Diastereomeric Alkenes

Yes, it's a new post. I know. I hardly posted at all this year with school, then finished that in June, made two posts, and disappeared. No posts in July or August. It was not my intention to put this project on hiatus. I am sorry I failed you. I don't even know what kept me away for most of this time, but recently, there has been one thing: I am planning a trip. It's a pretty big one, actually. I'll be going to Germany. Since this is going to happen fairly soon and since my German is not so good, working on becoming as fluent in the language as I can be takes priority over writing new posts here. I guess this means I have to go on an actual, planned hiatus. Or, if you prefer, this means I'll have to extend the current hiatus, albeit interrupted by this solitary post. Whatever.

My primary reason for choosing German as a language to learn in the first place was its connection to chemistry. I haven't written about that here, so I'll explain. Germany as a region has long been a powerful contributor to the body of scientific knowledge. This was especially true in the nineteenth century, a formative period for many branches of chemistry. Because of this, in the twentieth century, in American colleges that required students to study a foreign language, chemistry majors traditionally chose German and were often encouraged to do so because it would allow them to access chemical literature that had only ever been published in German. The utility behind this is probably all but vanished these days, as any text that's of practical use to a chemist is probably available in English. But I enjoy history and the prospect of some day being able to read old chemistry texts in German has some sort of allure for me.

The German classes I took this year in school didn't do much in the way of making me comfortable with the language (the instructor mostly just played videocassettes from the 1990's), but they did make me really want to visit Germany. And so, here we are, with me abandoning you yet again. I'm really quite sorry about that.

Rest assured, this blog will be back with a vengeance. After the chapter I've been covering, there's material that I really like and I am excited about presenting it to you. Just seeing this material in the book makes me brainstorm different ways to cover it. There's some really cool chemistry to come once I finish this chapter, which I wish I'd already done over the summer instead of putting it off. So yeah, I should be back home and caught up on everything else by, let's say, October 29th or so. Mark your calendars for late October. Back with a vengeance. Really. More frequent updates. Better updates. Awesome chemistry. But not yet. You'll have to wait for my return. It will totally be worth the wait, though.

In honor of my trip, I'm going to skip ahead a bit to something with an obvious connection to organic chemistry's German roots. We'll return the to the material we were on shortly after I get back. And really, now might be as good a time as any to talk about this. By now, I'm sure you have a decent grasp of one type of stereoisomerism. But as you might have guessed, having a chiral center isn't the only way for stereoisomerism to occur. There are other ways that have nothing to do with a carbon atom bonded to four different groups. In fact, I initially wanted to write a post introducing all of the different ways for this to happen that I knew of, but I couldn't find a way to make it work. I did, once upon a time, say this, though...

Constitutional isomers often have dramatically different chemical properties. Their physical properties differ too. They might have different functional groups. In contrast, stereoisomers don't exhibit such bold differences. Two compounds that are stereoisomers of each other not only have the same atoms, but the atoms are connected in the same way. Their properties are almost identical. But the spatial positions of the atoms are different.

And a chiral center isn't the only way for that to happen. So, here's one of the other types of stereoisomerism, and it's a lot easier to demonstrate graphically in two dimensions than the type you already know about. There's just one thing I've probably never mentioned here that you need to keep in mind: unlike single bonds, there is no rotation along a double bond. Behold...
Can you spot the difference? Don't get too excited about it: everyone else notices it too. Same atoms. The atoms are connected in the same way, with an A and a B connected to a carbon that is double-bonded to another carbon also connected to an A and a B. But the spatial arrangement is different, no matter how we oriented these in three dimensions (try it if you want). We have a notation system (cis/trans) that makes this pretty easy, as seen in the drawing I just made. The "cis" version of this molecule has both A's on one side of the double bond and both B's on the other side of it. The "trans" version has an A and B on each side. They're diastereomers, which you recall from my last post means that they are stereoisomers, but not mirror images of each other. Easy, right?

