CHEM 125a: Freshman Organic Chemistry I

Lecture 23

 - Valence Theory and Constitutional Structure (1858)


Youthful chemists Couper and Kekulé replaced radical and type theories with a new approach involving atomic valence and molecular structure, and based on the tetravalence and self-linking of carbon. Valence structures offered the first explanation for isomerism, and led to the invention of nomenclature, notation, and molecular models closely related to those in use today.

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Freshman Organic Chemistry I

CHEM 125a - Lecture 23 - Valence Theory and Constitutional Structure (1858)

Chapter 1. Archibald Couper: “Look to the Elements” [00:00:00]

Professor Michael McBride: So we’re going through the development of the theory and practice of organic chemistry, generation by generation. We started with Lavoisier, and then Berzelius and Gay-Lussac and Davy. And last time we did Dumas and Liebig and Wöhler. And now we’re onto the next generation, people that were born around 1830. And the new developments are going to be valence and benzene. Remember, in this list, things that are in boldface are experimental and things that are theoretical are in normal face. So valence and benzene is experimental, but what we’re interested in is the theory of benzene here. And the people who are associated with this are August Kekulé — and you’ve all heard of Kekulé structures and Kekulé benzene, and he’s memorialized on the front of the building here. He was born in 1829. But few of you probably have heard of Archibald Couper, who is not on the front of the building, and was born two years later in 1831. This is Couper as a student. He lived to a reasonable age, sixty-one-years-old, but everything he did, he did early. He was sort of a dilettante student. He was a native of Scotland and studied in Edinburgh, but also went to Berlin often. And he studied classics and metaphysics, logic, moral philosophy; went to a lot of concerts. But while in Berlin he got fascinated by chemistry.

So in 1855 he became a student chemist. And in 1856 he went to Paris to work with Wurtz, who was the other leading French chemist, and worked indeed on salicylic acid; as you have done. In 1858, while in Wurtz’s lab, he got fired, for reasons that you’ll see, and he went back to Scotland and tried to make a career there. But soon he had a mental breakdown, and the rest of his life he didn’t do anything. Okay? But what he did in 1858 is really something. Okay? So he didn’t get a stone on the front of our building. But on this building, which is “To Let,” or at least was a few years ago, in Kirkintilloch, which is about eight miles northeast of Glasgow, in Scotland, there’s a stone. And it says: “This plaque marks the birthplace of Archibald Scott Couper whose brilliant pioneering contributions to chemical theory have won for him international renown, and whose genius, stifled by an early illness, was denied the opportunity of consummation.” When he was recovering from his first breakdown, he went fishing on this Endrick Water, north of Glasgow, and got sunstroke, which did him in for the rest of his life. And that’s where he lived. His mother nursed him for his final 33 years in this house in Kirkintilloch.

So this is the paper. He tried to publish it in French. He submitted it to his boss, Wurtz, in the hopes that he would put it in the French journal, but Wurtz thought it was not such a — it was a little rash as a paper, and put it in his desk to cool off for awhile. And in the meantime Kekulé published, which irritated Couper no little, and he told Wurtz what he thought about it, which was why he got sacked. But he also published it in English, and later it was published in French. But so this is the English version, not the first version he wrote; by Archibald Scott Couper. And the first sentence already is fascinating. “The end of chemistry” (that is, the goal of chemistry) “is its theory.” Not many people would have said that. They would have had more practical motivations for chemistry. But Couper was a big thinker. He said:

“The guide in chemical research is a theory. It is therefore of the greatest importance to ascertain whether the theories at present adopted by chemists are adequate to the explanation of chemical phenomena, or at least, based upon the true principles which ought to regulate scientific research.”

So what were the theories that were prevalent at this time, by 1850? Name one.

Students: Dualism.

Professor Michael McBride: Shai?

Student: Dualism.

Professor Michael McBride: Dualism; okay, the radical theory. That was prevalent in Germany. How about elsewhere, like France?

Student: Substitution.

Professor Michael McBride: The substitution, or type, or unitary theory. Okay, so those were the big theories.

“Among those which have been lately developed, there is one, on account of its apparent numerous merits, which particularly claims investigation, and respecting which we deem that it would not be unprofitable were either new proofs of its scientific value furnished or, on the contrary, should considerations be adduced establishing not only its inadequacy to the explanation, but its ultimate detriment to the progress of science. I allude to the system of types as advocated by Gerhardt.”

