WEBVTT 00:02.500 --> 00:06.330 J. MICHAEL MCBRIDE: So the second part of the last 00:06.333 --> 00:08.733 lecture introduced the idea of 00:08.733 --> 00:11.603 alkenes, and we were talking about the stability of 00:11.600 --> 00:17.670 alkenes, cis versus trans, and the effect of substitution. 00:17.667 --> 00:21.827 We'll go on with that today, and then go on to addition 00:21.833 --> 00:24.103 mechanisms, which is a characteristic 00:24.100 --> 00:28.100 reactivity of alkenes. 00:28.100 --> 00:33.730 So if we look at the stabilities of these C6 alkene 00:33.733 --> 00:36.703 isomers that we talked about last time, we showed how you 00:36.700 --> 00:42.430 can get the numbers for those from the NIST website. 00:42.433 --> 00:48.273 It's also possible to get them from this book by Pedley. 00:48.267 --> 00:51.967 That's the one that Professor Wiberg, who's a big expert on 00:51.967 --> 00:56.227 this, recommended to me as a good source. 00:56.233 --> 01:03.473 So here's the heat of formation of the straight 01:03.467 --> 01:08.597 chain with a double bond in the number one position. 01:08.600 --> 01:10.570 What does heat of formation, what's that the 01:10.567 --> 01:13.727 heat relative to? 01:13.733 --> 01:17.803 The energy of this molecule compared to what? 01:17.800 --> 01:20.600 The heat of formation. 01:20.600 --> 01:23.330 Rehearse from last semester. 01:23.333 --> 01:24.173 Mary? 01:24.167 --> 01:27.167 STUDENT: Is it the heat of atomization to-- 01:27.167 --> 01:29.127 PROFESSOR: It's not the heat of atomization. 01:29.133 --> 01:31.303 How do you measure such things? 01:31.300 --> 01:33.130 How do you measure the heat of a molecule? 01:33.133 --> 01:34.833 STUDENT: You have to heat it up-- 01:34.833 --> 01:36.373 PROFESSOR: Can't hear too well. 01:36.367 --> 01:37.197 STUDENT: You have to heat it up until it's 01:37.200 --> 01:38.430 really, really hot. 01:40.300 --> 01:42.370 PROFESSOR: That was the way that you got the heat of 01:42.367 --> 01:46.827 formation of carbon atoms from graphite, remember? 01:46.833 --> 01:52.273 But that only needed to be done once, to get where carbon 01:52.267 --> 01:55.167 atom is relative to graphite. 01:55.167 --> 01:58.727 Then you need to know the energy of molecules relative 01:58.733 --> 02:00.273 to graphite. 02:00.267 --> 02:02.667 How do you do that? 02:02.667 --> 02:03.527 STUDENT: You want to break the bonds in graphite and then 02:03.533 --> 02:04.773 break the bonds-- 02:08.967 --> 02:11.567 PROFESSOR: But that's a very difficult thing to do, to 02:11.567 --> 02:12.597 break the bonds in graphite. 02:12.600 --> 02:14.330 Matt, do you have an idea? 02:14.333 --> 02:16.303 STUDENT: Do you combust graphite? 02:16.300 --> 02:18.400 PROFESSOR: You burn them. 02:18.400 --> 02:21.600 You compare them to CO2. 02:21.600 --> 02:24.370 So you take your molecule and burn it, we talked about this 02:24.367 --> 02:25.367 last semester. 02:25.367 --> 02:26.767 You burn graphite. 02:26.767 --> 02:30.667 You also burn H2, so you can get a hydrocarbon. 02:30.667 --> 02:34.127 What's the energy of your compound, compared to the 02:34.133 --> 02:36.533 amount of energy that was in hydrogen 02:36.533 --> 02:38.903 and carbon of graphite. 02:38.900 --> 02:44.570 So anyhow, the heat of formation is -10 or a little 02:44.567 --> 02:47.097 bit more kilocalories per mole. 02:47.100 --> 02:50.370 That means it's that much more stable than the elements would 02:50.367 --> 02:54.767 be in their defined standard state. 02:54.767 --> 03:00.497 Now if you look at it for the cis-3-hexene, the double bond 03:00.500 --> 03:06.130 beginning on the third carbon, it's a little bit more stable. 03:06.133 --> 03:08.903 And that doesn't surprise you, because we've talked about 03:08.900 --> 03:14.900 having more carbons as opposed to hydrogens on a double- 03:14.900 --> 03:16.230 bonded carbon 03:16.233 --> 03:19.603 makes the compound more stable. 03:19.600 --> 03:25.830 That is, that the CH bonds don't profit as much from the 03:25.833 --> 03:30.433 change in hybridization as C-C bonds seem to do. 03:30.433 --> 03:34.903 Now if you have the trans isomer, it's still more stable 03:34.900 --> 03:37.030 by about one and a half kilocalories. 03:37.033 --> 03:38.673 I don't know whether you had a chance to look at 03:38.667 --> 03:39.467 the problems yet. 03:39.467 --> 03:40.197 Probably not. 03:40.200 --> 03:42.030 They were for Wednesday. 03:42.033 --> 03:43.633 Just look at the generalities. 03:43.633 --> 03:48.133 But you find that the trans isomer is about one and a half 03:48.133 --> 03:50.673 kilocalories more stable than the cis. 03:50.667 --> 03:52.627 Can you think of any explanation for that? 03:55.433 --> 03:57.203 Amy? 03:57.200 --> 03:58.330 STUDENT: Less repulsion from the-- in cis there's repulsion 03:58.333 --> 03:59.603 from the hydrogens that are on the same side. 04:03.167 --> 04:05.697 PROFESSOR: The groups run into one another, it's a steric 04:05.700 --> 04:06.630 effect when they're cis. 04:06.633 --> 04:10.073 That's worth about one and a half kilocalories per mole. 04:10.067 --> 04:13.597 OK, now if you make three carbon-carbon bonds from the 04:13.600 --> 04:15.630 double bond-- 04:15.633 --> 04:19.933 incidentally there's a table that has those in it too in 04:19.933 --> 04:21.303 the Jones book. 04:21.300 --> 04:24.530 If you put three carbons on the double bond, then it's 04:24.533 --> 04:26.073 more stable still. 04:26.067 --> 04:28.527 And obviously if you put four, it's a little 04:28.533 --> 04:30.833 bit more stable still. 04:30.833 --> 04:36.703 Now if we come back and look at correcting for strain, 04:36.700 --> 04:40.770 which was what we attributed the difference between cis and 04:40.767 --> 04:46.327 trans to, we find that there's 3.6 kilocalories of molecular 04:46.333 --> 04:50.433 mechanics strain in the cis, and 2.5 in the trans. 04:50.433 --> 04:54.633 So it's what you said, Amy, that the molecular mechanics 04:54.633 --> 04:58.933 explains it, in terms of a steric effect. 04:58.933 --> 05:03.603 So the average then, for those two, is about there. 05:03.600 --> 05:06.500 And now if we make strain corrections for the other 05:06.500 --> 05:09.600 molecules as well, calculate the strain by molecular 05:09.600 --> 05:12.900 mechanics, there's 2.8 kilocalories of strain 05:12.900 --> 05:18.500 calculated there, 2.6 and 7.7 in the one that has four 05:18.500 --> 05:21.070 carbons on the double bond. 05:21.067 --> 05:24.697 And now you notice something interesting about the trend. 05:24.700 --> 05:29.670 Once you've corrected for strain, what do you notice? 05:29.667 --> 05:32.327 It's almost exactly linear according to the 05:32.333 --> 05:35.403 number of C-C bonds. 05:35.400 --> 05:39.170 So you gain in stability, about two and a half 05:39.167 --> 05:43.527 kilocalories per mole every time you substitute a C-H with 05:43.533 --> 05:48.773 a C-C, keeping the same number of carbons and hydrogens in 05:48.767 --> 05:51.927 the molecule, discussing isomers of one another. 05:51.933 --> 05:55.533 Because of course it makes no sense to compare heats of 05:55.533 --> 05:58.003 formation of molecules that are not 05:58.000 --> 06:01.570 isomers of one another. 06:01.567 --> 06:04.667 You're comparing apples with oranges. 06:04.667 --> 06:08.997 So that could be due to this hybridization. 06:09.000 --> 06:12.270 It could, in part, be due to hyperconjugation, that we 06:12.267 --> 06:17.127 talked about before, and we'll talk a little bit 06:17.133 --> 06:19.503 more about it later. 06:19.500 --> 06:24.530 Now let's think about the thermochemistry involved in 06:24.533 --> 06:26.673 alkene/HCl reactions. 06:26.667 --> 06:31.827 So here's HCl plus an alkene, to give the addition product, 06:31.833 --> 06:36.203 where H is added to one carbon, and Cl to the other. 