CHEM 125a: Freshman Organic Chemistry I

Lecture 25

 - Models in 3D Space (1869-1877); Optical Isomers


Despite cautions from their conservative elders, young chemists like Paternó and van’t Hoff began interpreting molecular graphs in terms of the arrangement of a molecule’s atoms in 3-dimensional space. Benzene was one such case, but still more significant was the prediction, based on puzzling isomerism involving “optical activity,” that molecules could be “chiral,” that is, right- or left-handed. Louis Pasteur effected the first artificial separation of racemic acid into tartaric acid and its mirror-image.

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Freshman Organic Chemistry I

CHEM 125a - Lecture 25 - Models in 3D Space (1869-1877); Optical Isomers

Chapter 1. Venturing into 3-D Arrangements of a Molecule’s Atoms [00:00:00]

Professor Michael McBride: Okay, I need two volunteers here at the beginning, while we’re waiting for everybody else to come in. You have to come up here and do something. Come on up. Okay. So each of you — actually I don’t want two people that are sitting together. So Lucas, you; and Julia stay. But Rick, why don’t you come up? So here, come here, come here. So your job is to grind — this is a spice — so you grind it up good, and then we’re going to take it back and show it to the people around you; we’re going to smell it. Okay? See if you can — you have to really grind it, like this, look. Okay? So we’ve got two spices here that we need to identify. Okay, you can go back and grind them at your seat and then show it your neighbors. Later on I’m going to ask you whether you know what it is.

Okay, so here’s how we’re going through the history of organic chemistry and the development of the molecular model. So generation by generation. And last time we got up to students of Kekulé, who actually did experiments to allow people to figure out something about structure, not only what was connected to what — that’s constitution; remember, the nature and the sequence of bonds. So we had composition, what elements are there, what ratio they’re in, how many atoms. Right? Then constitution, the nature and sequence in bonds. And now we’re going to see if you can go beyond that, without cheating and using X-ray diffraction, to figure out how things are arranged in space; so structure. And we talked last time about Koerner’s proof of positions in benzene, and his proof that all the positions in benzene were equivalent to one another. And today we’re going to go really to the next generation. Koerner wasn’t all that much younger than — or he was at least halfway in the generation there. But van’t Hoff is a full generation beyond. So we’re going to look at him. It says under him “tetrahedron”, and it says down here “tetrahedral carbon”. And also we’ll see something about Albert Ladenburg and about your, whatever, great-great-chemical-grandfather, Peter Kohler at Harvard.

Okay, so as we were closing last time we were looking at this definite picture of the arrangement of atoms in space, where the models are clearly real models, held together by rubber tubing, and they’re tetrahedral, and it explains why there can be three isomers of disubstituted — symmetrically disubstituted — ethane. Of course it was wrong. Do you know why it was wrong, why there are not three different models? Yeah. Because you can rotate about the middle bond freely; it’s not frozen in one rotation here. Or is it? We’ll see something about that later on. That turns out to be what’s called conformation. Remember, that’s one of the C’s. But now we’re in configuration, how things are attached to the tetrahedron. Okay, so after Paternó had published that, he got a letter from his former sponsor, who had moved on, Adolf Lieben, an Austrian who was at this time in Italy; first in Palermo and then, at this time I think he was in Northern Italy, I think, in Pisa — oh, Torino; it says up at the top, “Torino, 25th of June, 1869.” And you can see a translation there if you click. But anyhow it says:

“My dear Emmanuele.” (They corresponded, like several of these people who were great friends; were for a very long time.) “Permit me to make an observation regarding the isomerism pointed out by you and by Cannizzaro.” (It’s a little bit misleading to say “by Cannizzaro.” He was very cautious about this, as you know, but he introduced the paper.) “Between” (and he shows those two structures here. Which look like type structures, with those brackets. And you can see what’s fastened on the two carbons is H2Cl H2Cl, and H2Cl, H2Cl. But he wrote the Cl’s differently, obviously meaning to denote this difference in phase of rotation.) “Certainly it’s not absurd, this kind of isomerism, even without any differences among the four valences of carbon.” (Remember one could’ve been straight, one crooked, short, long or something like that.)