But what if instead of just having two different kinds of groups, we have three or four? This notation system doesn't have a way to deal with those situations! For that, we need the E,Z system as my textbook calls it or Z-E Isomerie as the German Wikipedia calls it. And it's at this point that I realize I've gotten ahead of myself. In order to explain this, I need to use information that would come from a post I haven't written yet, probably the post I was supposed to have written if I didn't skip ahead to this section because I remembered it having something German in it. So despite my failure here, this still seems like a good post to end on before my trip.

If I had class, I'd rewrite this post and just make the whole thing be about cis/trans notation and save E/Z for later. Since I don't, I'll find some way to squeeze the German connection into this post. E and Z are really just more comprehensive versions of cis and trans, which themselves come from Latin instead of German: "cis" means "on the same side" or something like that and "trans" means "across" probably. In the E/Z system, each group is assigned a priority, but I haven't yet written about the rules for determining priority. They're the same ones that are used in the notation system for chiral centers, which is yet another Latin-based system. But since this simpler system already has the Latin words, for the comprehensive system, we use the German words. Crazy, I know. Eventually, I hope to show some examples of names of molecules with "E" or "Z" in them. "Z" stands for zusammen (together) and "E" stands for the word entgegen (against). I leave it to you to figure out which one corresponds to "cis" and which one corresponds to "trans."

Examples of Naming Cyclic Alkanes

Once again, I steal some problems from my textbook and do them here.
It's a ring made of six carbons, so it's a cyclohexane. Only one of the positions in the ring has any groups attached to it, and both groups are methyl groups, so it's 1,1-dimethylcyclohexane.
Another cyclohexane, obviously. This one has two groups at two different ring positions. The positions are across from each other in a 1,4 relationship. But which group gets numbered "1" and which one gets numbered "4"? Well, one is a methyl group and the other is a butyl group (four carbons in a straight chain). Alphabetical order determines which one comes first, so this is 1-butyl-4-methylcyclohexane.
The chain is bigger than the ring this time. So this compound is, as far as naming goes, defined as a five-carbon chain with a group attached at the first carbon, and the group that is attached is a cyclopropane ring. Therefore, we have 1-cyclopropylpentane.
The ring is a cyclopentane. Three groups this time, and all of them methyl groups, which makes naming this easy. Almost so easy that you could do it by yourself. But how do we number these groups? It doesn't matter which way we count, the smallest number we can start with is 1. This compound is 1,2,3-trimethylpentane.
This one is trickier. We definitely have a cyclohexane ring, but what are those groups attached to it. Well, let's start with the smaller one. It's an isopropyl group. See that? Probably not. Well, I told you, so now you know. Isopropyl group. The other one has four carbons. You might remember that there are four such groups possible. And if you have really been paying attention, it's clear that this is a sec-butyl group. Alphabetical order again, but the only prefix that matters for that is "iso-." That means the sec-butyl group is first. So this compound is 1-sec-butyl-2-isopropylcyclohexane.

Well, that's way that I learned to name this. And it's even the name that my solutions manual gives. But ChemSketch generated a different name that I am guessing is the true systematic name using proper IUPAC rules. The only difference is that the groups can't be written as isomeric forms of their straight-chain versions. This makes the nomenclature a bit messier (but it also scales up nicely, while the shortcut I'm using doesn't.

And that, children, is how to name cycloalkanes. I don't actually know what topic I'll cover next. You'll just have to wait to find out.

Nomenclature of Cycloalkanes

This will not cover all cycloalkanes. In fact, for now we're only dealing with compounds that have a single ring. But then I didn't really cover all acyclic alkanes either. But what you should have with this post is a basic idea of cycloalkane nomenclature.

Rings themselves are named by how many carbons they consist of. So, for example, this molecule...
...is cyclohexane. But you already know that, of course. I mean, you do, right? You'd better, seeing as I already told you that this is cyclohexane. Yes, it was back in October, but so what? I mean, you are supposed to read and remember everything I write here. You know, I'm getting the feeling that you're not being much of a team player here. Yeah, it sure seems like I'm the one doing all the work. Look, it's just cyclohexane. It's not complicated. It's a simple molecule with a simple name. Four syllables. That's not too many. Cyclohexane. Cyclohexane. Cyclohexane. And don't you forget it.