06:36.200 --> 06:40.330 So notice we've converted a single bond and a double bond 06:40.333 --> 06:45.173 on the left into three single bonds on the right. 06:45.167 --> 06:48.267 Is that likely to be favorable or unfavorable? 06:48.267 --> 06:51.127 What's the energy of a double bond compared 06:51.133 --> 06:53.303 to two single bonds? 06:53.300 --> 06:54.530 That's what we've done here. 06:58.133 --> 07:04.773 You remember from our table we looked at in the first semester? 07:04.767 --> 07:07.127 Is the second bond of a double bond worth 07:07.133 --> 07:08.833 as much as the first? 07:08.833 --> 07:09.333 More? 07:09.333 --> 07:10.603 Less? 07:10.600 --> 07:11.530 STUDENT: Less. 07:11.533 --> 07:13.303 PROFESSOR: It's worth less. 07:13.300 --> 07:16.200 So if we look back at that table that we looked at last 07:16.200 --> 07:22.730 semester and put actual numbers to these bonds that 07:22.733 --> 07:27.603 we've drawn here, we see that the hydrogen chloride bond is 07:27.600 --> 07:30.400 worth 103 kilocalories per mole. 07:30.400 --> 07:35.400 The double bond, carbon-carbon double bond, is a 146. 07:35.400 --> 07:38.330 So there's 249 that we're losing in 07:38.333 --> 07:39.833 the starting material. 07:39.833 --> 07:42.603 So how much do we gain in the product making the three 07:42.600 --> 07:44.200 single bonds? 07:44.200 --> 07:53.700 So there's 104, and 83, and 81. 07:53.700 --> 07:54.770 So 268. 07:54.767 --> 07:58.627 In other words, it's profitable by 19 kilocalories 07:58.633 --> 08:00.803 per mole to do this addition. 08:00.800 --> 08:02.930 And we talked about this before, when we were talking 08:02.933 --> 08:08.073 about chain reaction addition of HCl, or HBr, 08:08.067 --> 08:09.367 of HI, and so on. 08:09.367 --> 08:13.027 All of them were exothermic by about that much. 08:13.033 --> 08:19.203 So it's favorable by about 19 kilocalories per mole. 08:19.200 --> 08:22.170 Now we could also look more specifically at these 08:22.167 --> 08:25.567 compounds by using that data that's in the table that we 08:25.567 --> 08:28.827 accessed last time from National Institute of Science 08:28.833 --> 08:29.173 and Technology [correction: National Institute of 08:29.167 --> 08:31.497 Standards and Technology], to look at the actual heats of 08:31.500 --> 08:36.430 formation of these particular molecules in the gas phase. 08:36.433 --> 08:39.873 That is, before we were just looking at average bond 08:39.867 --> 08:43.167 energies to get an order of magnitude idea of what's going 08:43.167 --> 08:45.827 on, or better than an order of magnitude. 08:45.833 --> 08:49.003 But these now will give us the precise values. 08:49.000 --> 08:52.530 And where we said it was 19 before, now we see that, for 08:52.533 --> 08:56.003 this specific set of compounds, it's 14.5. 08:56.000 --> 08:57.170 But we were about right. 08:57.167 --> 08:58.397 It was a good guess before. 09:03.000 --> 09:06.400 So this reaction should go to the right. 09:06.400 --> 09:08.970 But we've already talked about a reaction 09:08.967 --> 09:11.527 that goes to the left. 09:11.533 --> 09:14.833 What reaction would that be, the one that starts on the 09:14.833 --> 09:18.073 right and goes to the left? 09:18.067 --> 09:20.197 That's the elimination reaction. 09:20.200 --> 09:23.000 An E2 reaction goes from right to left. 09:23.000 --> 09:24.270 What's the difference? 09:27.900 --> 09:31.330 If this equilibrium lies to the right, how could we have 09:31.333 --> 09:34.633 spent all this time in the last quarter of the course 09:34.633 --> 09:37.933 talking about reactions going the other way? 09:37.933 --> 09:39.333 Isn't that uphill in energy? 09:43.433 --> 09:46.003 What reagent do you use to do the elimination? 09:46.000 --> 09:48.200 STUDENT: Base. 09:48.200 --> 09:49.800 PROFESSOR: Strong base, right? 09:49.800 --> 09:50.830 Like hydroxide. 09:50.833 --> 09:55.903 So let's see what happens if we put hydroxide into the mix. 09:55.900 --> 09:59.970 So in acid we get addition. 09:59.967 --> 10:03.327 And it's favorable by 14 kilocalories per mole, in this 10:03.333 --> 10:04.673 particular case. 10:04.667 --> 10:08.067 But if we put hydroxide in there, now what 10:08.067 --> 10:09.497 will it react with? 10:09.500 --> 10:14.470 Well, we talked about how it pulls off the proton at the 10:14.467 --> 10:17.497 same time chloride leaves in the E2 reaction. 10:17.500 --> 10:20.700 But if all we're doing is studying thermochemistry, we 10:20.700 --> 10:25.170 could look at where it is in the product, in the balance of 10:25.167 --> 10:26.867 the two sides of the equation. 10:26.867 --> 10:32.067 So what it does on the left is take the proton away from HCl, 10:32.067 --> 10:34.627 to give water and chloride. 10:34.633 --> 10:40.373 Now HCl is a much stronger acid than water is. 10:40.367 --> 10:44.467 So that's a very favorable process, to take the proton 10:44.467 --> 10:48.097 away from HCl, and to put it on hydroxide. 10:48.100 --> 10:53.230 And in fact, we could find out how different it is by looking 10:53.233 --> 10:57.633 up in that same table the proton affinities of OH minus 10:57.633 --> 11:00.933 and chloride, and we find they differ by 19 11:00.933 --> 11:02.833 kilocalories per mole. 11:02.833 --> 11:06.903 So adding hydroxide stabilizes the left side of this equation 11:06.900 --> 11:10.800 by 19 kilocalories per mole, and now it's favorable to go 11:10.800 --> 11:14.500 the other way by four and a half kilocalories per mole. 11:17.400 --> 11:19.400 So in base, you do elimination. 11:19.400 --> 11:21.230 You go the other way. 11:21.233 --> 11:25.473 Now are these really the reverse of one another? 11:25.467 --> 11:31.927 Do we just run the machine backwards to get elimination, 11:31.933 --> 11:34.673 or to get addition, as compared to the elimination we 11:34.667 --> 11:36.067 talked about before? 11:36.067 --> 11:39.467 Or is it a different mechanism because the 11:39.467 --> 11:42.067 conditions are different? 11:42.067 --> 11:45.327 The elimination was done in the presence of strong base. 11:45.333 --> 11:47.973 The addition is done in the presence of acid. 11:47.967 --> 11:52.497 So you might not be able to run the machine backwards. 11:52.500 --> 11:55.830 So incidentally, here's a problem for you to look at for 11:55.833 --> 11:57.003 Wednesday too. 11:57.000 --> 12:03.730 Use the pKa values for HCl and H2O to estimate-- 12:03.733 --> 12:08.633 here I looked it up, actually, the proton affinities in NIST, 12:08.633 --> 12:11.903 and got 19 kilocalories per mole. 12:11.900 --> 12:14.470 But you could have done the same thing out of your head, 12:14.467 --> 12:16.927 or out of a simple table. 12:16.933 --> 12:22.173 Does anybody remember what the pKa of HCl is? 12:22.167 --> 12:23.567 STUDENT: Maybe nine? 12:23.567 --> 12:25.997 PROFESSOR: The exam was Friday, I realize that. 12:26.000 --> 12:27.970 You can scrub that now. 12:27.967 --> 12:30.627 Anybody remember about what it is? 12:33.400 --> 12:34.870 I think it's minus eight. 12:34.867 --> 12:35.497 Isn't it? [correction: -3] 12:35.500 --> 12:37.870 How about the pKa of water? 12:37.867 --> 12:39.497 STUDENT: Negative 16. 12:39.500 --> 12:40.770 PROFESSOR: Right, negative 16 [correction: positive 16]. 12:40.767 --> 12:45.427 So from that difference, you can calculate what the energy 12:45.433 --> 12:47.373 of that reaction would be. 12:47.367 --> 12:49.927 So try it from that difference and see if you can do it. 12:49.933 --> 12:52.273 That's a good exercise for you. 12:52.267 --> 12:54.367 But at any rate, we have the question of the addition of 12:54.367 --> 12:55.097 the mechanism. 12:55.100 --> 13:01.100 Is it the reverse of the E2 or E1 mechanism? 