“I believe, however, that I don’t err when I suppose that the same cause which keeps me from accepting it will also keep all the other chemists from doing so, in a silent agreement with one another. You perhaps haven’t thought that accepting this kind of isomerism crosses the Rubicon which separates speculation that’s considered legitimate about the mode of combination of atoms from speculation which is less legitimate about the position of the atoms in space.” (And notice here that he added as an afterthought, vero; the true position of the atoms in space, rather than just chemical position. Right?) “In all speculations that have been made up to this time, people have always been careful never to let enter in either the relative positions in space (Distances and surroundings of the atoms). This idea doesn’t even enter into the speculations of Kekulé about isomerism and the aromatic series.” (So he draws the picture of the hexagon and numbers 1 to 6.) “Kekulé believes that chlorine 1, chlorine 2 positions is different from chlorine 1,3 and from 1,4, because in the first case the two carbon chlorines find themselves combined with one another, but in the second case they’re separated by CH, and in the third case they’re separated by two CH’s. And to accept this, one has no need to make hypotheses about the surroundings of the atoms.”

That’s really weird isn’t it? He says in one case they’re connected to one another, in another case there’s a CH in between, and in another case there are two CH’s in between. But to talk about this you don’t have to talk about the surroundings of the atoms. That doesn’t make sense to us, right? But it’s the difference between drawing a graph, which is just a graph, and drawing something that’s supposed to show something in space. He goes on and says,

“It could be that the two chlorines in positions 1, 2 are in truth more distant from one another than the ones in positions 1, 4!” (And he draws an exclamation point there. Right?) “This just doesn’t enter into Kekulé’s hypothesis. Making hypotheses about the relative positions of the atoms in space would make possible two isomers of dichloromethane, even accepting the symmetry of the four valences…” (That is, that all the valences of a single carbon are equal to one another.) “…and the existence of a single isomer of methyl chloride. For example,”

And he draws the two structures here, the ones where the chlorines are far from one another and when they’re near one another; but that’s just the graph, that’s not where they are in space. Okay. He added as an afterthought between the lines here, “There would also result two isomers of ethyl chloride.”

“I repeat that I don’t consider this kind of isomerism absurd. But since we have no means to know the topographic position of the atoms, while we have many to know how they are connected to one another, I consider it a bit dangerous for science. Shooting off into space in search of atoms, one risks losing the ground under his feet.” (Right? So be careful former student.) “Greeting to Cannizzaro, and Koerner, Amato. I’ll write soon to Compisi. Your friend, Adolf Lieben.”

Okay, so be careful. This isn’t what structures are about, right? They’re just graphs that show constitution. So here’s Koerner, you remember, before he went south, in Ghent, with Kekulé. And on the right of this group is Albert Ladenburg who, once you know, once you’ve read what he wrote, you can sort of read into his attitude there, that he was a hard guy to deal with. Okay? He thought he knew a lot, and he did know a lot, but not as much as he thought. So he published this paper the next year, in 1869; or the same year that Paternó published his thing.

“Kekulé adopts two hypotheses: (1) The 6 hydrogen atoms of benzene are equivalent;” (that’s what Koerner actually proved) “and (2) Each hydrogen atom of benzene has two sets of two others which are symmetrically arranged with respect to it; that is, 1.2 is the same as 1.6, and 1.3 is the same as 1.5, although 4 is unique with respect to 1.” (Okay? So there are two that are symmetrically arranged.) “Already several years ago I had occasion to make Kekulé aware that his graphical formula for benzene is not adequate, since here 1.2 and 1.6 must be inequivalent, while one could have various views about whether positions 3 and 5 are equivalent or not.”

Why would one and two be certainly — two and six, that is, be certainly inequivalent, but three and five might or might not? Yeah, Kate?

Student: Because between one and two is a double bond and between one and six is a single bond.

Professor Michael McBride: Both have a single and a double. But what about 1.3 and 1.5? Are they 100% equivalent?

Student: [inaudible].

