Let’s start with the name. Quite a mouthful, isn’t it? Believe me, that one’s pretty chewy even for experienced organic chemists. We see lots of more complicated nomenclature, of course, but this one some features some speed bumps, that make you go back to make sure that you’re reading it correctly. I’ll take you through my own thoughts as an example.
You skip to the end in chemical names – Mark Twain would have felt about them the same way he felt about the German language. But this brings me up short, because very few chemists could walk up to the board and draw an isowurtzitane. And I am not among their number. I have a vague picture of these “wurtz” compounds being funky three-dimensional cage structures, and that much only from having probably read too many photochemistry papers over the years. So the only thing that “isowurtzitane” calls to mind is some complicated framework of fused rings, looking like one of those wire sculptures that unexpectedly fold up flat when you pull on them.
Moving on out, as you do in a systematic name, I see that this is a hexaaza variation, which makes the picture a bit fuzzier. That’s a lot of nitrogens substituted for carbons, and the first thought is that this must be some weirdo condensation product of ammonia, some aldehyde, and who knows what. You can get some pretty funny-looking structures that way, like hexamethylenetetramine (which I’ve actually used a couple of times). I don’t know where those nitrogens are, I think to myself, but I’ll bet that’s how they got there, because any other pattern would be a synthetic nightmare. So far, so good. But now comes the unexpected habanero.
Hexanitro? Say what? I’d call for all the chemists who’ve ever worked with a hexanitro compound to raise their hands, but that might be assuming too much about the limb-to-chemist ratio. Nitro groups, as even people who’ve never taken a chemistry class know, can lead to firey booms, and putting six of them on one molecule can only lead to such. And since there are six nitrogens and six nitro groups, the first assumption must be that these are all bonded to each other. I mean, come on, leaving the nitro groups attached to the carbons is for wimps. So that means that someone, somewhere, has perversely made a poly-N-nitro cage compound, as if they’d been dared to cram the most bond energy into the smallest space.
That, as it happens, is exactly the case. Hexanitrohexaazaisowurtzitane, or CL-20, was developed as a highly energetic, compact, and efficient explosive. What makes it unusual is not that it blows up – go find me a small hexa-N-nitro compound that doesn’t – but that it doesn’t actually blow up immediately, early, and often. No, making things that go off when someone down the hall curses at the coffee machine, that’s no problem. Making something like this that can actually be handled and stored is a real accomplishment.
Not that it’s what you’d call a perfect compound in that regard – despite a lot of effort, it’s still not quite ready to be hauled around in trucks. There’s a recent report of a method to make a more stable form of it, by mixing it with TNT. Yes, this is an example of something that becomes less explosive as a one-to-one cocrystal with TNT. Although, as the authors point out, if you heat those crystals up the two components separate out, and you’re left with crystals of pure CL-20 soaking in liquid TNT, a situation that will heighten your awareness of the fleeting nature of life.
Stabilized or not, I still won’t get near it. For one thing, I’m a drug discovery chemist, and if you think a structure like this is going to be a drug, then you must be on some strong ones yourself. No, the thing about these compounds is that they can be handled as long as they’re very pure and formulated just right. The side products from their synthesis, well, those might not be so nice. And if a batch gets contaminated, or doesn’t come out so clean, well, that might not be so nice, either. Synthesizing polynitro compounds is no chocolate fondue party, either: if you picture a bunch of guys wheeling around drums of fuming nitric acid while singing the Anvil Chorus from Il Trovatore, you’re not that far off the mark. You really have to beat the crap out of a molecule to get that many nitro groups on it, which means prolonged heating of things that you’d really rather not heat up at all.
No, I’ll leave the can-you-top-this nitration chemistry to those that love it. You guys just go ahead and stuff as many energetic bonds as you can into the smallest tangles; I’ll be over here in the bunker cheering you on, and jumping a foot in the air every time someone sneezes. I’m not cut out for hexanitro anything.
Addendum: it’s an odd thing, but when you search for information on this compound, a significant number of the Google hits are for its environmental effects. This is an explosive, meant for munitions and destruction, but there are all kinds of studies on its effects on earthworms, fish, soil microorganisms, and so on. Steven Pinker must be right when he says that violence is getting tamer all the time.