13:01.100 --> 13:03.400 So we're going to talk about addition to alkenes, and we've 13:03.400 --> 13:05.500 already done that to a certain extent. 13:05.500 --> 13:09.000 We talked about the SOMO mechanism, the free radical 13:09.000 --> 13:12.670 chain addition of HBr, and you remember what that is, that 13:12.667 --> 13:18.397 the bromine atom adds to give the more stable radical, which 13:18.400 --> 13:21.230 then attacks HBr, taking off the hydrogen 13:21.233 --> 13:23.973 atom, to give the product. 13:23.967 --> 13:28.097 It gives that product, and there's a reference in the 13:28.100 --> 13:33.130 textbook, it gives that product, not the one above. 13:33.133 --> 13:35.333 And it doesn't give that one. 13:35.333 --> 13:37.333 It's regioselective. 13:37.333 --> 13:42.703 And remember we called this anti-Markovnikov. Markovnikov 13:42.700 --> 13:46.970 had the idea, as we talked about last time, that the 13:46.967 --> 13:50.467 carbon that has more substituents gets more. 13:50.467 --> 13:54.297 So if you had two methyl groups already on one of the 13:54.300 --> 13:57.070 double-bonded carbons, it would be the one that gets the 13:57.067 --> 14:00.367 bromine, that the hydrogen goes to the other one. 14:00.367 --> 14:06.197 But this is anti-Markovnikov, it does the opposite. 14:06.200 --> 14:07.700 So we've already talked about that. 14:07.700 --> 14:11.630 And the key feature in the regioselection is getting the 14:11.633 --> 14:13.803 more stable carbon radical. 14:16.800 --> 14:20.070 But it's also conceivable that there could be a HOMO-LUMO 14:20.067 --> 14:24.527 mechanism, a concerted one, where the new group comes up 14:24.533 --> 14:27.333 to the double bond, and the partners just 14:27.333 --> 14:29.133 exchange like this. 14:29.133 --> 14:33.073 Suppose you had H2 to react with, with the carbon-carbon 14:33.067 --> 14:36.897 double bond, a very exothermic reaction. 14:36.900 --> 14:41.300 So you could just shift electron pairs like that, and 14:41.300 --> 14:44.470 perform the addition. 14:44.467 --> 14:46.427 Now let's look at the orbitals that would 14:46.433 --> 14:49.233 be involved in that. 14:49.233 --> 14:52.733 What's the HOMO of the C-C double bond? 14:58.600 --> 14:59.330 Ellen? 14:59.333 --> 15:01.273 STUDENT: Pi? 15:01.267 --> 15:02.497 PROFESSOR: Pi. 15:04.267 --> 15:07.367 And the LUMO is pi-star. 15:07.367 --> 15:10.497 So let's try to bring the LUMO and the HOMO of 15:10.500 --> 15:14.370 the H2 down on top. 15:14.367 --> 15:15.997 Look good to mix HOMO and LUMO? 15:18.700 --> 15:20.070 What do you call that situation? 15:28.367 --> 15:29.827 Rahul? 15:29.833 --> 15:31.333 STUDENT: It's out of phase? 15:31.333 --> 15:32.873 PROFESSOR: They're out of phase. 15:32.867 --> 15:35.367 In fact, there's a specific word to talk about things that 15:35.367 --> 15:38.967 don't overlap for this symmetry kind of reason. 15:38.967 --> 15:40.827 Remember what it is? 15:40.833 --> 15:42.073 They're orthogonal, Anurag says. 15:43.500 --> 15:47.930 So there's no mixing of HOMO of one with LUMO of the other. 15:47.933 --> 15:52.473 Of course you could shift them so that they mix. 15:52.467 --> 15:55.197 But then there's nothing to be gained, because on the left, 15:55.200 --> 15:56.770 you're mixing filled with filled. 15:56.767 --> 15:57.667 That's repulsive. 15:57.667 --> 15:59.797 And on the right, the orbitals are empty. 15:59.800 --> 16:02.030 There's nothing that has whatever energies are 16:02.033 --> 16:03.903 available for those orbitals. 16:03.900 --> 16:06.300 So you can't do this reaction. 16:06.300 --> 16:10.730 You can't just bring AB single bond down on top of a double 16:10.733 --> 16:14.033 bond and change partners, because the symmetry is wrong 16:14.033 --> 16:14.803 of the orbitals. 16:14.800 --> 16:16.070 They won't mix that way. 16:16.067 --> 16:18.297 You can't draw those two arrows. 16:18.300 --> 16:22.770 However, it turns out that if you use certain catalysts, in 16:22.767 --> 16:27.127 particular metals like platinum, they make it 16:27.133 --> 16:31.503 possible to do this, because the platinum gets involved. 16:31.500 --> 16:33.930 It gets its orbitals involved, and there can 16:33.933 --> 16:35.573 be favorable overlap. 16:35.567 --> 16:37.667 And we'll be talking about that pretty soon. 16:37.667 --> 16:43.167 You see it in the Jones book: it's in chapter 10. 16:43.167 --> 16:46.127 So that would be a concerted reaction, where you make and 16:46.133 --> 16:48.333 break bonds at the same time. 16:48.333 --> 16:51.533 But it's also possible to do it stepwise. 16:51.533 --> 16:55.233 That's why the reactions are called electrophilic, because 16:55.233 --> 17:02.003 you have an acid, for example HBr, and the HOMO attacks the 17:02.000 --> 17:07.300 sigma-star LUMO, attacks the proton to give off bromide. 17:07.300 --> 17:11.230 And then the bromide can attack the 17:11.233 --> 17:12.533 intermediate cation. 17:12.533 --> 17:14.833 So it's a two step reaction. 17:14.833 --> 17:19.003 So the initial reaction is the reaction of an electrophile, 17:19.000 --> 17:20.400 something that wants the electrons. 17:20.400 --> 17:26.300 A low LUMO, the sigma-star of HBr is the reagent that reacts 17:26.300 --> 17:29.470 with the double bond. 17:29.467 --> 17:33.527 And notice that that one works for any hydrogen halide. 17:33.533 --> 17:36.573 Remember all the hydrogen halides were exothermic for 17:36.567 --> 17:38.467 their addition to the double bond. 17:38.467 --> 17:42.627 But only HBr would work by the top mechanism here, the SOMO, 17:42.633 --> 17:45.833 because both steps were exothermic to compete with 17:45.833 --> 17:47.933 termination. 17:47.933 --> 17:52.073 Whereas any of the hydrogen halides will do this stepwise 17:52.067 --> 17:54.567 electrophilic reaction. 17:54.567 --> 17:57.867 And notice, in this case, that it does follow 17:57.867 --> 17:59.167 Markovnikov's rule. 17:59.167 --> 18:00.767 It's not that one. 18:00.767 --> 18:04.797 It's the Markovnikov, where bromide goes to the more 18:04.800 --> 18:08.270 substituted carbon, and it goes there because that carbon 18:08.267 --> 18:10.097 is more stable in the inter... 18:10.100 --> 18:15.330 that carbon cation is more stable as an intermediate. 18:15.333 --> 18:20.533 So that's stuff that mostly we've mentioned before. 18:20.533 --> 18:23.633 Now let's think about the orbitals and where they should 18:23.633 --> 18:24.973 get attacked. 18:24.967 --> 18:28.397 So here's ethylene, an alkene, 18:28.400 --> 18:31.900 and we can look at the surface potential to see the energy of 18:31.900 --> 18:34.330 a proton on the van der Waals surface. 18:34.333 --> 18:37.603 So a proton likes to be up above where the double bond 18:37.600 --> 18:39.970 is, and it doesn't like to be around where 18:39.967 --> 18:42.897 the hydrogens are. 18:42.900 --> 18:48.030 So this electrostatic feature of the surface of the molecule 18:48.033 --> 18:51.533 is important in positioning the fragments before they 18:51.533 --> 18:53.003 begin to react. 18:53.000 --> 18:56.700 But new bonding requires mixing orbitals, not just 18:56.700 --> 18:58.170 positioning things electrostatically. 19:00.767 --> 19:04.267 So we should look at the HOMO and the LUMO. 19:04.267 --> 19:10.067 So the LUMO is this pi-star orbital and the 19:10.067 --> 19:11.397 HOMO is the pi orbital. 19:14.433 --> 19:16.803 So the addition, as we've said, to alkenes is 19:16.800 --> 19:19.370 electrophilic, it goes after the pi 19:19.367 --> 19:21.367 electrons, the high HOMO. 19:21.367 --> 19:23.897 So the proton, with its low vacant orbital, 19:23.900 --> 19:26.300 electrostatically likes to be up on top. 19:26.300 --> 19:28.430 That's the best place to be, if you're on the surface of 19:28.433 --> 19:29.573 the molecule. 19:29.567 --> 19:36.397 And when it's there, it can overlap with this pi orbital. 