doing addition 40:34.867 --> 40:36.327 to that double bond. 40:36.333 --> 40:39.703 Notice that as I drew it there, it's random which end 40:39.700 --> 40:43.070 of the styrene got attacked. But it would also be possible 40:43.067 --> 40:45.427 to imagine that it always goes head to tail, 40:45.433 --> 40:46.803 always the same way. 40:46.800 --> 40:48.930 Why would that be advantageous, 40:48.933 --> 40:50.203 always to go this way? 40:55.533 --> 40:58.103 Because in fact, that's what it does. 40:58.100 --> 41:00.330 It goes just head to tail. 41:00.333 --> 41:01.773 Why? 41:01.767 --> 41:04.427 Why don't you do additions like this, where you add to 41:04.433 --> 41:05.903 the center and leave that radical? 41:09.433 --> 41:11.433 Why do you want to leave this radical instead? 41:14.067 --> 41:14.497 Amy? 41:14.500 --> 41:15.330 STUDENT: It's secondary. 41:15.333 --> 41:20.403 PROFESSOR: It's secondary, but much more important, it's 41:20.400 --> 41:24.070 benzylic, it's like allylic. It has the double bonds to 41:24.067 --> 41:26.727 interact with the SOMO. 41:26.733 --> 41:28.973 For both reasons it does that, even if it were a methyl 41:28.967 --> 41:31.297 group, it would tend to do this, because it would be 41:31.300 --> 41:33.070 secondary the way Amy said. 41:33.067 --> 41:36.527 But with phenyl, it's ever so much more so, so that's 13 41:36.533 --> 41:42.303 kilocalories more stable, than adding the other way. 41:42.300 --> 41:44.300 There's also the question of tacticity, which 41:44.300 --> 41:47.000 we talked on before. 41:47.000 --> 41:48.700 You can have isotactic-- 41:48.700 --> 41:52.770 these are all head to tail, notice, the methyl group is on 41:52.767 --> 41:54.097 every other carbon. 41:54.100 --> 41:57.600 But you can have isotactic, syndiotactic, where isotactic 41:57.600 --> 42:00.730 they're all the same, syndiotactic they alternate, 42:00.733 --> 42:02.573 and atactic is random. 42:02.567 --> 42:07.627 That's what you get, the regiochemistry is always head 42:07.633 --> 42:10.503 to tail with the free radical, but the stereochemistry is 42:10.500 --> 42:12.570 random, so it's atactic. 42:12.567 --> 42:15.397 But, you can get isotactic polypropylene from 42:15.400 --> 42:20.200 Ziegler-Natta catalyst, that we talked about last lecture 42:20.200 --> 42:23.100 or the lecture before and syndiotactic, remember, if use 42:23.100 --> 42:26.970 this fancy Kaminsky catalyst, that comes from one side, then 42:26.967 --> 42:28.867 the other, then one side, and then the other, and it has a 42:28.867 --> 42:34.067 mirror plane, so you change the configuration each time. 42:34.067 --> 42:37.197 There's another question of selectivity when you have two 42:37.200 --> 42:40.330 different monomers in there, for example methyl 42:40.333 --> 42:44.133 methacrylate and styrene. So you could have block 42:44.133 --> 42:47.073 copolymers, where you have a bunch of one coming together, 42:47.067 --> 42:51.497 and then a bunch of the other one in a single chain. 42:51.500 --> 42:55.000 Another possibility is to have them strictly alternate, A B A 42:55.000 --> 43:00.300 B A B, and you can do that. And the reason you can do it, 43:00.300 --> 43:03.870 is because of the rates of adding different radicals to 43:03.867 --> 43:05.497 the alkenes. 43:05.500 --> 43:08.030 So, if you have that particular radical at the end 43:08.033 --> 43:13.533 of the chain, it adds to styrene faster than it adds to 43:13.533 --> 43:15.673 methyl methacrylate. 