So Gerhardt and Laurent, who were slightly second-level French chemists, were the ones who really came up with the idea of the type theory. But Dumas adopted and advocated it, and was the leader. So this was the theory that was going on in the lab where he was present, in Paris. So page 106 of this paper, on the French type theory, he said:

“Should the principle which is therein adopted be applied to common events of life, it will be found that it is simply absurd. Suppose that some one were to systematize the formation of letters into words that formed the contents of a book. Were he to begin by saying that he had discovered a certain word which would serve as a type” (remember there were four types; water, ammonia, HCl and hydrogen) “and from which by substitution and double decomposition all the others are to be derived, that he by this means not only could form new words, but new books, and books almost ad infinitum; he would state certainly an empirical truth.”

That is, you can take some word and substitute letters, or pairs of letters, or sets of letters, for other letters, and get any word you want. Okay? “At the same time, however, this method would, judged by the light of common sense, be an absurdity. But a principle which common sense brands with absurdity is philosophically false and a scientific blunder.” So this was not likely to make him popular in Paris. So here’s what he was doing, actually, right? The Emperor’s new clothes. He’s a new guy on the block, right? He’s only been a chemist for two years, only begun studying chemistry two years before, and says what everybody’s doing is nonsense, in France. Well how about in Germany? That’s where he was before, in Berlin. “I can only remark that it is not merely an unprofitable figure of language, but is injurious to science, inasmuch as it tends to arrest scientific inquiry by adopting the notion that these quasi elements…” What does he mean by the quasi elements; what are they?

Student: Compound radicals.

Professor Michael McBride: Compound radicals, right? “…contain some unknown and ultimate power, which it is impossible to explain.” That is, okay, so you got radicals. What holds the radicals together? Right? “It stifles inquiry at the very point where an explanation is demanded, by putting the seal of elements, of ultimate powers, on bodies which are known to be anything but this.” They’re definitely not elements, because you can burn them, of course, and get water and CO2 and so on. Okay? So this was absolutely true; on both counts, both the French and the German theory. But it was stated pretty undiplomatically. Right? And especially for someone who’d only been studying chemistry for two years. Maybe it took someone who was only studying chemistry two years to see how stupid it was. Right? But at any rate, he got in trouble because of this. What did he offer as an alternative? To look at the properties of the elements themselves, not of radicals, not of these planetary systems that were the types. “Science demands the strict adherence to a principle in direct contradiction to this view. That first principle, without which research cannot advance a step, dare not be ignored; namely, that a whole is simply the derivative of its parts.” So you have to understand the elements, and then you can understand assemblies of elements from that. That’s not obviously true. In fact, I think you could make a pretty good argument that as you get things more complicated, they’re not simply a sum of their parts. But that’s what he said.

“As a consequence of this, it follows that it is absolutely necessary to scientific unity and research to consider these bodies as entirely derivative, and as containing no secret ultimate power whatever, and that the properties which these so-called quasi elements possess are a direct consequence of the properties of the individual elements of which they are made up.”

So study the elements, not the radicals. And he began by saying he’ll focus on carbon; which is not surprising for an organic chemist. “In applying this method, I propose at present to consider the single element, carbon. This body is found to have two highly distinguishing characteristics. (1) It combines with equal numbers of hydrogen, chlorine, oxygen, sulfur, etc.” Would you agree with that? How could he say so? It’s because he wasn’t dealing with the right atomic weights, right? Okay? “It enters into chemical union with itself.” So those are the two things: that it’ll unite with itself, not just with other things, the way dualism would like you to do; and it combines with — it has a certain combining power. “These two properties, in my opinion, explain all that is characteristic of organic chemistry.”

Chapter 2. Tetravalence and Self-Linking of Carbon [00:10:25]

So 1858 marks a new frontier in organic chemistry; a completely new revelation: The tetravalence and self-linking of carbon. That’s points one and two, right? That it combines with a certain number of other things and it can link to itself. Right? Now here he shows the formula of methane, of CO2, and of carbon tetrachloride. But those are the atomic weights he’s using, in order to get these formulas. Notice he says: “Here the whole four of hydrogen are not bound by a mutual affinity.” You know that they’re not all collected together because you can substitute them one by one; one chlorine, two chlorines, three chlorines. “For each element of hydrogen can be substituted for one of chlorine in regular series, beginning with the first and ending with the last. The atoms of oxygen are, on the contrary, united in pairs.” What does he mean by that, that the oxygens are united in pairs? Why are they united in pairs in his formulas, drawn by those curly brackets, like they had in type theory? Yes Angela?

Student: Probably one oxygen, because the — they were probably one oxygen because his weight is half of it.