19:36.400 --> 19:40.270 So in the various textbooks you have, you could read about 19:40.267 --> 19:42.197 this electrophilic addition to alkenes. 19:42.200 --> 19:45.400 I guarantee you there are all these sections in there. 19:45.400 --> 19:50.570 One about hydrogen halide addition by R+, by the cation 19:50.567 --> 19:52.167 intermediate, what we just talked about on 19:52.167 --> 19:53.527 the previous slide. 19:53.533 --> 19:57.303 There'll be discussion of the regiochemistry, which end of 19:57.300 --> 20:00.370 the H+ adds to, to give the more stable cation 20:00.367 --> 20:01.527 intermediate. 20:01.533 --> 20:05.733 And there'll be discussion of hydration, where instead of 20:05.733 --> 20:09.503 the X minus being the thing that adds to the H+, [correction: C+] 20:09.500 --> 20:14.030 it's the solvent water, the unshared pair on oxygen can 20:14.033 --> 20:18.303 attack the cation as well, and then lose a proton, so that it's 20:18.300 --> 20:22.030 OH that goes on instead X minus. 20:22.033 --> 20:24.173 So you can be reading that in your texts. 20:24.167 --> 20:30.127 And this, just to show an example, is from the lecture 20:30.133 --> 20:33.133 that I mentioned was given by Professor Siegel last year, 20:33.133 --> 20:37.573 where he talks about how you can do this Markovnikov 20:37.567 --> 20:42.127 addition, chloride going to the more substituted, OH going 20:42.133 --> 20:45.803 to the more substituted, Br going to the more substituted, 20:45.800 --> 20:49.670 when it's clean, that is when you don't have free radical 20:49.667 --> 20:52.197 initiators there. 20:52.200 --> 20:57.030 So all of these form by proton first going here to get this 20:57.033 --> 20:59.833 more stable cation intermediate. 20:59.833 --> 21:04.033 And in this case, with dilute sulfuric acid, it was water 21:04.033 --> 21:07.773 that came on to do this. 21:07.767 --> 21:10.097 And that mechanism is shown here. 21:10.100 --> 21:15.070 Incidentally, on the exam, remember the last question was 21:15.067 --> 21:18.467 to draw a very careful curved arrows and everything. 21:18.467 --> 21:22.327 I put this up, in part, to give you fellow feeling, 21:22.333 --> 21:24.703 because we graded that very rigorously. 21:24.700 --> 21:28.730 When you draw a curved arrow to denote electron pair 21:28.733 --> 21:31.533 shifts, where do you start the arrow? 21:31.533 --> 21:33.303 STUDENT: Where the electrons are. 21:33.300 --> 21:35.930 PROFESSOR: Where the electrons are in the starting material. 21:35.933 --> 21:39.233 And where do you end the point of the arrow? 21:39.233 --> 21:40.333 STUDENT: [INTERPOSING VOICES] 21:40.333 --> 21:42.573 PROFESSOR: Where those electrons are in the product. 21:42.567 --> 21:45.997 Sometimes they will be on an atom, become an anion, or 21:46.000 --> 21:47.930 becoming an unshared pair. 21:47.933 --> 21:50.333 Sometimes they will be between two atoms, because you're 21:50.333 --> 21:52.173 forming a new bond. 21:52.167 --> 21:55.467 And if you want to be thinking really clearly about it, you 21:55.467 --> 21:57.427 should draw those arrows clearly. 21:57.433 --> 22:00.103 That's why I asked it that way in the question. 22:00.100 --> 22:03.100 But notice that Professor Siegel here, in drawing some 22:03.100 --> 22:06.800 of these arrows, drew it going on to the hydrogen. 22:06.800 --> 22:10.770 He drew the X minus going on to the carbon, where I would 22:10.767 --> 22:13.497 expect you to draw this curved arrow ending 22:13.500 --> 22:15.670 between X and the carbon. 22:15.667 --> 22:18.927 And to position those two things that are going to react 22:18.933 --> 22:22.433 with one another in your drawing, such that they're 22:22.433 --> 22:25.033 close enough to one another that it makes sense to end the 22:25.033 --> 22:28.273 arrow between them, not to have it off here and the arrow 22:28.267 --> 22:31.597 terminating in space, just because it's halfway between. 22:31.600 --> 22:36.530 So that's a lesson that I hope will become clear. 22:36.533 --> 22:38.803 So this shows the case of water intervening. 22:41.500 --> 22:44.630 So there are sections of the book about the addition of 22:44.633 --> 22:47.933 hydrogen halides, about regioselectivity, about the 22:47.933 --> 22:51.473 intermediate cations that are involved, about cation 22:51.467 --> 22:54.467 stability, and about cation rearrangements. 22:54.467 --> 22:56.927 And I'll talk about a few of those things here, but you can 22:56.933 --> 23:00.303 review them in your book as well. 23:00.300 --> 23:02.130 First about cation stability. 23:02.133 --> 23:03.973 I took this from the text we used a couple of 23:03.967 --> 23:05.427 years ago by Loudon. 23:05.433 --> 23:08.073 So he shows primary, secondary, and tertiary 23:08.067 --> 23:12.867 cations, how many carbons are substituted on them, and says 23:12.867 --> 23:16.567 of the stability of carbocations, that tertiary is 23:16.567 --> 23:19.867 more stable than secondary is more stable than primary. 23:19.867 --> 23:22.897 This of course is the explanation for Markovnikov 23:22.900 --> 23:24.130 orientation. 23:25.933 --> 23:29.233 Now we can look at the rationale for that, rehearse 23:29.233 --> 23:30.073 it once again. 23:30.067 --> 23:35.527 Hyperconjugation says its better to have a carbon on the 23:35.533 --> 23:38.803 carboncation with its hydrogen, or whatever else 23:38.800 --> 23:41.700 it's bonded to, than to have just the hydrogen. 23:41.700 --> 23:46.400 And the reason it's better is that there's some kind of HOMO 23:46.400 --> 23:49.770 that's in position to have its electrons stabilized by that 23:49.767 --> 23:54.467 unusually low-energy LUMO, the one associated with the 23:54.467 --> 23:57.267 positive charge, the vacant orbital on carbon. 23:57.267 --> 23:59.867 And in the case of hydrogen, there are no electrons to be 23:59.867 --> 24:02.967 stabilized, those are orthogonal to this But in this 24:02.967 --> 24:07.027 case, the sigma electrons that are in the bond, even though 24:07.033 --> 24:10.233 they aren't particularly high, can interact with a really 24:10.233 --> 24:13.973 unusually low orbital, and get some stabilization. 24:13.967 --> 24:17.197 So we've talked about this before, this hyperconjugation. 24:17.200 --> 24:21.430 And it could be denoted by writing a resonance structure, 24:21.433 --> 24:26.073 which has a double bond between the carbons and an H+, 24:26.067 --> 24:29.197 or R+ if it's an R group out there. 24:29.200 --> 24:33.400 The other rationale for this order of cation stability is 24:33.400 --> 24:34.330 bond energies. 24:34.333 --> 24:38.303 And we've talked about this several times, changing CH to 24:38.300 --> 24:44.430 CC bonds when you have this especially advantageous sp 24:44.433 --> 24:47.533 square hybridization of the carbon. 24:47.533 --> 24:53.073 But those are theory, and it's different to know what the 24:53.067 --> 24:56.897 evidence is that they actually have that order, tertiary more 24:56.900 --> 24:58.430 stable than primary. 24:58.433 --> 25:01.373 So the first part is the rationalization. 25:01.367 --> 25:05.227 Now we have to say where do we actually get the evidence that 25:05.233 --> 25:08.073 their stability is that way? 25:08.067 --> 25:12.397 And several people on the exam, when asked for evidence, 25:12.400 --> 25:17.030 provided theory, rationale like this, rather than actual 25:17.033 --> 25:20.673 numbers, where the evidence comes from. 25:20.667 --> 25:25.127 So to get evidence, you have to compare something. 25:25.133 --> 25:31.403 So you could compare products or transition states, that is 25:31.400 --> 25:33.200 you could look at reactivity. 25:33.200 --> 25:37.530 If you have a very unstable cation, you would expect it to 25:37.533 --> 25:39.533 be more reactive. 