OK, since I’m a medicinal chemist, I have an excellent excuse to avoid the nitro functional group. It’s metabolic trouble, and although there are indeed drugs with nitros on them, they’re exceptions. Some of them, in fact, are antibacterials that rely on that metabolic activation to work, in the same way that there are nitrate esters that rely on hydrolysis of that group to exert their vasodilating effect. And in a way it’s too bad, because stereoelectronically there’s nothing quite like a nitro group, and if it weren’t for such difficulties I’m sure we drug researchers would have found many more uses for the things.
But you know who loves them, nitros and nitrates and maybe both at the same time, because why not? Energetic materials chemists, naturally. High-nitrogen compounds of all kinds are their delight, since these can easily turn into rapidly expanding clouds of gas with the release of catastrophic amounts of free energy. You’ve got your azides, your poly-azoles, your nitros and nitrates (which bring along their own oxygen for extra boomification), and more. Trinitrotoluene is merely the most famous of the polynitro explosives, and there are things out there that make TNT look like toothpaste by comparison. I’ve written about some of them in the past, and two new papers prompt me to write about them again. First up is this one from the Shreeve group at Idaho, in collaboration with the Naval Research Laboratory. Her chemistry is often partnered with people who have an abiding interest in explosions; the group is well-known on the short list of those that try to see just how much craziness you can pack into a single molecule’s structure before it flings itself apart out of sheer joie de vivre.
No, that’s a perfectly accurate statement of their research program: this new paper’s introduction includes the phrase “In our continuing efforts to introduce as many nitro groups associated with a tetrazole ring as possible. . .” and to most organic chemists that’s roughly equivalent to saying something like “In our continuing efforts to spray as much graffiti on the snouts of salt-water crocodiles as possible. . .” Because if that were your research program, you’d seek out the most humungous reptiles available and position yourself at the best angle to give them a cloud of Krylon straight up the ol’ nostrils, right? Same difference.
You don’t think so? Here, take a look at the this paper’s target compound at right. Think about preparing that puppy on scale and ask yourself if you might not rather reach for some swim trunks and a spray can of metallic purple (gotta keep the crocs looking stylish). You’re not going be able to cram more nitro groups onto a tetrazole system that small, darn it all, that’s all there is and there ain’t no more. It’s fair to ask whether this hexanitro beast can even exist. As you read the paper the answer turns out to be “Just barely”.
The team can detect its formation, but it decomposes within minutes on standing, which honestly is probably for the best. Although I will confess to some curiosity about its properties, because the corresponding tetranitro compound is actually a slightly better explosive than the pentanitro (which can be isolated, as well as the similarly energetic salts of both of these, since that remaining terrified carbon-acid protein between the two nitro groups flees easily to give you the anions). The pentanitro and most of its salts, though, are described as “very sensitive to impact and friction”, which I’m sure isn’t the half of it. Some of the tetranitros are (relatively) more tractable, which makes you figure that the hexanitro shown would be a hoppin’ good time indeed, if you were so foolish as to try to prepare any amount of it. And if it weren’t departing this world while you watch, and fit to take you with it, too.
The other paper I wanted to mention is this one from the Baran group at Scripps, and I have to say that I didn’t know that they were into this stuff, but here they are along with the US Army Research Laboratory. It’s titled “Impact of Stereo- and Regiochemistry on Energetic Materials”, which is a good one, because these things will give you the chance to study all sorts of impacts if you let them. But the paper has a point: not many people have looked at what the stereochemistry of such compounds does to their energetic properties, because current calculation methods have them coming out pretty much the same. But can that be right? Stereoelectronic properties influence so many other things, why not the propensity for lab-shattering explosions, too?
The group prepared all four possible meso stereoisomers of the tetranitrate ester shown at right (and some related compounds with quaternary carbon branches off the ring as well). Making the corresponding primary alcohols in stereoselective fashion was not always so easy, and the group had to pull in both photochemical and electrochemical reaction steps to get some of the intermediates. Nitration of all of them, though, seems to move right along, as you’d expect from the example of nitrocellulose, nitroglycerine, and other polynitrates. This exact compound had never been investigated, although some other small-ring polynitrates have been looked at over the years, only to be abandoned as explosives when it was found that their melting points were too low or the vapor pressures of their liquified forms was too high – that last one is particularly alarming when you consider the likelihood of depositing a sublimed layer of high-purity high explosive crystals on every nearby surface. Noooo, thank you.
The calculated properties (detonation velocities, detonation pressures, heats of formation, and specific impulses) of these stereoisomers were almost identical, as expected, and experimentally they aren’t far off either. But their other physical properties are all over the place. For example, the “three up one down” isomer turns out unexpectedly to be a liquid, and doesn’t even solidify at -40C, while the “two up two down” one (same sides, not alternating) melts at over 100C. Their sensitivities to friction, impact, electrostatic discharge and the like vary quite a bit, too (and to be sure, these are generally experimentally determined properties and not so subject to calculation). Their explosive properties compare well with widely used stuff like PTEN and RDX, so such compounds have possibilities in mixtures for melt-casting and so on. As the paper says, “Such tunability has the potential to cater to both the explosives community and the propellants community”, although in my own work I try very hard to avoid catering to either one, and the community of those of us who aren’t trying to figure out how quickly things can blast through the ceiling will just have to sit these guys out.
Oh, and in case you’re wondering about how these tetranitrates compare to the Shreeve group’s polynitrotetrazoles (just typing that out gives me the willies), the latter have higher detonation velocities and pressures, and are more friction-sensitive. So if you are faced with the choice of handling one or the other, go with the cyclobutane polynitrates. But do consider the alternative, if it’s available, of slathering up with sunscreen and diving in to tag those crocodile snouts instead.