Professor Michael McBride: Ah ha, because he had half the atomic weight. So they always came in pairs; to get oxygen sixteen, if he thinks oxygen is eight. “The atoms of oxygen are in pairs which will be more fully developed hereafter.” Not in this paper. But there was no hereafter, because of his breakdown, okay? “And only for two atoms of oxygen two chlorines can be substituted; thus.” So you can start with CO2, get a phosgene and carbon tetrachloride. “In the same manner with bodies that contain multiples of C2 united to hydrogen.” So carbon is the same deal. It always comes in pairs, because its atomic weight is six instead of twelve. Okay. “Take the inverse of this. If the four atoms of hydrogen were bound together, we could evidently expect to form such bodies as H4 Cl4.” Right? If the four were all held together and could make four bonds, then you could get H4 Cl4, or these other compounds. “One would naturally expect to find carbon substituted for chlorine, and find bodies like H4, Cl2, C2, and so on. These bodies are not only unknown, but the whole history of hydrogen might be investigated and not a single instance be found in favor of the opinion that it has any affinity for itself, when in union with another element.”

Okay? Remember, getting all the chlorines together was Schwindler’s idea. Okay, so here, now that he have the idea that these atoms associate together, it makes sense to talk about a structure. Right? What’s connected to what’s connected to what? You know, the toe-bone and the ankle-bone and all that stuff. Okay, so now you could write structural formulas. So these were the very first structural formulae, because before this there was no such thing as structure. Okay? Thus methylic alcohol has this formula, and ethylic alcohol has the other. So now do you see the theme here? What’s he using to denote bonds?

[Students speak over one another]

Professor Michael McBride: Three dots are a bond. So he has a carbon bonded to three H’s, and also to OOH. That’s because oxygen is doubled, right? So we would write it this way. Or ethyl alcohol we would write that way. Or this compound. Now, this one has a printer’s error in it. What is it, and what’s the printer’s error? What’s the substance? Seth, can you read it off to me? Start from the left, the bottom left; what is it? What’s the first group? There’s a carbon. That’s a single carbon. Yeah, methyl, CH3. Right? He doesn’t draw the three bonds to hydrogen separately, but it’s clearly C bonded to three hydrogens. What’s next? Keep going. It’s a C bonded to what? The top left. It’s bonded to the methyl, of course. What else?

Student: Oxygen and hydrogen.

Professor Michael McBride: Bonded to oxygen, to a single oxygen; although he wrote two, right? And what else is that carbon bonded to?

Student: Hydrogen.

Professor Michael McBride: How many? How many hydrogens is it bonded to? The top left carbon.

Student: Two.

Professor Michael McBride: The two. So it’s CH3CH2O. Now, can someone help me out with the rest of it? Ryan?

Student: Well wouldn’t that only just be one O at the top because —

Professor Michael McBride: It should be one O, but he’s got the different atomic weight. John?

Student: Shouldn’t the C’s on the far right be bonded?

Professor Michael McBride: Ah, the C’s on the far right should be bonded to one another. The printer left that out. So it’s symmetrical. It’s diethyl ether. Okay? And it’s the correct structure for it, except for the printer’s error. Okay? Now, he says: “There is no reaction found where it is known that C2 is divided into two parts. It is only consequent therefore to write, as Gerhardt does, C2 simply as C, it being understood that the equivalent of carbon is twelve.” So he changed it in this paper to be twelve. So now his formulas are going to be more like ours, without the C2. So he has a formula for glycerin, and one for glyceric acid. So let’s do glyceric acid. What do you see up at the top? What’s the top carbon? Max, what do you say? What’s it bonded to, the carbon at the top, on the right?

Student: Bonded to a hydrogen.

Professor Michael McBride: One hydrogen.

Student: OH group.

Professor Michael McBride: An OH group.

Student: And another —

Professor Michael McBride: Pardon me?

Student: Another OH group.

Professor Michael McBride: Two OH groups, and the carbon below. Right? Because carbon can bond to carbon; that’s the special thing about it. Okay, so here’s the one on the left, in a formula nowadays, and there’s the one on the right. And it turns out he guessed a bit wrong, because he put two OH’s on the top part and none on the middle one; and it should be the other way around. So in truth he should’ve changed those two. Of course, he had no way of knowing it. And the same on the right. Okay? So he’s pretty close with his structure. But probably the most remarkable thing is his structure for glucose, which is shown here, and which — now, he doesn’t show the vertical bonds between carbons. He uses this curly bracket. So that’s a little reminiscent of type theory. But clearly that’s what he meant was the carbons to be bonded to carbon. So that’s the formula he wrote. And that’s the correct structure for glucose, if it’s hydrated. Okay?

So you can add — glucose is an aldehyde, CHO on the bottom, double bond O. So you can add water to the aldehyde; that actually happens. And you get the structure he wrote here. So it’s right, if the glucose is in water. Right? So try yourself. This is a good exercise for reviewing what we’ve been doing. Depending on how you define it, it’s a two or a three-step sequence of HOMO/LUMO interactions that take water plus an aldehyde into a diol; which is what you have here. So try that one out. Okay? And in the French version of the same paper, which as we said appeared a little later, he drew it this way. And what do you notice is different now? There are two things different actually.