Speaking of earlier posts, in this one I showed cyclopropane. Unless you're as awful at geometry as you are at chemistry, you should be able to make the connection that if the triangle is cyclopropane and the hexagon is cyclohexane, a square is cyclobutane and a pentagon is cyclopentane. Yes, and a heptagon is cycloheptane and so on. All we're doing is using those chemical numeric prefixes I showed earlier and counting the numbers of carbon atoms making up the ring. You can count, right? You can at least do that much.

But watch out. Not everything in the molecule is necessarily part of the ring...
That is methylcyclohexane (in glorious 3-D). Seven carbons, but only six of them form a ring. The pesky seventh one is attached to the ring. And if you've already forgotten how skeletal structures work, the hydrogens attached to the carbons are not drawn in. All but one of the ring carbons has two hydrogens. One of them has only one hydrogen and is also bonded to that carbon outside the ring, which itself has three hydrogens. So it's a methyl group. Hence the name: methylcyclohexane.

Of course multiple groups could be attached to the ring. In that case, we use numbers. This compound...
...goes by the name 1-ethyl,3-methylcyclohexane. And if you think in terms of the rules you learned for acyclic alkanes, this makes sense. We have to number the positions on the ring somehow. So we're starting at first substituent alphabetically. Here, I'll even put the numbers in...
This system of numbering positions on rings will be used a lot in the future, so you should be comfortable with it. But it seems straightforward enough to me, so I'm not going to reiterate it further.

We might also end up with two groups attached at the same position on a ring. Not to worry...
That's 1,1-dimethylcyclopentane. The same general principles from naming acyclic alkanes still apply. This does run into limitations of course. I won't be covering those now. But I do think that I should to a follow-up post in which I name some examples from homework problems in the textbook. And just so that you can follow along, there is one more tiny little thing that you need to know about cycloalkanes. If a ring is attached to a hydrocarbon chain that is longer than the number of positions in the ring (like if a cyclopentane ring had an octane chain attached to it), the compound is named based on the chain (so that example I just made up would be 1-cyclopentyloctane).

Examples of Naming Acyclic Alkanes

As promised, here are some right out of the textbook.

The first one is in condensed notation: CH₃CH₂CH(CH₃)CH₂CH₃

Since I am so good, I immediately recognize that the third carbon has a one-carbon branch. Other than that, this is a straight chain. But let's not get ahead of ourselves. We are doing this the right way. We start with the last part of the name. With no heteroatoms, this is a hydrocarbon. With no multiple bonds, it's an alkane. With no rings, it's an acyclic alkane. We know the name must end in "-ane." Next, what's the longest carbon chain? Five. So the parent name is pentane. Branches? Yes, at the third carbon (counting either way). And the branch is a methyl group. Therefore, the name of this compound is...

...3-methylpentane. And you know what else? I checked the answer in the study guide and I was right! Woo hoo, Stephen got something right. Anyway...

(CH₃)₃CCH₂CH(CH₂CH₃)₂

This one is harder. First we have three methyl groups attached to one carbon. That carbon links to another that links to another, which is attached to two ethyl groups. Which methyl group and which ethyl group is considered part of the longest carbon chain does not matter because the groups are identical (that is, the methyl groups are identical to each other and the ethyl groups are identical to each other). So adding those three carbons to the rest of the chain, we find that the longest carbon chain is six carbons long, so this is a hexane.

Which group gets priority? In this case, we go in alphabetical order. "E" comes before "M." So this should be...

...3-ethyl-5,5-dimethylhexane. Or not. Oops. I started at the wrong end. It's actually 4-ethyl-2,2-dimethylhexane. It's that instead of the one I thought it was because 2 is lower than 5. It doesn't matter that 3 is lower than 4 because the method that gives the lowest number period is the one that gets priority, not the one that gives the lowest sum or anything like that. I hope you learned your lesson. Moving on.