Professor Michael McBride: Ah ha! because one is single-double and the other is double-single, going the other way. So that might or might not be equivalent, Ladenburg says. “Both conditions could, however, be fulfilled through alternative formulas which, so far as I know, have not been proposed before.” So here are Ladenburg’s formulas. And the one in the middle, the triangular prism, actually came to be called, 100 years later, or a little, seventy years later, Ladenburg benzene, right? Because someone was able to synthesize that. It’s also called prismane, because it looks like a prism, right? And then there’s this funny structure on the right.

Chapter 2. Exchanges between van’t Hoff and Ladenburg on Aromaticity and Chirality [00:11:41]

Okay, so this is 1872, and we’re going to — there’s van’t Hoff, who is an eighteen-year-old student, come to Bonn to see the great Kekulé. And he writes, in 1876, at the age of 24, a reply to Ladenburg. “Proof that the prism formula” (of Ladenburg) “suffers from the same problem as the hexagon with fixed double bonds removes the previously asserted superiority from the former” (from the prism) “and makes the original Kekulé notation not only simpler, but also the presentation with which the facts conform best.” Because Kekulé had the idea that what these lines really meant was how often the bonds collide — how often the atoms collide with one another. They collide more often, he thought, when it’s a double bond than when it’s a single bond. So he thought that in this case, although you would draw single, double, single, double, that the number of collisions actually was the same one way and the other. So you’d draw both formulas to be in between what we would call now resonance formulas. So anyhow, this young student, twenty-four years old now, says Ladenburg’s off base in saying the prism is better, because it has the same problem and you can’t get out of it with this vibration thing.

Okay, “The 1,2; 5,6; and 3,4 derivatives are completely similar, although differing from 4,5; 2,3; 6,1.” So 1,2; 5,6; and 3,4 are completely similar. So here’s 1,2; 3,6; — 1 5,6; and 3,4. Right? And you can see that just by rotating by 120° you go from one to the next. So those he says “are completely similar, although differing from 4,5; 2,3; 6,1.” Do you see how they differ? 4,5; 2,3; 6,1. 4,5; 2,3; 6,1. Now are the red and the blue different or not? The reds, obviously you just rotate and you get from one to the next. How about from a red to a blue? Can you rotate to get from one to the other, from a red to a blue? Say the one’s in front? What happens if you rotate to put a red line on a blue line?

Student: It’s sitting on a square face.

Professor Michael McBride: Sam?

Student: It’s sitting on a square face instead of —

Professor Michael McBride: Ah, then it’s sitting on a square face instead of triangular face. It’s not the same anymore. Everybody see that? Right? This, as drawn, it has a triangle on top. If you turn it 180° [correction: 90°], so as to superimpose the blue in front, on the red, then you no longer have a triangle on top. It’s not the same thing, van’t Hoff, twenty-four years old, says. What do you think Ladenburg would say about that? Well we’ll see. But van’t Hoff goes on. He says: “A 1,3 product is different according to whether A or B occupies position 1.” (In the right, there’s single double, double single, when you go from A to B. Okay?) “Exactly the same thing happens in Ladenburg’s formula.” (With the prism.) “An adequate consideration shows that I and II are absolutely different.” So one. And notice he puts dashed lines in. Why? Why is one bond shown dashed? Says so, right? Because it’s behind, it’s hidden. So he’s definitely showing this in three dimensions. And if it’s in three dimensions, then AB goes left to right, in one, and right to left in the other, and you can’t superimpose them, they’re not the same thing; according to van’t Hoff. Now what would Ladenburg say about this? He wouldn’t take it lying down. He wrote back and said, “van’t Hoff finds the two formulas below absolutely different.” How is what he wrote different from what van’t Hoff wrote? How are Ladenburg’s formulas different from van’t Hoff’s? Pardon me? It’s not what?

Student: It isn’t in 3D.

Professor Michael McBride: How do you know whether it’s in 3D or not?

Student: Well, then he’d use a dashed line.

Professor Michael McBride: Ah, he doesn’t use the dashed line. Okay? So he says they’re absolute — that van’t Hoff finds them absolutely different. But he’s just drawing constitution, what’s linked to what. Right? “I cannot agree with him in this. Van’t Hoff is dragging something into the formulas which I, together with most chemists, expressly exclude. I refer to the arrangement in space.” (Right? So keep that out. Okay?) “The formula takes account of the composition, molecular weight, and mode of union of the atoms.” So composition and what else? What other C?