Student: Straight lines.

Professor Michael McBride: Yeah, now he’s drawing single lines, straight lines for bonds. So this is the first time that was done; none of those dotted lines anymore. But there’s a typo. Instead of drawing OH here — and notice — yeah, instead of drawing OOH here, for a second, he added — he’s got the wrong formula, he’s got H2 there. But this is really amazing, that he got the correct structure for such a complicated molecule, in the very first paper where the idea of structure, or of bonds altogether, was proposed. So this is really amazing. And we owe to Couper the idea of bonds, and the notation we use to do them. So here’s Old Aisle Cemetery in Kirkintilloch in Scotland, yesterday. And there you see that this is the gravestone of Archibald Scott Couper. And down here is your thank-you note. [Students react] And those are the flowers — that’s the heather you bought to put on his tombstone. So that was our secret thank-you note. And we thank Susan Frew for being our agent and delivering this yesterday. [Applause]

Chapter 3. Kekule’s Advancements in Chemical and Molecular Notation [00:19:40]

Okay. But on the front of the building Couper doesn’t get any notice at all. It’s only you guys that are in the know that can do that. Right? On the front of the building they say Kekulé. So let’s talk about Kekulé. And we’ll also talk about Hofmann, who we’ve mentioned before as the guy in the top-hat on the right end of that picture in Liebig’s lab in Giessen. And Cannizzaro. Okay, this is a drawing that Kekulé made at the age of thirteen. By my standards, that’s a pretty darn good drawing. And here’s one he made at the age of eighteen. And he went to Giessen University to study architecture, but while he was there he went to Liebig’s lectures and got fascinated by chemistry and became a chemist. Okay. He studied with Liebig and after he finished he wanted to go to Paris to study with the people there. Liebig, remember, had worked with Gay-Lussac. So he knew from Paris. What do you think he would tell Liebig, the young guy about to go off to Paris?

Student: Stay away from Dumas.

Professor Michael McBride: Stay away from Dumas perhaps, right? What he said was, “There you will broaden your horizons, there you will learn a new language, there you will learn to know the life of a great city, but there you will not learn chemistry.” But he went anyhow. He also went to England and became widely acquainted with all the leading chemists of the day. Then he got a job in Heidelberg, where he was from 1856 to ‘58, and he did research on cacodyl; that’s that free radical involving arsenic. A remarkable — his student, who worked with him on that was Baeyer, who we’ll talk about a lot later. That was Bunsen, who was the big cheese in Heidelberg, was the one Kekulé was associated with. And he did the work, not in the laboratory, but in the kitchen in his apartment, this work on arsenic.

So in 1857, while he was there, he proposed a new type, based on carbon. So carbon could have four things attached to it, which you could substitute one for another. Right? So this was where he got the idea of carbon being something special, the basis of a new type, and that it was tetravalent. And ultimately he got onto the idea, a year later, that carbon could link to itself. So he had proposed the marsh gas type before Couper’s paper. But this other one was essentially simultaneous; just slightly later written but earlier published. Here are his Observations on Mr. Couper’s New Chemical Theory.

“In fact, in two memoirs which have appeared in Liebig’s Annalen, I have put forward different views, which, in my opinion, should furnish a clearer insight into the constitution of chemical compounds.” [Clearer than Couper’s.] “I may be allowed to indicate that [my first paper] lays down the principle…which I have called the basicity of atoms.” [We’d say valence.] “If Mr. Couper thinks he has discovered the cause of this difference of basicity in the existence of a special kind of affinity, I am the first to admit that I have no right to contest his priority in this.”

But he was quick to say that he was the one that had the idea first. And he went to quite a successful career, in contrast to Couper. Here he is in Ghent, the period during which he proposed the structure for benzene, with a bunch of people whom we will talk about later; or at least several of them we’ll talk about later, as his students. And then he became the leader in Bonn, in a big new chemistry institute there. Here he is in 1872. And we’ll talk a lot about this guy on the right here. So he became a really prime leader in chemistry.