25:39.533 --> 25:41.903 So that would be one way that you could do it. 25:41.900 --> 25:48.030 Another way would be compare with a starting alkene. 25:48.033 --> 25:52.833 If it's easier to put a proton on, that would indicate that 25:52.833 --> 25:55.873 the cation you're leading to is more stable. 25:55.867 --> 26:00.397 The cation is uphill in energy typically, but if it's a more 26:00.400 --> 26:02.800 stable cation, it should be easier to form. 26:02.800 --> 26:06.230 So the reactions, where you protonate the 26:06.233 --> 26:07.473 alkene, should be easier. 26:07.467 --> 26:11.767 Or in the reactions we were just talking about, the SN1 26:11.767 --> 26:15.497 substitutions, the ease of losing the anion, in order to 26:15.500 --> 26:18.170 generate the leaving group that is to say, in order to 26:18.167 --> 26:21.167 generate the carbon cation should be easier when the 26:21.167 --> 26:22.727 cation is more stable. 26:22.733 --> 26:24.603 So we could do either of those. 26:24.600 --> 26:32.130 Or we could directly compare the cations with one another, 26:32.133 --> 26:35.733 to see which one is more stable. 26:35.733 --> 26:41.603 Now here's a table taken from the textbook, which gives a 26:41.600 --> 26:46.830 table of the heats of formation of carbon cations in 26:46.833 --> 26:47.773 the gas phase. 26:47.767 --> 26:51.667 So they're very hard to make in the gas phase. 26:51.667 --> 26:56.067 And it says, "secondary vinyl cations are more stable than 26:56.067 --> 26:58.367 primary vinyl cations." 26:58.367 --> 27:00.167 Now what's the basis for that here? 27:03.567 --> 27:06.367 In the table, what's the basis for saying that secondary 27:06.367 --> 27:09.627 vinyl cations are more stable than primary vinyl cations? 27:14.633 --> 27:18.573 Well we look up there and we see a primary vinyl cation is, 27:18.567 --> 27:23.197 in fact, the highest energy of any of these. 27:23.200 --> 27:27.830 And the secondary vinyl cation is also pretty high. 27:27.833 --> 27:30.733 It's substituted, it's secondary, which 27:30.733 --> 27:31.533 should make it better. 27:31.533 --> 27:34.233 And indeed, it is better. 27:34.233 --> 27:37.373 So it says it's better by 54 kilocalories per 27:37.367 --> 27:40.527 mole, in the gas phase. 27:40.533 --> 27:45.273 Now there's a problem with using this table in this way. 27:45.267 --> 27:49.527 And the problem is compared to what? 27:49.533 --> 27:53.703 Is this really saying that a primary vinyl cation is more 27:53.700 --> 27:57.830 stable than a secondary vinyl cation? 27:57.833 --> 27:59.103 Compared to what? 28:01.900 --> 28:06.130 These heats of formation, what are they compared to? 28:10.533 --> 28:15.503 So this is the energy of the vinyl cation compared to what? 28:18.267 --> 28:19.427 What's the heat of formation? 28:19.433 --> 28:20.333 Helen? 28:20.333 --> 28:22.833 STUDENT: Carbon dioxide and [UNINTELLIGIBLE] 28:22.833 --> 28:24.603 PROFESSOR: That's the heat of combustion. 28:24.600 --> 28:26.100 It could have been heat of combustion. 28:26.100 --> 28:27.900 We would add something to these, to get heat of 28:27.900 --> 28:29.970 combustion. 28:29.967 --> 28:32.327 Actually what's given is the heat of formation. 28:32.333 --> 28:34.603 STUDENT: Compared to the carbon and hydrogen? 28:34.600 --> 28:36.730 PROFESSOR: Yeah, compared to carbon and graphite and 28:36.733 --> 28:39.603 hydrogen and H2 gas, at standard temperature and 28:39.600 --> 28:40.970 pressure and so on. 28:40.967 --> 28:42.567 That is compared to the elements, in 28:42.567 --> 28:44.127 their standard states. 28:44.133 --> 28:48.003 But notice that there are different numbers of atoms in 28:48.000 --> 28:48.570 these things. 28:48.567 --> 28:51.197 They're not isomers of one another. 28:51.200 --> 28:53.970 So it's not that you can convert one to the other and 28:53.967 --> 28:58.067 say that this heat difference shown here 28:58.067 --> 29:00.197 takes one to the other. 29:00.200 --> 29:02.100 You're comparing apples and oranges. 29:02.100 --> 29:04.230 They're just not the same thing. 29:04.233 --> 29:07.733 If you want to compare with heats of formation, they have 29:07.733 --> 29:11.003 to be things that are isomers of one another, where you know 29:11.000 --> 29:14.100 their energy relative to one another, rather than relative 29:14.100 --> 29:16.230 to something completely different, different numbers 29:16.233 --> 29:18.503 of atoms. 29:18.500 --> 29:21.630 So this particular table doesn't help you at all 29:21.633 --> 29:25.473 directly in figuring out, except that there might be 29:25.467 --> 29:28.127 some cases here, like this one. 29:28.133 --> 29:35.173 Notice that this is C4H9, and this one is also C4H9. 29:35.167 --> 29:38.797 So here's a primary and a secondary which differ by 20 29:38.800 --> 29:41.230 kilocalories per mole in the gas phase. 29:41.233 --> 29:44.503 And that really does show that secondary is 29:44.500 --> 29:46.370 more stable than primary. 29:46.367 --> 29:49.097 But this table definitely doesn't show that secondary 29:49.100 --> 29:52.930 vinyl cations are more stable than primary vinyl cations. 29:52.933 --> 29:55.703 So you have to think carefully about compared to what. 29:55.700 --> 29:58.570 Here you're comparing to different numbers of atoms in 29:58.567 --> 30:03.367 their standard states, not relative to each other, nor 30:03.367 --> 30:06.197 relative to their respective starting materials or 30:06.200 --> 30:08.430 products, as we talked about last time. 30:08.433 --> 30:12.203 So this table, in general, is irrelevant to the question of 30:12.200 --> 30:15.570 cation stability in the sense that we were talking about it 30:15.567 --> 30:18.367 in the previous slide. 30:18.367 --> 30:22.097 Now on the other hand, in Loudon here, Table 4.2, gives 30:22.100 --> 30:27.200 the heats of formation of isomeric butyl cations. 30:27.200 --> 30:29.730 So the same atoms, they're being compared to the same 30:29.733 --> 30:33.003 thing, so differences between them now do tell us the 30:33.000 --> 30:35.070 difference between primary, secondary, 30:35.067 --> 30:36.627 and tertiary cations. 30:39.167 --> 30:42.127 They got it from the same numbers, heats of formation. 30:42.133 --> 30:44.703 Although, in fact, you'll find that these numbers are 30:44.700 --> 30:49.330 slightly different from those in the previous table. 30:49.333 --> 30:53.073 Neither book says what the source of these numbers is. 30:53.067 --> 30:56.597 These NIST values have been pretty carefully gone through. 30:56.600 --> 31:00.600 So if push comes to shove, I often look at those, or ask 31:00.600 --> 31:04.400 Professor Wiberg what the best value is. 31:04.400 --> 31:08.630 But here you see that the relative energy of the 31:08.633 --> 31:13.733 tertiary butyl cation is taken as zero, the most stable. 31:13.733 --> 31:16.873 And now the others are 16 or 32 or 37 31:16.867 --> 31:20.067 kilocalories less stable. 31:20.067 --> 31:23.927 So tertiary is more stable than secondary is more stable 31:23.933 --> 31:24.973 than primary. 31:24.967 --> 31:28.527 And here are two different primaries. 31:28.533 --> 31:32.703 Now if we look at those two, we see that going from primary 31:32.700 --> 31:38.570 to secondary is a difference of 21 kilocalories per mole. 31:38.567 --> 31:43.827 37 to 16, primary is less stable by 21 31:43.833 --> 31:47.273 kilocalories per mole. 31:47.267 --> 31:52.327 Now remember that the reason for this could be things like 31:52.333 --> 31:56.203 hyperconjugation, or the number of carbons bonded to 31:56.200 --> 32:01.230 the sp squared carbon, the same factors that we talked 32:01.233 --> 32:04.703 about in the case of alkenes, making more substituted 32:04.700 --> 32:06.330 alkenes more stable. 