CH₃(CH₂)₃CH(CH₂CH₂CH₃)CH(CH₃)₂

A propyl group? No, that's part of the longest carbon chain. They're trying to trick us. Starting from the left we have a carbon and then a string of three more, so that's four in a row. Then there's another (five) with that propyl group branching off. If we count going up the propyl group we get three more (eight). If we treat the propyl group as a branch, we get another carbon with two methyl groups, one of which would be a branch, making the total length seven. Sneaky textbook. This is actually an octane.

If we start from the end of what's being labeled as a propyl group (but is actually part of the chain) we get a branch at the fourth carbon. Starting from the left makes it at the fifth, so we start from the end of the propyl group instead. The branch consists of three carbons and two of them are attached to the other, which is where the branch connects, so it's an isopropyl group, meaning the compound is...

...4-isopropyloctane. And I'm right. I rule.

Enough of these condensed structures!
I used MS Paint because it was a small one and it's kind of hard to make them look less awful on ChemSketch. Anyway, this one seems easy to me. Five carbons long means pentane. Two methyl branches at the second carbon and two at the fourth. Therefore...

...2,2,4,4-tetramethylpentane. And I am right again. Excellent.
It's seven carbons long, but there are a couple of different ways to arrive at that. The one that give the lowest number to a branch is the one that simply starts on the far left, for a methyl group at the second carbon. There's another one at the fifth carbon and an ethyl group at the third, so this is...

...3-ethyl-2,5-dimethylheptane. And I'm right yet again. Three in a row! Let's do one more.
I moved back to MS Paint again when I perhaps should not have. But ChemSketch was being annoying (it kept trying to put rings into this). Obviously this one is larger than the other ones so far, but the principle is the same. The longest carbon chain is ten. The fastest we can get to a branch with it is on the second carbon, again counting from the far left. From there we label the other branches and put them in the proper order. About that, the branch on the fifth carbon is a sec-butyl group. When alphabetizing the branch names, this is treated as a "B" and not as an "S." The same would be true for tert-butyl but not for isobutyl. Unnecessarily confusing, I know. But in this case it does slightly affect the name, which is...

...5-sec-butyl-3-ethyl-2,7-dimethyldecane. And that's pretty much all there is to it. The study guide I used to check my answers breaks the process into three steps.

1. Name the parent chain by finding the longest C chain.
2. Number the chain so that the first substituent gets the lower number. Then name and number all substituents, giving like substituents a prefix (di, tri, etc.).
3. Combine all parts, alphabetizing the substituents, ignoring all prefixes except iso.

It takes some getting used to, but this is the basis for how other compounds, even ones with multiple functional groups, are named.

Nomenclature of Acyclic Alkanes: Prefix

I hope you have the other component of naming alkanes down, because I am never reviewing it again (just kidding). Now for the prefix. While the parent name identifies the longest carbon chain, the prefix tells us where on that chain branches occur and what the branches look like. Depending on how much branching (and what kind) is going on, the prefix may be anywhere from nonexistent (no branches, which we sometimes denote by using "n" as a prefix) to ridiculously long.

Firstly, the location of a branch is denoted using Arabic numerals. A branch at the second carbon in the longest carbon chain gets a "2" and a branch at the third carbon gets a "3" and so on. Some carbons in the longest carbon chain might have two branches. When that happens, its number gets used twice.

Often, there are multiple possible places to start from. With alkanes, the correct starting carbon is the one which, when started from, yields the lowest possible number being named first. If we start counting on one end of a chain and the first number that comes up is for a branch at the fourth carbon, but counting from the other end of the chain would make our first branch be at the second carbon, then it is the end that would make the first branch be at the second carbon that is the correct starting point.

Also, numerals are separated from each other by commas and from the rest of the name by hyphens. That's not just for alkanes. That's a universal rule. Commit it to memory, slave.

Anyway, to specify how long a branch is, we use the wonderful numerical prefixes I introduced in my last post. You know, the ones that are mostly Greek, but not really. A branch that is only one carbon is a "methyl" group. Two carbons is an "ethyl" group, etc. A branch that is seven carbons long is a "heptyl" group (and since it's not part of the longest carbon chain, that means the longest carbon chain must be really long). This all works nicely for branches that are themselves straight. But what about branches that have branches of their own? That's the hard part. Kind of. In order for considerable branching to occur, the molecule itself has to be pretty big. I've never had to deal with such compounds myself. The textbook is covering substituents with up to four carbons and that's always been good enough for what I've had to do. There are not very many. Here we go...