Now, facts, ideas and words. Words means nomenclature. So these new ideas needed new words, and also not just words written out with letters, but notation; what symbols are you going to write for these things? And a further development to that is actual physical models that you could put together to show what you are talking about with molecules; if they have structure. So Hofmann was great with language. He actually went and became a leading professor in the Royal College of Chemistry in London, because Prince Albert — you know, Victoria and Albert — Albert was a German. So he had a lot of connections with German chemistry and went to Liebig. Liebig said, “Hofmann’s a great guy, you should get him.” So they got him. So he came to London, and was completely fluent in English, and very good at languages altogether. And he’d had the idea of systematizing the names of hydrocarbons. So he was going to base it on Latin roots. So four will be “quart,” right? What do we say for four? Butane, right? We talked about that last time. So “quartane” is the start. And then you can start pulling hydrogens off and have names for the radicals. So you could pull one hydrogen off; that’s “quartyl.” Because yl, remember, is the root that means radical. And then you can pull a second hydrogen off; then you have “quartene.” And then you pull a third hydrogen off, it’s “quartenyl.” And then “quartine, quartinyl, quartone, quartonyl, quartune, quartunyl.” What was his system? How can you remember which one is “quartune”? Yeah?

Student: It’s the vowels.

Professor Michael McBride: It’s the vowels in order, a-e-i-o-u. And we preserve the a, the e and the i; although we write y, instead of i, for alkyne. Right? That’s where it came from, from Hofmann. But the quart didn’t stick, because but was well established by then. Okay, now how about Kekulé’s — so that’s words. How about notation? So in the paper where he proposed benzene, in 1865, while he was Ghent, he says, in a footnote:

“For greater clarity I am presenting at the end of this note a table giving graphical formulae for most of the substances mentioned. The idea that these formulae are designed to express is rather well known now; so it will not be necessary to dwell upon it. I am keeping the form that I adopted in 1859 when expressing for the first time my views on the atomic constitution of molecules.”

So 1859 is at the time he was proposing valence and so on; 1858, as I remember. So this is now five, six years later, and he’s applying it to benzene. “This form is nearly identical with that which Monsieur Wurtz used in his beautiful lectures on chemical philosophy. It seems to me preferable to the modifications proposed by Messieurs Loschmidt and Crum-Brown.” So let’s first look at what Loschmidt did, and Crum-Brown. So here’s what they said, to which Kekulé considers his superior. Well first let’s think, what should a formula show? Right? What’s the very first thing a formula should show? It has to correspond with the facts, right? So what facts do you deduce about a molecule? What should the formula show? Kevin?

Student: What elements are in the —

Professor Michael McBride: What elements are there: carbon, hydrogen, oxygen, chlorine, nitrogen, whatever. Okay, what else? Shai?

Student: The ratio of elements.

Professor Michael McBride: The ratio of the elements, right? Berzelius also had this. But what’s going to be new? Composition: the elements and the number of atoms. What next; now, if we’ve got the new theory, the valence theory? Sherwin?

Student: The structure.

Professor Michael McBride: The structure. What do you mean by the structure?

Student: The relative positioning.

Professor Michael McBride: You mean like x,y,z coordinates? You could write numbers. Here’s the origin; I’m going to give this 1.238 and so on. What?

Student: Then which atoms are bonded.

Professor Michael McBride: Ah, which ones are connected to one another. Right? Not their positions in space; but this is connected to this, this valence goes there, and so on. Okay, so Berzelius was already fine for composition. But now we also need to show Constitution — that’s the second C is constitution — which means the nature and sequence of bonds. Now sequence is clear. This is bonded to this, bonded to this one. What do I mean by the “nature” of the bond? How could bonds differ in their nature?

Student: Strength.

Professor Michael McBride: Single, double, triple, right? So, and you need to be able to show isomers, which Berzelius couldn’t show. Okay? So here’s Lohschmidt. This is Lohschmidt’s formula, 1861, for acetic acid. Can you understand it? Andrew, what do you say? No hope. What do you think the stuff on the left is?

Student: CH3.

Professor Michael McBride: Pardon me?

Student: CH3.

Professor Michael McBride: CH3, right? So the size is something about the weight, right? So it’s CH3. What’s it bonded to? C; same size. What’s it bonded to?

Student: O.

Professor Michael McBride: O, H; and what’s special?

Student: Double bond.

Professor Michael McBride: Double bond. So he’s got a way of showing a double bond, right? So we can easily understand it. Okay, now here’s Crum-Brown. Now that’s benzoic acid in Crum-Brown’s paper. So the COOH is even clearer than it was in Loschmidt, right? What’s a double and single bond. The same thing we use now. Okay? What’s different is the C6 part and five hydrogens attached to it. But we don’t really know how the C6 is arranged. Although he also showed this one, for phenol, which is a benzene with a hydroxyl group on it, OH. So there you see a ring. It’s double, single, double, single, double, single; and H is attached to all the carbons, except for one, which has OH. So that’s easy to understand. Shai?