Student: [inaudible].

Professor Michael McBride: Composition, that’s composition of molecular weight he says; and the “mode of union,” right? Which is constitution. It doesn’t talk about arrangement in space. Right?

“If van’t Hoff’s view of space really will not tolerate the two formulas above to be identical, then I invite him, for his own special purposes, to use a different benzene formula, which was proposed by me in the year 1869, when I compared the hexagon and the prism for the first time, together with the prism, to which it is for me certainly identical. It is the so-called Cross of David. Even van’t Hoff will have to find that these two formulas are absolutely identical.”

Okay, now how could the prism be identical to this? There’s the prism. They certainly don’t look identical. But watch this. Let’s twist it; keep all the bonds, but twist it. Right? So as long as you don’t have dashed lines for 3D, they’re the same; one is just twisted. Okay? So Ladenburg is expressly keeping this out. Now this point of view survived for quite awhile. Here’s some notes taken by a former Yale colleague, R. M. Fuoss, in the 1920s, when he was a student at Harvard in Kohler’s course. His notes say: “Benzene gives one monosubstitution product and three disubstituted. Cyclohexatriene” (if you had the ring with double single, double single) “apparently requires four disubstitution products. [So 1,2 and 1,6] should be different, it was argued.” But then he shows another example of an experiment that’s been done here, with this molecule. Now these hydrogens can be — it turns out that those hydrogens we’ll call later α hydrogens, because they’re the first, next to a carbonyl group. So here’s a carbon-oxygen double bond, and then a C that has the H; carbon-oxygen double bond, a C that has the H. Those are called α. And it was known that it’s possible to easily substitute α carbons with other things; more easily than other carbons typically, other hydrogens. So you’d expect to get two possible structures. You could do this α one and get the R in that position, or this one and get R in that position. So you’d get two products. Right? Those two.

“We can alkylate” (that is, put the R group on) “glutaconic acid in several ways, but we always get the same product 1. 2 cannot be made, no matter how we go. Hence it is not surprising that only one ortho disubstituted product for benzene exists. The double bonds shift until the stable system is reached.”

Right? So you can’t make one. Or if you make it, it immediately transforms to the other one. So there could be two, but there aren’t. Okay? So this is what? This is forty years later, almost half a century later, and he’s still saying the same thing. Okay?

Now, is it really that? Well you can calculate by Spartan that the minimum energy really is symmetrical with equal bond distances, but it vibrates. It can vibrate like this — right? — in and out, and the frequency of that is 1276 wavenumbers. You’ve taken spectra, you’ve seen that kind of thing. Although this particular one is not active in the infrared. You can’t see it in IR. It’s called breathing. [breathes loudly] Right? Okay, but there’s another way it can vibrate, like this: distort in that direction, then back to symmetrical, and in the other direction. See what that is? That’s distorting in the Kekulé direction, single double, single double. Right? So that one is at 1367 wavenumbers; the Kekulé distortion. But it’s around a single minimum. So at any given time the thing is probably vibrating. So it is single double, single double. But the lowest energy is all the same intermediate distance.

Okay, now might you be able to push on benzene and make it favor the Kekulé structure, with alternating bond distances? Well that’s a challenge. So a chemist named Jay Siegel and his colleagues synthesized this one, which you’ll notice has a four-membered ring, with funny bond angles, attached to one side of the benzene. That means those angles are going to be stretched and so on. So they’re going to distort and make that bond different. And indeed that bond, 1,6, is a little different. But on the other side… if it made that one — let’s say it’s a little bit long; make it single and the neighbors double, because of the way it’s being stretched — then if that propagated around the ring, single double, single double, single double, the ones on the left would be different lengths. But in fact they’re the same. Right? So it doesn’t happen here. So that’s the kind of thing that people sometimes do to test out the limits of these kinds of theories.

