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."