32:06.333 --> 32:09.203 Here more substituted cations are more stable. 32:09.200 --> 32:11.700 But the difference is much bigger here. 32:11.700 --> 32:15.300 21 kilocalories per mole versus two and a half 32:15.300 --> 32:18.100 kilocalories per mole, from the slope of that plot we made 32:18.100 --> 32:20.670 for the alkenes at the beginning of the lecture. 32:20.667 --> 32:24.327 So it's ever so much more important for cations than it 32:24.333 --> 32:28.673 is for alkenes, whatever the factor is that's doing this. 32:28.667 --> 32:30.727 So it could be hyperconjugation. 32:30.733 --> 32:34.333 Would you expect that to be more stable, more important 32:34.333 --> 32:38.303 for the cations shown here, or for the alkenes, where the 32:38.300 --> 32:40.630 carbon is a carbon-carbon double bond? 32:40.633 --> 32:44.473 You have the same factor, that the carbon carbon double bond 32:44.467 --> 32:47.127 here would have a low LUMO. 32:47.133 --> 32:50.073 This has that p orbital on carbon. 32:50.067 --> 32:53.567 That has a pi-star orbital, a low LUMO, and 32:53.567 --> 32:56.367 the same deal here. 32:56.367 --> 33:00.797 Would you expect the effect of that mixing, HOMO-LUMO mixing 33:00.800 --> 33:04.730 in hyperconjugation, to be the same as when you 33:04.733 --> 33:06.003 have a double bond? 33:11.567 --> 33:15.297 If one stabilization is bigger than the other, which? 33:15.300 --> 33:20.930 So here we have the electrons in a sigma bond. 33:20.933 --> 33:23.033 And here we have a vacant orbital. 33:23.033 --> 33:26.773 In the case of the cation, we have just a vacant p orbital. 33:26.767 --> 33:31.667 In the case of the double bond, we have pi-star orbital, 33:31.667 --> 33:33.297 plus minus. 33:33.300 --> 33:35.200 Which one's going to give more 33:35.200 --> 33:39.100 stabilization of these electrons? 33:39.100 --> 33:41.570 What determines it? 33:41.567 --> 33:44.727 What determines how much stabilization you get out of 33:44.733 --> 33:47.373 mixing orbitals? 33:47.367 --> 33:48.297 Megan? 33:48.300 --> 33:49.230 STUDENT: Overlap? 33:49.233 --> 33:51.903 PROFESSOR: Overlap is one thing. 33:51.900 --> 33:55.100 So it's a p orbital on carbon in either case here. 33:55.100 --> 33:57.830 In one case, that p orbital is also associated 33:57.833 --> 33:59.873 with another p orbital. 33:59.867 --> 34:02.227 So each of them is 1 over the square root of 2. 34:02.233 --> 34:03.973 So it's smaller in one case. 34:03.967 --> 34:08.867 So there indeed would be less overlap in the case of the 34:08.867 --> 34:11.497 carbon-carbon double bond, rather than just a full 34:11.500 --> 34:12.930 fledged p orbital. 34:12.933 --> 34:14.003 So that's one factor. 34:14.000 --> 34:15.570 How about another factor, Megan? 34:15.567 --> 34:16.867 STUDENT: Energy? 34:16.867 --> 34:19.297 PROFESSOR: How about the energy match? 34:19.300 --> 34:21.270 STUDENT: When you have a positive charge 34:21.267 --> 34:23.697 it raises the energy. 34:23.700 --> 34:26.370 PROFESSOR: Think about that carefully. 34:26.367 --> 34:27.827 Say it again. 34:27.833 --> 34:28.803 STUDENT: It lowers the energy? 34:28.800 --> 34:31.370 PROFESSOR: The positive charge makes the energy fabulously 34:31.367 --> 34:33.267 low for the cation. 34:33.267 --> 34:37.397 The pi-star is low, compared to sigma-star But it's not 34:37.400 --> 34:40.700 really all that low, compared to the vacant orbital. 34:40.700 --> 34:43.870 So both on the basis of overlap, and on the basis of 34:43.867 --> 34:47.027 energy match, you expect a much bigger factor for the 34:47.033 --> 34:48.633 cation than you do for the alkene. 34:48.633 --> 34:50.073 And in fact, that's what you see. 34:50.067 --> 34:51.567 It's worth a heck of a lot more. 34:54.133 --> 34:57.503 But the bond energy factor might be more or less similar 34:57.500 --> 35:00.600 between the two, because in both cases, you're changing CH 35:00.600 --> 35:02.730 to C-C single bonds. 35:02.733 --> 35:04.973 But this is the one that should make a really big 35:04.967 --> 35:05.627 difference. 35:05.633 --> 35:09.673 But there's another factor as well, actually, which is that 35:09.667 --> 35:11.867 these are measured in the gas phase. 35:14.433 --> 35:16.303 So we have this positively charged 35:16.300 --> 35:19.230 carbon in the gas phase. 35:19.233 --> 35:23.573 We saw, when we talked about the ionization of water, how 35:23.567 --> 35:29.127 much effect solvation has on the ability to form ions. 35:29.133 --> 35:33.373 So here we're trying to form an ion on this carbon. 35:33.367 --> 35:40.827 Can you see another difference among these, about 35:40.833 --> 35:46.473 stabilization by this positive charge, other than 35:46.467 --> 35:49.597 hyperconjugation and bond energy? 35:52.833 --> 35:53.403 Nathan? 35:53.400 --> 35:56.270 STUDENT: The top one's more polarizable. 35:56.267 --> 35:56.827 PROFESSOR: Right. 35:56.833 --> 35:59.833 The other stuff in the molecule, the other three 35:59.833 --> 36:04.533 carbon atoms and their hydrogens, are polarizable. 36:04.533 --> 36:08.003 But remember, there's a very strong distance dependence. 36:08.000 --> 36:10.400 You have to be really close to get a lot of 36:10.400 --> 36:12.030 advantage out of that. 36:12.033 --> 36:16.033 So with t-butyl, you have all the other atoms really close 36:16.033 --> 36:21.833 to the positive charge to be stabilized by polarization. 36:21.833 --> 36:25.173 Here you have that one much further away. 36:25.167 --> 36:28.627 Here you have two of them significantly further away. 36:28.633 --> 36:31.203 And here you have this one significantly further away, 36:31.200 --> 36:34.900 and this one off in left field. 36:34.900 --> 36:38.030 So we can see that there's going to be "intramolecular 36:38.033 --> 36:42.273 solvation" you might call it, but nobody else calls it that. 36:42.267 --> 36:45.867 But this effect from polarizability in the gas 36:45.867 --> 36:47.797 phase is going to be very important. 36:47.800 --> 36:52.200 But if you go into solution, it's just the reverse. 36:52.200 --> 36:58.930 Because the primary one up here has the positive charge 36:58.933 --> 37:00.503 very exposed. 37:00.500 --> 37:04.270 So neighboring molecules could get very close to that. 37:04.267 --> 37:09.527 So in the gas phase, it's the atoms within the molecule that 37:09.533 --> 37:13.203 are solvating the positive charge, and their electrons 37:13.200 --> 37:14.670 being stabilized. 37:14.667 --> 37:17.197 But if you go into regular solution, it turns around. 37:20.667 --> 37:25.697 So these differences in the gas phase are much bigger than 37:25.700 --> 37:28.170 the effects that you would actually see in a solution 37:28.167 --> 37:34.267 reaction, where the primary cations are more stabilized by 37:34.267 --> 37:35.927 the surrounding molecules. 37:35.933 --> 37:40.473 So for example, if you do SN1 reaction, t-butyl bromide is 37:40.467 --> 37:45.327 only about 5 kilocalories per mole easier to ionize than 37:45.333 --> 37:47.073 isopropyl bromide. 37:47.067 --> 37:49.067 That we saw in that-- 37:49.067 --> 37:54.097 remember where methyl, ethyl, isopropyl t-butyl. 37:54.100 --> 37:58.400 t-Butyl was unusually easy to substitute, 37:58.400 --> 38:01.270 because it went by SN1. 38:01.267 --> 38:05.897 But if you looked at the isopropyl, it also was going 38:05.900 --> 38:07.700 partly by SN1. 38:07.700 --> 38:10.900 And it turns out that it's about 5 kilocalories harder 38:10.900 --> 38:12.300 for isopropyl. 38:12.300 --> 38:19.270 But here the difference between t-butyl and the 38:19.267 --> 38:20.897 isopropyl-- 38:20.900 --> 38:22.170 the equivalent of isopropyl-- 38:24.967 --> 38:30.727 is 16 kilocalories per mole, or even more. 38:30.733 --> 38:34.973 So it's very strongly attenuated in solution. 