Chapter 3. In-Class Observations and Experiments on Chirality [00:22:58]

Okay, so we’ve seen the position determination. And now more about tetrahedral carbon. And to get there we’re going to step back just a bit to look at Louis Pasteur, who was in a different line of business at this time. Now it’s hard to prove that two samples are identical because there could always be some other test in which they appear to be different, even though everything you’ve done so far shows them the same. But let’s take carvone as an example. Here’s a sample of carvone, a bottle of carvone. And if I crack it a little bit, I can smell it. It has a very nice smell. Here, I’ll pass it around here. Don’t spill it obviously. Do you recognize the odor? Actually I have another bottle too. Here let’s start this one over here. I’ll start it with Catherine. So just smell that one. Don’t put your nose right down in it. But it’s not that strong. It’s very pleasant actually. Now, this stuff is called carvone. There’s a reason it’s called carvone. How are you doing Rick, with your grinding? Can you smell anything on it?

Student: Definitely can smell it.

Professor Michael McBride: What does it smell like?

Student: It smells to me like — I’m not sure if I know the exact name but it’s a little like sesame seeds —

Professor Michael McBride: A little like sesame seeds. But there’s another — pass it to Shai next to you here, let him smell. Is it familiar? How about anybody? How about the bottle here coming? Do you notice it? Is it a familiar scent?

Student: Smells like peppermint.

Professor Michael McBride: It’s spearmint, not peppermint. But you’re close. Shai, how are you doing? Does it smell like peppermint to you?

Student: I couldn’t say what kind mint it smells like.

Professor Michael McBride: A little bit like mint. Okay, how are doing Yoonjoo? You’re grinding up a different stuff. You’re grinding dill seed, in fact. Does it smell?

Student: Yes it does.

Professor Michael McBride: What does it smell like? Dill seed maybe, right? But carvone comes from — this is a familiar scent. I’m surprised you didn’t get it, Shai. Did anybody else recognize? Did you get it, Andrew? No. How about the bottle that’s coming along the back row there, that little bottle? Yeah, notice it?

Student: Rye.

Professor Michael McBride: It’s rye. Rye bread has that. It’s caraway, those seeds are caraway. And caraway is — carv is the Latin root for it. It’s like caraway, right? So carvone, this substance we’re passing around, carvone, caraway. Okay? Now, so let’s look at the — suppose we took these two bottles and you distilled them, found the boiling point. Both bottles have 230-231°. Right? The densities are exactly the same. The refractive indices for those two bottles, how much they bend light when it goes through, exactly the same. The infrared spectra are identical; to the extent that any IR spectra are identical. You know from experiment that if you put more in one sample than in another they don’t look the same. Right? Did you notice that in lab? If you ground up more stuff and less stuff, the spectra don’t really look the same, but then you can find there is a peak to peak correspondence; the intensities are never going to be exactly the same. But to the extent IR spectra can be the same, these are the same. If you take their NMR spectra they’re the same. So they must be the same. Or is there any property in which they differ? Any property that those two bottles differ in?

Students: Smell.

Professor Michael McBride: The smell is different. Now, those bottles — they’re in dark bottles, to protect them from light. But if I poured them out — they should be colorless liquids but I bet anything they’re brown — right? — because they’ve sat around and a little bit is oxidized. So it’s very difficult with scent, because little bits of other things can make them smell a little different. Right? You don’t need much stuff for scent of the proper thing. So it’s hard to be confident that this difference, that one of them is rye bread, caraway, and the other is spearmint; it’s hard to be sure that that’s a difference. But it turns out there’s another property you can measure too. And Karo syrup has that property. And I’m going to show it to you. It’s the ability to rotate polarized light. Now — oh, did I bring? — oh I forgot to bring my Polaroids. So what I’m going to do is ask a TA to go back to my office, and on the table in my office there’s a brown package about that big that has two sheets of Polaroid in it. There’s the key. Yeah, it’s not here.

Sorry. Let me think a second, what we can go on to. I’ll tell you the measure of this, which is you have light that’s polarized. Have you all dealt with polarized light? You probably have polarized sunglasses and you can turn it and see different light reflecting off things. Have you done that? Okay, and that’s why you have polarized sunglasses, because light that reflects doesn’t go through the particular direction that’s in the sunglasses. So you get rid of reflected light. Okay, so but what you measure is — when light goes through a polarized filter, it’s as if it were — you were shaking a rope and it was going through a picket fence. It can vibrate one way but it can’t vibrate the other way. What vibrates in light? What is vibrating? What’s light? We went through this before.