Student: Where does the idea of double bonds come from? I feel like it’s just —

Professor Michael McBride: Because if you say carbon has four, then you have to figure out where the fourth one is. Right? If it’s associated with three atoms, but can make four bonds; it must make two to one of them. That’s the idea anyhow. Or it could be that they go to the center of the ring, the fourth one and so on. There are other ideas too. Okay, or here is a reaction in Crum-Brown’s notation. So it’s an aldehyde. You add the HOMO of cyanide, and a proton to the O, and you get that compound; a cyanohydrin, as it’s called. So this is almost identical — what difference is it from our current, how different is it? We don’t draw circles around the elements anymore; that makes it faster to draw. Otherwise it’s essentially the same. Okay? Or here, he says there are two kinds of alcohols: “true” alcohol, which can lose water and give an olefin; or olefin hydrate, which can lose water and give the same olefin. What’s the difference between the true alcohol and the olefin hydrate? How are they isomers? Russell?

Student: The position of the OH.

Professor Michael McBride: Right, the position of the OH relative to the R. Okay? So he’s explaining isomers here. So those are pretty good. But Kekulé says his is ever so much better. This is Kekulé’s structure of benzene, from that same paper. Remember he says at the end, I’m going to give my superior formulas. So what in the heck is he talking about? Eric, you got any idea? So what do you think that is? Eric? No idea? Well you don’t have much to work with in benzene. [Laughter] What do you have?

Student: Carbon and hydrogen.

Professor Michael McBride: It’s C6H6. Right? So that’s got to either be carbon or hydrogen. Which do you think?

Student: Hydrogen.

Professor Michael McBride: The top left there. No idea. Anybody got an idea? Why do you say carbon Kate?

Student: Yeah, because that one’s showing double and single bonds all together.

Professor Michael McBride: Ah, it’s showing up above here, the schematic one is showing double bonds between carbons and then a single bond; then a double bond and a single bond; then a double bond. Well let’s go — that’s open chain, and this is closed chain. What are these arrows on the end?

[Students speak over one another]

Professor Michael McBride: Ah, those are the ones that loop back and attach to one another, somehow, to make a ring. Okay? Do you think he’s trying to give — to imply Cartesian coordinates, for the location of every atom, by where he draws these symbols? Is he trying to show x,y,z coordinates? What do you think Nate? Think he thinks he’s showing that, or not?

Student: I don’t know.

Student: Which Nate?

Professor Michael McBride: Either of you.

Student: He’s got x,y. There, he’s got an x,y.

Professor Michael McBride: Oh you could say, right, here’s an x-axis and there’s a y-axis. He says at a certain point it’s that, that, that, that, that. Do you think he thinks that if he had a microscope and could see it, that’s where those atoms would be, in fact? Is it physical position he’s showing?

Student: They look kind of like [inaudible] to me.

Professor Michael McBride: Kelsey, you got an idea? Do you think he thinks he’s showing where these atoms are actually located in space? What is he showing? Yeah Sherwin?

Student: The sequence.

Professor Michael McBride: What’s bonded to what, and whether it’s single bond or double bond. He’s showing Constitution, the nature and sequence of bonds; not positions of atoms, certainly not. Okay, and then the little ones are hydrogens, right? So the length of one of these things is how many valences it has. Right? Four long, for carbon; only one long for hydrogen. And tangency then means a bond. Right? So this is C-H double bond; C-H single bond; C-H double bond, and so on. And then you get to the end, you go back to the beginning. Okay? Now he then shows chlorinated benzene, because chlorine is also monovalent. Right? So you can substitute for hydrogen, and then you could have dichlorobenzene too, or tri-, or tetra-, and so on. Right?

Now is he showing isomers here? That is, in this case, if you substituted this one for the chlorine, instead of that one; same thing or different things? If you believe that this was a picture of things in space, they’d be different. Right? But if all you’re interested in is what’s connected to what, by single and double bonds, then it’s the same. You have the same pattern either way. How about for the dichloro? Would there be isomers of that?

Student: Yes.

Professor Michael McBride: Ah, you could get different sequence, right? They could be adjacent carbons that have chlorines, or next adjacent, or beyond that. Right? So you could use this system to talk about isomers. So how many isomers? Now what do you think Kekulé thought? What do you think he thought about how many isomers of chlorobenzene there are? Do you think he thought there was many? Just what — guess. I’m going to show you. Okay, he thinks his system is superior. Now he also used his system — in this same paper he did a lot of formulas. But he showed these compounds. Like this one. What’s that group, on the left?