38:34.967 --> 38:40.967 People often talk about gas phase being intrinsic values 38:40.967 --> 38:44.167 for things you can measure, like the heats of formation of 38:44.167 --> 38:46.367 these things, that that's the ideal. 38:46.367 --> 38:48.767 The reason it's ideal is you can compare it with 38:48.767 --> 38:50.467 calculation. 38:50.467 --> 38:52.927 But it's not an ideal reference if what you're 38:52.933 --> 38:56.203 interested in is predicting reactions in solution, where 38:56.200 --> 38:58.370 that solvation is going to make a lot of difference. 39:01.533 --> 39:07.603 Now here's addition of HCl to an alkene. 39:07.600 --> 39:11.300 So is this Markovnikov or anti-Markovnikov addition? 39:20.867 --> 39:24.697 Natalie, do you remember what Markovnikov and 39:24.700 --> 39:25.200 anti-Markovnikov means? 39:25.200 --> 39:29.700 STUDENT: Your anion goes to the more substituted-- 39:29.700 --> 39:33.570 PROFESSOR: Does the anion, the non-hydrogen thing, go to the 39:33.567 --> 39:37.397 more substituted carbon in this case? 39:37.400 --> 39:38.330 Yeah, it did. 39:38.333 --> 39:39.173 And why? 39:39.167 --> 39:42.397 Of course, what we would expect is that it would be 39:42.400 --> 39:46.800 attacked, and this would give the more substituted cation, 39:46.800 --> 39:49.770 rather than attacking the central carbon, and having the 39:49.767 --> 39:52.527 primary cation as the intermediate. 39:52.533 --> 39:56.733 Then chloride goes over and forms a new bond, and you have 39:56.733 --> 39:57.603 the product. 39:57.600 --> 40:01.430 So that's Markovnikov addition. 40:01.433 --> 40:04.033 However that's only 17% of the product. 40:09.200 --> 40:12.770 The 83% of the product is that. 40:16.233 --> 40:18.473 Can anybody see what's funny about that product? 40:18.467 --> 40:22.397 Is that the anti-Markovnikov addition? 40:22.400 --> 40:23.330 Natalie? 40:23.333 --> 40:27.173 STUDENT: No, it's also just as equally substituted-- 40:27.167 --> 40:28.627 PROFESSOR: I can't hear very well. 40:28.633 --> 40:31.233 STUDENT: That carbon is equally substituted as in the 40:31.233 --> 40:32.533 first case. 40:32.533 --> 40:34.733 PROFESSOR: Ah, the substitution of 40:34.733 --> 40:37.473 the carbons is different. 40:37.467 --> 40:38.727 What has happened? 40:42.033 --> 40:44.933 We saw this before. 40:44.933 --> 40:45.403 Chris? 40:45.400 --> 40:46.830 STUDENT: A methyl shift. 40:46.833 --> 40:48.333 PROFESSOR: A methyl has shifted. 40:48.333 --> 40:49.103 It's rearranged. 40:49.100 --> 40:54.070 In fact, what shifts is a methyl group, but it's a 40:54.067 --> 40:57.227 methyl with its electrons. 40:57.233 --> 41:02.103 It's a methide shift when you're talking about cations, 41:02.100 --> 41:03.330 as you will see here. 41:03.333 --> 41:08.303 So it's shifted a methyl across, so what's happened is 41:08.300 --> 41:12.800 that those electrons--it's like hyperconjugation, that 41:12.800 --> 41:16.000 those sigma electrons are stabilized by the vacant 41:16.000 --> 41:18.400 orbital on the carbon. 41:18.400 --> 41:21.270 But they're stabilized so much that the methyl with its 41:21.267 --> 41:23.727 electrons, with those bonding electrons, the 41:23.733 --> 41:25.833 methide, shifts across. 41:29.733 --> 41:33.073 If you want to have fun, sometimes when you draw a 41:33.067 --> 41:35.797 rearrangement, you draw a little loop in the arrow, 41:35.800 --> 41:38.100 isn't that cute? 41:38.100 --> 41:41.630 So the methide has shifted across to give this more 41:41.633 --> 41:44.633 stable cation, this was secondary, this one is 41:44.633 --> 41:50.903 tertiary, and then that gives 83% of the product. 41:50.900 --> 41:54.830 So what this says is that the rearrangement, the methide 41:54.833 --> 41:59.803 shift, competes with reaction, with the collapse of the ions, 41:59.800 --> 42:02.600 chloride attacking the positive carbon. 42:02.600 --> 42:08.830 So both things happen, and they have similar rates. 42:08.833 --> 42:14.903 Now suppose you had a lot of some other nucleophile there, 42:14.900 --> 42:16.930 like water. 42:16.933 --> 42:20.403 Now the unshared pair of water could fill the role that 42:20.400 --> 42:26.970 chloride filled here, then lose protons, so you got H2O+, 42:26.967 --> 42:28.497 then it loses a proton. 42:28.500 --> 42:31.600 So you can get this alcohol, or you can get 42:31.600 --> 42:33.700 the rearranged alcohol. 42:33.700 --> 42:36.630 So the fact of this rearrangement is, of course, 42:36.633 --> 42:39.973 additional evidence that you had the cation intermediate as 42:39.967 --> 42:45.827 it was in SN1 reactions, where the rearranged skeleton showed 42:45.833 --> 42:47.403 that there'd been a cation intermediate. 42:51.033 --> 42:58.403 So when you do acid-catalyzed hydration of a double bond, 42:58.400 --> 43:03.200 adding H and OH, you have to be aware that there's the 43:03.200 --> 43:04.800 possibility of rearrangement. 43:04.800 --> 43:08.970 If the skeleton is one where you would form one cation, but 43:08.967 --> 43:14.127 then a methide shift, or a hydride shift, could give a 43:14.133 --> 43:16.933 more stable one. 43:16.933 --> 43:18.773 Now let's look at the influence of 43:18.767 --> 43:23.327 halogen in these cations. 43:23.333 --> 43:29.133 So here we start with acetylene and add HCl. 43:29.133 --> 43:34.203 So we've started to give the more substituted vinyl cation. 43:34.200 --> 43:38.530 Remember the table, it said primary vinyl cations are less 43:38.533 --> 43:42.773 stable than secondary vinyl cations, even though the data 43:42.767 --> 43:45.527 was not the appropriate data to support that, it's 43:45.533 --> 43:50.603 absolutely true that vinyl cations are very hard to make. 43:50.600 --> 43:53.330 So this is not such an easy process. 43:53.333 --> 44:00.203 But then it can add chloride, so you have this product, 44:00.200 --> 44:03.300 which is the Markovnikov product. 44:03.300 --> 44:06.230 The proton added to the terminal carbon to make the 44:06.233 --> 44:09.473 more stable secondary, rather than adding to the central 44:09.467 --> 44:12.497 carbon to make the less stable primary cation, so the 44:12.500 --> 44:14.270 chloride ends up there. 44:14.267 --> 44:17.427 And this is perfectly reasonable. 44:17.433 --> 44:20.933 But it went by way of this unstable vinyl cation. 44:20.933 --> 44:25.233 So it turns out that this addition, this kind of 44:25.233 --> 44:31.833 electrophilic addition to alkenes, is 100 to 1,000 times 44:31.833 --> 44:35.803 slower than it-- 44:35.800 --> 44:40.470 to alkynes is 100 to 1,000 times slower than to alkenes. 44:40.467 --> 44:43.397 It's much harder to add to triple bonds than to double 44:43.400 --> 44:47.830 bonds, because you get these unfavorable vinyl cation 44:47.833 --> 44:49.173 intermediates. 44:49.167 --> 44:51.027 Well fine. 44:51.033 --> 44:52.673 But now we have a double bond. 44:56.367 --> 44:59.867 We have Markovnikov regiochemistry, but if you 44:59.867 --> 45:05.527 have excess HCl, you can add it again, and get a second 45:05.533 --> 45:07.203 Markovnikov addition. 45:11.800 --> 45:15.770 Now suppose you didn't have an excess of HCl. 45:15.767 --> 45:18.327 Suppose you just had one equivalent of HCl. 45:21.233 --> 45:24.703 What product would you expect to get? 45:24.700 --> 45:25.500 Jack? 45:25.500 --> 45:26.470 STUDENT: Still that. 45:26.467 --> 45:26.967 PROFESSOR: Pardon? 45:26.967 --> 45:27.427 STUDENT: Still that. 45:27.433 --> 45:28.633 PROFESSOR: Why still that? 45:28.633 --> 45:32.373 STUDENT: Because the double bond is easier to attack. 45:32.367 --> 45:34.497 PROFESSOR: Ah! The double bond is easier to attack than the 45:34.500 --> 45:35.500 triple bond. 45:35.500 --> 45:39.700 So if you didn't have enough HCl, still this would be much 45:39.700 --> 45:40.930 more reactive than this. 45:40.933 --> 45:44.803 After you form any of that, it will be reacting 100 to 1,000 45:44.800 --> 45:48.270 times faster per molecule than that did. 45:48.267 --> 45:50.167 So you're still going to get this product. 