Student: Electromagnetic —

Professor Michael McBride: Electromagnetic radiation. What’s vibrating?

Student: Electric field.

Professor Michael McBride: The electric field. And it’s set up so one of these, if you have a Polaroid filter, gets absorbed, and the other one comes through. Okay, so light that comes through is vibrating this way. And if you put another one, that’s crossed with it, like this, what do you get — what do you see?

Student: Nothing.

Professor Michael McBride: Nothing, because the first one stops one and the second one stops the other one. If you turn it like this, then it goes on through. Okay? But suppose that you put something in the middle that rotated the plane of polarization. So the light’s vibrating like this. Suppose it’s going like this. Right? Suppose by the time it gets to the second filter it’s like this. Right? Now you have the second one like this. What do you see? Light comes right on through, because that’s the way it’s vibrating now; if you rotated the plane of polarization. Okay? That’s what “rotation” means. Okay, now you measure this property of a substance and call it specific rotation; which is how many degrees it rotates. And you have to say what light rotates, so that the units of α are degrees — how many degrees it changes, per gram per milliliter. If you put more stuff in, then you get a bigger effect. Right? So it depends on the concentration. So how many degrees per mole, you might say. Okay? But then it’s per decimeter, because the longer path you go, if it’s rotating, the further you go, the further it twists. So the old style cells were — oh, no, that’s not it. Nice try. This is really embarrassing. It’s an envelope.

Teaching Assistant: Oh it’s an envelope. Oh okay.

Professor Michael McBride: Did you see it?

Teaching Assistant: I can take another look.

Professor Michael McBride: Okay, good [laughs] Okay, let me show you the other things I’m going to show you. Okay here — actually here’s — we’ll do a trick. Here are models of carvone. Okay? Now I’m going to give them to you guys, and I want you to compare and see if they’re the same. Stand up and let people see what — so decide how you — start maybe at the top, with the methyl group. Each of you — turn — both of you turn and face the thing, so they can see. Now hold — start with the methyl group at the top. It starts with the methyl group. Now Russell, you describe it and then check and see whether — Eric, you check and see whether your model is the same as Russell’s. Okay? While you’re doing that, I’ll go help find the thing. So tell me when I get back.

[Professor McBride leaves the class briefly]

Professor Michael McBride: Okay what’s the answer? Are they the same?

Student: One of them’s pointing down. I can’t see it on the other one.

Professor Michael McBride: So is that real, do you think? Is that just a peculiarity of the models, or does that relate to molecules?

Student: I think it should relate to the smell of the thing.

Professor Michael McBride: Okay. Well anyhow — if I can catch my breath — these are Polaroid sheets. So here you see me, and here you don’t.

Students: Ohhh.

Professor Michael McBride: Okay? Now, let’s see a case of rotation. So here you see crystals and I’ll put this — I’ll get light coming through. And there’s the Polaroid filter, and here’s another Polaroid filter. Right? So I turn it 90°. And now let me put the crystals in there. Now, so there, most of it’s coming through. But now watch. What do you notice?

Student: [inaudible].

Professor Michael McBride: What’s happening?

Student: [inaudible].

Professor Michael McBride: What’s surprising about this? Obviously they’re rotating the plane of polarization — right? — so that I have to set a different angle in order to stop the light. But what’s special about this set of crystals?

[Students speak over one another]

Professor Michael McBride: Some are right-handed and some are left-handed. Right? That’s sodium chlorate. That’s the stuff that Gay-Lussac used. Right? So this property was discovered, of crystals, early in the century. Now how about liquids? Will liquids do it? Well here’s glass. Glass doesn’t do anything special, it’s just like the air around it. But let me pour water in here, and see if water does the trick. Nothing special, right? So let’s try Karo syrup. It’s not quite January, but it takes a little while. It’s a little bit yellow. It sat around awhile. What’s happening? Well there’s an underlying yellow that biases this. But we can get different colors. So different colors in the spectrum are being blocked out. How can that be, for different angles different colors? So the Karo syrup is rotating the polarization, but it’s rotating different colors different amounts. Right? So we block off different colors at different angles.