Methyl group: R—CH₃
Ethyl group: R—CH₂CH₃
Propyl group: R—CH₂CH₂CH₃
Isopropyl group: R—CH(CH₃)₂
Butyl group: R—CH₂CH₂CH₃
sec-Butyl group: R—CH(CH₃)CH₂CH₃
Isobutyl group: R—CH₂CH(CH₃)₂
tert-Butyl group: R—C(CH₃)₃

If you find the condensed structures confusing for those four-carbon groups, here are some links to pictures (off-site) for the butyl variations...

Butyl, sec-butyl, isobutyl, and tert-butyl.

And that's all. Now you know how to name acyclic alkanes. Oh, one more thing. If two or more of the same type of branch exists in a molecule, those branches get named together and get a Greek numerical prefix just to confuse you even more. But really, that's it. Stay tuned for next time, where I'll do a follow-up post with some examples of naming alkanes using problems from the textbook. Oh wait, this isn't a radio. You can't tune anything. Whatever.

Nomenclature of Acyclic Alkanes: Parent Name

I mentioned the IUPAC systematic nomenclature system before. I think I did, anyway. This project has been on hiatus for a while and I can't remember. But I'm back now! Really. I hope. Anyway, today we are going to learn how to name some alkanes. It's easy to do, and you need to know it to name other compounds. So learn it. I command you.

Let's start at the end. That's a good place to start, right? The last part of the name of any alkane is, get ready for this...

...it's "-ane." That should be quite easy to remember, even for you, because "alkane" itself ends in "-ane." If a compound is an alkane, its name ends in "-ane" and, conveniently enough, if a compound is not an alkane, its name will not end in "-ane." I know. Chemistry is so hard.

Next, we find the longest carbon chain. This is actually very easy, but teachers love trying to trick beginning students with odd drawings where they make part of the longest carbon chain look like a branch to people who are not paying attention. If this were a real chemistry class and I were the teacher (that would be bad), I would totally do this to you because I think it's hilarious. For now, I'll just give you the benefit of the doubt and assume that you are paying attention and can tell what the longest carbon chain in a molecule is.

Really? I shouldn't do that? Fine.
How long is the longest carbon chain? If you answered eight, congratulations, you did not fall for the dumbest trick in chemistry class. If you answered some other number, you were not paying attention or you cannot count or you're just a moron or something. I don't know. Shame on you anyway. You're bad (unless you got the right answer).

Once we know how long the longest chain is, we convert that into a numerical prefix, then attach it to our "-ane" suffix. Convert it into a numerical prefix? Yes, it's easy. No really. It is easy, just so long as you already know the Greek numerical prefixes—and use the Latin one for "nine" just to mess things up—and forget the first four prefixes and make up new special ones that are specific to chemistry. It was easy for me though! Here, I'll give you the first ten and we'll worry about going higher later.

1 = "meth"
2 = "eth"
3 = "prop"
4 = "but" (pronounced like the word "butte" just to confuse you even more)
5 = "pent"
6 = "hex"
7 = "hept"
8 = "oct"
9 = "non" (pronounced so that it rhymes with "tone" and not some other way)
10 = "dec"

Memorize them now. I command you. Done? Good. See, that wasn't so bad. Now, there's just one more tiny thing. Then we'll be all done and you'll know how to name acyclic alkanes. We have straight chains covered (unless they're longer than ten carbons long, but shut up). So a hydrocarbon that is a straight chain with five carbons would be "pentane" and one with nine would be "nonane" and so forth. Everything is fine, and then branches come and mess it all up. Not to worry: the IUPAC has an elaborate set of rules for us to denote where on a chain the branches lie and what the branches look like using prefixes and attaching them to the parent name (which simply describes the longest carbon chain. Well, it's elaborate enough that I'll save it for my next post, anyway. For now, just have the whole parent name part down.