[Students speak over one another]

Professor Michael McBride: What’s the group here? What is that? What’s this atom? CH3. What’s this? CH2. OH; O is two units long, right? So CH3CH2CH2OH. Alcool propylique; propyl alcohol. Get it? Now what’s this one? CH3, CH3, CH, OH. It’s alcool méthyle-éthylique; methyl ethylic alcohol. So this would be ethyl alcohol, the part on the top. But we’ve got a methyl substituted in it. And notice, he’s having to change his formula, because now these are horizontal tangencies that are bonds, but this vertical tangency doesn’t count as a bond. So it’s getting a little complicated to explain to students. Right? Okay, what’s the next one? CH3. What’s this?

Student: C-O.

Professor Michael McBride: C double bond O. CH3. So that’s our old friend acetone. And what’s this one? CH3. CH-OH. Right? CH3. Alcool acétonique. So it’s the alcohol you get by adding two hydrogens to acetone. Now what do you think about this? Do you see anything interesting? This one is CH3CHOHCH3. This one is CH3CHCH3OH. Sophie, what do you say?

Student: They’re the same thing.

Professor Michael McBride: Ah, they’re the same nature and sequence of bonds. Right? That one is what we know. That’s that one. That one, that one. But the last, and number twenty-eight here, are the same thing, but he draws different formulas for them. So his system is not showing isomerism properly. Right? He drew these different. He didn’t say that they were the same. Right? And they were thought to be different. Because when you do one — when you prepare it one way and when you prepare it another way, you get a different boiling point say, or something. I actually don’t know what the data was, but there was some difference that said there were these two different compounds. Did you ever get a wrong boiling point? So did they. Right? It just turned out the experiment was wrong. But that shows that his system is not as good as these other systems; whatever he said.

Chapter 4. 3-D Molecular Models: From Brass Strips to Croquet-Balls [00:38:32]

Okay, now how about physical models of molecules? Well here’s a great patent that was issued by the U.S. Patent Office to Samuel M. Gaines of Glasgow, Kentucky, in 1868. And we can read about his description of what was so good about this. He says,

“Now, if a class thus instructed be at the recitation bench” [So here’s our recitation bench, right?] “and one of them is requested to form carbonic acid, he will walk quickly to the table in front, where the box is open, and place on one of the shelves a cube marked 6, and two marked 8. Carbon, one, and oxygen,n two. If requested to form lime, he will place on the shelf a cube marked 20, (calcium,) and another one marked 8, (oxygen.) these being the elements of lime. These cubes being all placed in contact, and the pupil being asked, ‘What have we now?’ answers ‘carbonate of lime’.” Right? Okay. “Another pupil may then be asked to give the proximate analysis for carbonate of lime. And the cubes before him will indicate the answer and fix it in his memory: ‘It is composed of one equivalent of carbonic acid, (22) and one of lime, (28)’.” [You just pull them apart, and you see what the proximate analysis is.] “Another may be requested to give the ultimate analysis, and by the same means he is furnished with the answer. ‘Carbonic acid is composed of one atom of carbon, two of oxygen, and lime is composed of one atom of calcium and one of oxygen.’ The intelligent educator will see at a glance, that in this chemical alphabet he has the means of at once arresting the attention of his class and converting what had hitherto been an irksome task into a pleasant recreation. The atomic theory, the characteristics of affinity, the law of multiple proportions, the nomenclature, isomerism, etc., may all be learned in half the time required heretofore, and learned so as not likely ever to be forgotten.”

Now what do you think? Would this show the atomic theory that molecules are substances that are composed of atoms? Does it show it?

Student: Yes.

Professor Michael McBride: Yeah. Okay, how about the characteristics of affinity; single, double, triple bond, valence and so on, show that? Not in any obvious way that I can see. So I doubt that one. How about the law of multiple proportions? Remember what the law of multiple proportions was? You have the same elements — if you have a parts of A and b parts of B, and so on, all that stuff, then what AD will be, and so on. [Here and in the following Professor McBride confuses the Law of Equivalent Proportions with the Law of Multiple Proportions, which the Gaines Models do show.] Yes Max?

Student: Why did Kekulé only show one oxygen, in his notation?

Professor Michael McBride: Pardon me?

Student: Did Kekulé know what the exact mass of oxygen was? Because he only had one in his notation.

Professor Michael McBride: Yes, but notice everything here is halved.

Student: Oh okay.

Professor Michael McBride: So it’s okay, the right proportions, right? Okay? But multiple proportions, you have to know something about how many of one associates with how many of another; which isn’t obvious here. Okay? Nomenclature? Well maybe, calcium oxide and so on, CO2. Isomerism, that you can have the same things differently arranged? Well obviously you can put the blocks on top of one another or beside one another. But how do you know how many you should do? Right? Not very clear to me that this system works well. So those models are good for some things, but not good for an awful lot of things.