Okay, so if we’re going to make a measurement of the angle, how much it rotates, we’re going to have to say what color we’re talking about. And that’s what the D is. The D is the color. There’s a so-called D line of sodium, a yellow color. The yellow street lights are that color. Right? So that’s the color that was used, because it was easy to generate in the old times. And twenty is how many degrees; because it’s different for different temperatures. Okay, so you can measure the specific rotation of a sample. And if it’s in a solvent, you have to say what solvent it is, because it can vary with solvent. So anyhow, back to our comparison. These odors were the same, but we might not believe it. But if we measure the specific rotation, it’s equal and opposite. One is +62° and the other is -62°. So these things somehow are mirror images of one another. Now here is what you’ve already seen. You can write the structure of carvone, either longhand or shorthand. And there are the two models. And you correctly identified a difference between them, that one has a hydrogen pointing backwards and the other coming out toward you. And is this difference real? Of course there were differences in the sausage formulas too, but those weren’t real; remember, those two compounds turned out to be the same compound. The question is, is this difference real? Are there two carvones, but with different impurities in them, so they smell different, or is there just one? Oh pardon me, vice-versa from what I said there.

Chapter 4. Louis Pasteur’s Artificial Separation of Racemic Acid [00:39:14]

Okay, so tartaric acid turns out to be the same thing. Remember, Berzelius coined the name isomer to talk about tartaric acid and racemic acid, which both came as byproducts of the wine industry and had very, very different melting points. So obviously they’re quite different from one another. And one of them turns out to rotate light; if you make an experiment like this. It rotates light to the right, by 13° per gram, per milliliter, per decimeter. Okay. But if you heat it, you get another compound called pyrotartaric acid, or sometimes meso-tartaric acid. Meso doesn’t mean anything, except “between”. Do you know any cases where meso means between? Mesopotamia means between the Tigris and the Euphrates River, right? So it just means in between. So there was another one in between tartaric and racemic acid. It wasn’t between in melting point; it had 140° as its melting point. And it didn’t rotate light either. So there were these three different forms of tartaric acid, but all were found to have the same constitutional formula; the same things linked to the carbons.

So here’s a problem with the model, in terms of isomer numbers. Okay? Sometimes too many isomers are predicted, like Paternó’s dibromoethane. He had different phases of rotation; said those were different. In fact, they weren’t different, it was just experimental errors of someone who thought they were different. So sometimes the model predicts too many isomers. But often, or sometimes at least, it predicts too few; as in the case of maleic and fumaric acid where you have a double bond. There are two different substances with different melting points and properties. With lactic acid — remember that Scheele had found lactic acid from milk, but Liebig had found it from meat — and they were the same, it was found out about this time by a guy named Wislicenus. They had the same constitution, but one rotated to the right and the other rotated to the left. There was tartaric and racemic acid; which was even more complicated because there was also meso-tartaric and also what came to be called l-tartaric acid. And that was the discovery of Pasteur.

So in 1848, which was another revolutionary time in France, Pasteur was twenty-six years old and was becoming an interdisciplinary scientist: chemistry, physics, crystallography, all he was studying. And he wrote a paper called “On the relations that can exist among crystalline form,” (so crystallography) “chemical composition,” (chemistry) ” and the direction of rotatory polarization.” That is this stuff we’re talking about, which is physics. Now remember we talked about Mitscherlich and the ability to measure angles on crystals and tell — distinguish crystals by their angles. So these are pictures that Pasteur was using to draw his salts of tartaric acid. In particular sodium — it’s a diacid, and so you can have different cations; sodium ammonium tartrate is this one. And there’s his picture that he drew of it. And he said — mostly you see the faces that are shown in that picture, but sometimes you can see other little faces. Right? And you notice that the crystals have symmetry. They have a horizontal plane of symmetry and also a vertical plane of symmetry. Right? So if you can see one of those edges, you should see the others that are related by symmetry this direction, symmetry this direction, and also symmetry this direction. So you should see all eight, in the tartrate. But always only four of them are observed, in sodium ammonium tartrate; not all eight. Mitscherlich had studied this stuff, and he was sort of the father of measuring angles like this. But when Pasteur was trying to learn the technique and repeat it, he noticed that you don’t very often get these edges all together, these little tiny ones. It usually looks like the original picture did. But when those sharp edges are truncated by little additional faces, you only see four, you don’t see all eight.