45:50.167 --> 45:51.427 Jack's right. 45:55.233 --> 45:59.303 Under certain conditions, these are the yield you got. 45:59.300 --> 46:02.700 It was 56% and 44%. 46:02.700 --> 46:06.330 I suspect that if the reaction had been carried on longer, 46:06.333 --> 46:09.333 you would do what Jack did, well you'd 46:09.333 --> 46:10.933 go all the way there. 46:10.933 --> 46:14.703 But under certain conditions at least, the reaction was 46:14.700 --> 46:19.600 interrupted, and that was the product distribution. 46:19.600 --> 46:23.030 Now it went obviously by way of this cation intermediate, 46:23.033 --> 46:28.633 the second step, protonated to give this substituted cation. 46:28.633 --> 46:34.873 Now we have a question: is the halogen, as a substituent, 46:34.867 --> 46:38.267 favorable or unfavorable for purposes 46:38.267 --> 46:39.527 of forming the cation? 46:50.433 --> 46:51.773 Jack help us out here. 46:55.467 --> 46:59.167 Was this second step what you expected to 46:59.167 --> 47:00.497 be in terms of rate? 47:05.867 --> 47:07.327 STUDENT: No. 47:07.333 --> 47:09.833 PROFESSOR: Is it unusually fast or unusually slow 47:09.833 --> 47:13.303 compared to what you expected? 47:13.300 --> 47:14.800 STUDENT: Slow? 47:14.800 --> 47:15.300 PROFESSOR: Fast? 47:15.300 --> 47:16.270 Slow? 47:16.267 --> 47:19.127 What did you say before? 47:19.133 --> 47:21.303 This should react 100 to 1,000-- 47:21.300 --> 47:22.600 being a double bond-- 47:22.600 --> 47:25.670 should react 100 to 1,000 times faster than this. 47:25.667 --> 47:29.267 Did it react 100 to 1,000 faster? 47:29.267 --> 47:29.697 STUDENT: No. 47:29.700 --> 47:30.170 PROFESSOR: No. 47:30.167 --> 47:35.327 It didn't react any faster at all, or roughly the same. 47:35.333 --> 47:41.233 So for some reason this was slowed down by some several 47:41.233 --> 47:43.673 powers of 10. 47:43.667 --> 47:46.297 What could've slowed it down? 47:46.300 --> 47:49.470 STUDENT: The chlorine could've stabilized the cation. 47:49.467 --> 47:52.097 PROFESSOR: Because of the chlorine, it could be hard to 47:52.100 --> 47:53.730 get a cation. 47:53.733 --> 47:55.333 Would that make sense to you? 47:55.333 --> 47:56.303 Why? 47:56.300 --> 47:57.800 STUDENT: Because the chlorine has 47:57.800 --> 47:59.270 electrons that can overlap. 48:02.600 --> 48:04.930 PROFESSOR: The question is how hard is it going to be to form 48:04.933 --> 48:08.803 this cation, which we did by putting a 48:08.800 --> 48:10.670 proton on that carbon. 48:10.667 --> 48:14.367 The alkene carbon gets a proton gets that cation. 48:14.367 --> 48:16.727 So is it harder? 48:16.733 --> 48:18.003 Sebastian, what do you say? 48:21.300 --> 48:24.200 What effect do you think the chlorine had? 48:24.200 --> 48:26.700 STUDENT: The electron induction pulls the electron 48:26.700 --> 48:27.700 away from-- 48:27.700 --> 48:31.300 PROFESSOR: The sigma bond is electron withdrawing. 48:31.300 --> 48:35.000 The electrons around the carbon are lower in energy 48:35.000 --> 48:38.130 because they've been sucked away by the chloride, so it's 48:38.133 --> 48:41.233 harder to form the cation. 48:41.233 --> 48:46.103 That would explain why we get this ratio here. 48:46.100 --> 48:48.530 That this second step, even though it's an alkene and 48:48.533 --> 48:52.073 should be enormously faster, is not any faster, because 48:52.067 --> 48:53.767 this chlorine is slowing it down by 48:53.767 --> 48:55.267 pulling electrons away. 48:55.267 --> 48:58.267 It can't provide electrons to the proton. 48:58.267 --> 49:00.227 Now Sebastian, keep going for us. 49:00.233 --> 49:01.733 How about the regiochemistry? 49:04.767 --> 49:08.197 So the second step is slow, that's what we were just 49:08.200 --> 49:09.400 talking about. 49:09.400 --> 49:14.230 How about the regiochemistry of the second step? 49:14.233 --> 49:18.833 That is, is it Markovnikov or anti-Markovnikov? 49:18.833 --> 49:23.003 That is, the proton could have added here, to give this 49:23.000 --> 49:26.930 cation, or it could have added to the central carbon to give 49:26.933 --> 49:28.573 the other cation. 49:28.567 --> 49:29.767 Which did it do? 49:29.767 --> 49:32.897 Did the new chlorine go on the more substituted or the less 49:32.900 --> 49:33.300 substituted? 49:33.300 --> 49:35.030 STUDENT: On the more substituted. 49:35.033 --> 49:36.873 PROFESSOR: On the more substituted. 49:36.867 --> 49:42.767 So the chlorine slows it down, but it's still Markovnikov. 49:42.767 --> 49:45.967 This cation is better to get then the other cation. 49:49.533 --> 49:53.733 STUDENT: It reacts better with the chlorine. 49:53.733 --> 49:57.103 PROFESSOR: Now you might say, aha. 49:57.100 --> 50:00.570 The choice here is between a secondary, whatever the 50:00.567 --> 50:02.267 chlorine is, and a primary. 50:02.267 --> 50:04.897 And primaries are bad to get, you don't want to protonate 50:04.900 --> 50:07.800 the center one, because it would be a primary cation. 50:07.800 --> 50:14.370 But notice here with HBr, when it's symmetrical. 50:14.367 --> 50:16.927 Still the two bromines go to the same place. 50:19.700 --> 50:23.400 So that it looks like the chlorine is welcoming the 50:23.400 --> 50:25.970 formation of a cation. 50:25.967 --> 50:28.867 Or at least the bromine, in this case, is welcoming 50:28.867 --> 50:33.427 formation of that cation, rather than that cation in adding 50:33.433 --> 50:35.603 the second proton. 50:35.600 --> 50:37.070 So the answer is yes. 50:40.167 --> 50:42.367 Is it favorable or unfavorable? 50:42.367 --> 50:45.127 It's both. 50:45.133 --> 50:48.903 It's hard to make the cation because of this sigma electron 50:48.900 --> 50:49.800 withdrawal. 50:49.800 --> 50:55.470 But if you form the cation, where do you want to form it? 50:55.467 --> 50:56.597 Next to the chlorine. 50:56.600 --> 50:58.830 Why? 50:58.833 --> 51:01.373 Chris, do you have an idea? 51:01.367 --> 51:06.197 If you're forced to make a cation, say in this case when 51:06.200 --> 51:07.930 you put that second bromine in, you could've made the 51:07.933 --> 51:10.073 cation either here or here. 51:10.067 --> 51:13.427 Both of them with respect to the carbon would be secondary, 51:13.433 --> 51:16.073 but this one, the one you actually form, where the 51:16.067 --> 51:20.367 bromide comes in has a bromine on it. 51:20.367 --> 51:24.927 It's hard to form it because the sigma bonds are electron 51:24.933 --> 51:25.573 withdrawing. 51:25.567 --> 51:28.167 But if you form it, form it next to the bromine. 51:28.167 --> 51:28.797 Why? 51:28.800 --> 51:30.700 STUDENT: It's polarizable, so it makes 51:30.700 --> 51:31.530 it even more effective. 51:31.533 --> 51:33.173 PROFESSOR: Polarizability is a good point. 51:33.167 --> 51:34.927 That wasn't what I was thinking of, 51:34.933 --> 51:36.233 but it's a good point. 51:36.233 --> 51:37.633 What else? 51:37.633 --> 51:39.003 STUDENT: Lone pairs on the halogen. 51:39.000 --> 51:42.100 PROFESSOR: Lone pairs on the halogen can be stabilized by 51:42.100 --> 51:45.230 overlapping, so you get resonance stabilization. 51:45.233 --> 51:48.573 So if you're going to make it, make it where you can get help 51:48.567 --> 51:50.327 in the pi system. 51:50.333 --> 51:51.733 So that's really, I think, 51:51.733 --> 51:54.333 interesting about the halogens. 51:54.333 --> 51:58.073 That they could be deactivating, but still want 51:58.067 --> 52:00.597 to have the cation next to them, because of the different 52:00.600 --> 52:04.330 role of the sigma electrons, and the pi electrons. 52:04.333 --> 52:07.233 OK, thanks for sticking around here.