43:15.667 --> 43:18.497 This unit here was methyl methacrylate that added to a 43:18.500 --> 43:21.670 radical there, so now it's at the end of the chain, and it 43:21.667 --> 43:25.267 would prefer by a factor of two to one to react with its 43:25.267 --> 43:29.067 own kind, pardon me, to react with styrene rather than with 43:29.067 --> 43:31.627 it's own kind. 43:31.633 --> 43:33.573 What would you expect if you have 43:33.567 --> 43:37.467 styrene as the last radical? 43:37.467 --> 43:39.397 Notice what's interesting, here. 43:39.400 --> 43:42.170 I was wondering what this one was here, it's this. 43:48.600 --> 43:51.700 Now notice it's a factor of two in the other direction. 43:51.700 --> 43:53.300 One radical-- 43:53.300 --> 43:56.700 each radical, prefers to react with the other monomer, isn't 43:56.700 --> 43:57.330 that interesting. 43:57.333 --> 44:00.803 That's why it goes A B A B A B. Why should that be? 44:00.800 --> 44:04.970 This isn't surprising, because this radical is resonance- 44:04.967 --> 44:09.167 stabilized as Amy was telling us. 44:09.167 --> 44:11.927 That's the fastest of all these reactions, and it's a 44:11.933 --> 44:13.873 good radical, so that's reasonable. 44:13.867 --> 44:17.397 But why does this one prefer not to form the good radical, 44:17.400 --> 44:21.770 but to form what appears to be the less good radical? 44:21.767 --> 44:27.227 Here's our radical starting, and it can add to this, to 44:27.233 --> 44:31.103 give that, and that turns out it would be something like 44:31.100 --> 44:33.630 that, and it's 20 kilocalories per mole exothermic. 44:36.633 --> 44:43.303 If we react with the other alkene, then it gives this, 44:43.300 --> 44:45.900 which is less stable. 44:45.900 --> 44:48.930 This one doesn't have as much allylic or benzylic 44:48.933 --> 44:52.933 stabilization that you have down here. 44:52.933 --> 44:54.173 But, it's faster. 44:58.333 --> 45:00.973 So it's faster to go to the higher-energy product. That's 45:00.967 --> 45:04.027 not the Hammond postulate, right? 45:04.033 --> 45:05.103 Why? 45:05.100 --> 45:09.330 Why is it twice as fast to go to the less stable product? 45:09.333 --> 45:12.433 The transition state for one looks like that, the 45:12.433 --> 45:16.003 transition state for the other looks like that. 45:16.000 --> 45:19.030 What's good about the bottom one? 45:19.033 --> 45:22.473 There's the top one, there's the bottom one, and notice in 45:22.467 --> 45:28.627 this one the carbonyl is good at stabilizing high HOMOs. 45:28.633 --> 45:31.373 So, you could have a resonance structure with a minus charge 45:31.367 --> 45:33.797 here, and a plus charge here. 45:33.800 --> 45:36.970 You can't do that up here as well, this isn't specifically 45:36.967 --> 45:40.497 good at stabilizing anions. 45:40.500 --> 45:42.630 So you could have these resonance structures here, 45:42.633 --> 45:44.033 that you can't have there. 45:47.367 --> 45:49.797 So, the ionic resonance structure, you could say, 45:49.800 --> 45:52.470 stabilizes the transition state, or if you wanted to 45:52.467 --> 45:55.367 talk about it in terms of orbitals, you'd say that the C 45:55.367 --> 45:58.197 double bond O gives an unusually low LUMO, so that's 45:58.200 --> 46:02.470 good when the SOMO is not so low. 46:02.467 --> 46:06.027 This special stability applies only at the transition state. 46:06.033 --> 46:08.503 Once you've formed this bond, then you can't have a 46:08.500 --> 46:11.270 resonance structure like that. 46:11.267 --> 46:14.327 The product is not stabilized by that kind of thing, but the 46:14.333 --> 46:16.933 transition state is, which is what makes these things turn 46:16.933 --> 46:23.073 around and go A B A B A B. OK, now I was going to go on to 46:23.067 --> 46:26.067 acetylenes, but we'll do that next time.