So Mitscherlich had reported that the — now how…? — That was tartrate, right? Now they had the idea that racemate, which remember didn’t rotate light — Pasteur’s idea was that molecules were twisted in a right-hand or a left-handed helix, and therefore they had this effect on light, to make it twist. Right? But one’s that aren’t twisted, like racemic acid — Where the molecules aren’t twisted, then, at least in those, you should see all eight, because you will have mirror images. Okay? So he thought untwisted molecules would be what’s called “holohedral”; that is, have all their faces. So he studied the sodium ammonium racemate and was quite surprised to find that sometimes he saw the red ones, sometimes he saw the green ones, but never on the same crystal. Some crystals were right, red, and some were left, green. So he separated those crystals, the right ones from the left ones. He wrote: “I carefully separated the right from the left crystals and observing their dissolution separately, with Monsieur Biot’s polarization apparatus” (the thing that measures the rotation) “I saw with surprise and delight that the right crystals deviated the plane of polarization to the right and the left to the left.” So why did racemic acid not rotate light?

Student: [inaudible]

Professor Michael McBride: Because it was a 50:50 mixture of things that would rotate it one way — it’s not that it was untwisted — it was that some of them would rotate it one way and some would rotate it the other, and they’d cancel out. Okay. Oops I improved this and forgot to take that one out; sorry. Okay, so now he knows what’s going on, that racemic acid is actually a 50:50 mixture of right-handed, dextro-tartaric acid, d, that has a plus rotation, and left-handed tartaric acid, l, levo, which has a minus rotation, but the same melting point, the same rotation, but opposite in direction. So those are mirror images of one another. And the racemic acid is a 50:50 mixture. Now this was 1848. So that’s a long time from where we’ve gotten to. Right? That was ten years before the idea of bonds. Okay?

So that was “resolution”, the separation of a mixture into the two components. Right? But what is meso-tartaric acid? Why doesn’t it rotate? So now we go forward, twenty-four years, to Bonn, and there’s van’t Hoff studying with Kekulé, at the age of twenty. So that’s a quarter of a century. That’s a long time, right? It’s longer than you’ve lived. So here’s van’t Hoff. He had some opinion of himself. He won the first Nobel Prize — I mean, he had a high opinion that was completely justified — in 1901, the first Nobel Prize in Chemistry. And he didn’t get it for this, which he should’ve gotten it for. He got for inventing physical chemistry. The most admired trait he had was imagination. So he liked poets and artists. In fact, his hero was Lord Byron. Look at that. Okay? And in 1874, as we said, he was a student. And he wrote this, a pamphlet, in Dutch, which was then translated into German and called “The Arrangement of Atoms in Space,” and was published in 1877. It was fifty-three pages long and heavily illustrated. And look at the website for this. One of the Wikis I’ve assigned is that website. And also this website about criticism of van’t Hoff by a guy named Kolbe.

So here’s Hermann Kolbe. He was twice, more than twice van’t Hoff’s age, and a pillar of traditional chemistry. He wrote, in a review of this book: “It is completely impossible to criticize this booklet in any detail because the fancy trifles in it are totally devoid of any factual reality and are completely incomprehensible to any clear-minded researcher. The brochure begins with the words: ‘The modern chemical theory has two weak points. It says nothing either about the relative position or the motion of the atoms within the molecules.’” This is absurd, right? But he was wrong. Right? Now am I like Kekulé [correction: Kolbe]? Because I’m more than twice as old as you are. Right? So those are three websites you should read about this. But, in fact, it’s not true that van’t Hoff was devoid of any factual reality, because we’ve already seen some of his evidence, and we’ll see more next time.

[end of transcript]

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