Okay, now James Dewar in Scotland, a student, had the idea of using brass strips to show models. And, in fact, he could put these black balls on. He said, “To make the combination look like an atom, a thin round disc of blackened brass can be placed under the central nut.” I think that’s really cute, to “look like an atom,” as if he knew what an atom looked like. Right? Okay, so then he drew — he made these models from his strips of possible structures of benzene. Right? You can see where the atoms are, and the four valences on every carbon, and what they — which direction they go. That particular one, that compound, that structure, was actually prepared about 100 years later, and when it was, it was called Dewar benzene. Right? That one’s interesting. It’s got a four-center bond, four carbons all sharing the same bond. Right?

Hofmann, the guy, remember, who was good at language and was in London; his last lecture in London was at the Royal Institution, the place where Faraday was, and where Davy had been and did the experiments on electrolysis. So he gave this public lecture where he used croquet balls for atom models, with sticks stuck in them, and he colored them, and the system he chose is the one we still use for what color the atoms are: white for hydrogen, red for oxygen, blue for nitrogen, black for carbon. Okay? So here’s part of the lecture he gave. So he would build molecules before your very eyes. He’d start with a hydrogen, put an oxygen on, and another hydrogen; lo, he’s made water. Right? Or he could make marsh gas, methane. Or he could show the oxides of HCl. So you start with HCl, but he could take it apart and put an oxygen in between, or put two oxygens, or three or four oxygens in between. You’d go from hydrochloric to hypochlorous to chlorous to chloric to perchloric. So this is like Lavoisier, going to different levels of oxidation.

Unfortunately, those aren’t the structures. Right? What really happens is that the unshared pair of chlorine can share with oxygen to give this, which is the active ingredient in Clorox bleach. Or you can use another unshared pair of chlorine to put another oxygen. Or another for another, and get the oxidizing agent that Gay-Lussac used, the chlorate. Or you can put a last one. And this is an explosive. In fact, ammonium perchlorate, AP, is a component of military explosives. Notice that in Hofmann’s thing you could’ve put fifteen oxygens in a row if you could have reached high enough. But you can only go to four here, because that’s how many unshared-pairs chlorine has — choride has to use.

Okay, but then he showed the chlorination of marsh gas. So monochlorinated, dichlorinated, trichlorinated and tetrachlorinated marsh gas. Now this is interesting because for the dichloro, you could imagine having two isomers with these models. Right? The chlorines could be near one another or opposite one another. Right? Now some years later there was an exhibition in London and they showed molecular models. By this time Hofmann was back in Germany. But they got a guy in Spain to make some Hofmann-style models. Manuel Gonzalez Rodriguez from Madrid made this set. And here’s chloromethane made from that set. What’s the difference between these — it says “according to Hofmann” here, “Segun A.W. Hofmann”. Right? What’s the difference? Alex?

Student: Bonds on the carbon aren’t just- aren’t 90° from each other.

Professor Michael McBride: They’re not 90° from one another. They’re 109.5° from one another. It’s a tetrahedron, not a square plane. Right? Now what does that say about isomers? So suppose we substitute another chlorine. There’s a dichloro; there’s a different dichloro; there’s a different dichloro. How about it?

Student: No.

Professor Michael McBride: They’re just rotated from another. They’re not different, right? So depending on whether it’s square plane or tetrahedral, you’ll get different numbers of isomers. That’s neat, because it gives you a tool for telling whether it’s square plane or tetrahedral. Right? So you can get structure from analysis. So, but these constitutional models try to show the nature and sequence of bonds for Hofmann, not the arrangement in space; and not for Hofmann, and not for the guy that made these models. They weren’t trying to explain isomers in that way. All they were trying to show was the same thing you draw in these pictures: the nature and the sequence of bonds; whether they’re single or double, triple, and what’s associated with what. So that comes a little later, trying to get to the arrangement in space. But the pièce de résistance of Hofmann’s lecture was this: that if you take these croquet ball models and make the olefiant gas, which we talked about last time, it’s got two spare valences, and if you add chlorine you can get the dutch liquid, the dichloroethane.

But remember what we saw last time. He called olefiant gas “unfinished” or “not saturated”; we now call it an unsaturated molecule. And it’s reactive because of these dangling valences. And we now know why it’s that way. That he’s got the wrong structure; it should be CH2CH2, not CHCH3. And we know about HOMOs and LUMOs now. But that was what he really showed, was that the model had something completely new that no one anticipated. It could explain what, that’s new? What new qualities has he associated with the models? What elements are there, what ratio they’re in. Right? How they’re linked to one another, single, double, triple bonds. What’s he adding? Reactivity. Right? So it’s something new. So the whole history is adding new things, new properties to the models. And there are more to come. Okay. Okay, that’s it.

[end of transcript]

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