The latest addition to the long list of chemicals that I never hope to encounter takes us back to the wonderful world of fluorine chemistry. I’m always struck by how much work has taken place in that field, how long ago some of it was first done, and how many violently hideous compounds have been carefully studied. Here’s how the experimental prep of today’s fragrant breath of spring starts:
The heater was warmed to approximately 700C. The heater block glowed a dull red color, observable with room lights turned off. The ballast tank was filled to 300 torr with oxygen, and fluorine was added until the total pressure was 901 torr. . .
And yes, what happens next is just what you think happens: you run a mixture of oxygen and fluorine through a 700-degree-heating block. “Oh, no you don’t,” is the common reaction of most chemists to that proposal, “. . .not unless I’m at least a mile away, two miles if I’m downwind.” This, folks, is the bracingly direct route to preparing dioxygen difluoride, often referred to in the literature by its evocative formula of FOOF.
Well, “often” is sort of a relative term. Most of the references to this stuff are clearly from groups who’ve just been thinking about it, not making it. Rarely does an abstract that mentions density function theory ever lead to a paper featuring machine-shop diagrams, and so it is here. Once you strip away all the “calculated geometry of. . .” underbrush from the reference list, you’re left with a much smaller core of experimental papers.
And a hard core it is! This stuff was first prepared in Germany in 1932 by Ruff and Menzel, who must have been likely lads indeed, because it’s not like people didn’t respect fluorine back then. No, elemental fluorine has commanded respect since well before anyone managed to isolate it, a process that took a good fifty years to work out in the 1800s. (The list of people who were blown up or poisoned while trying to do so is impressive). And that’s at room temperature. At seven hundred freaking degrees, fluorine starts to dissociate into monoatomic radicals, thereby losing its gentle and forgiving nature. But that’s how you get it to react with oxygen to make a product that’s worse in pretty much every way.
FOOF is only stable at low temperatures; you’ll never get close to RT with the stuff without it tearing itself to pieces. I’ve seen one reference to storing it as a solid at 90 Kelvin for later use, but that paper, a 1962 effort from A. G. Streng of Temple University, is deeply alarming in several ways. Not only did Streng prepare multiple batches of dioxygen difluoride and keep it around, he was apparently charged with finding out what it did to things. All sorts of things. One damn thing after another, actually:
“Being a high energy oxidizer, dioxygen difluoride reacted vigorously with organic compounds, even at temperatures close to its melting point. It reacted instantaneously with solid ethyl alcohol, producing a blue flame and an explosion. When a drop of liquid 02F2 was added to liquid methane, cooled at 90°K., a white flame was produced instantaneously, which turned green upon further burning. When 0.2 (mL) of liquid 02F2 was added to 0.5 (mL) of liquid CH4 at 90°K., a violent explosion occurred.”
And he’s just getting warmed up, if that’s the right phrase to use for something that detonates things at -180C (that’s -300 Fahrenheit, if you only have a kitchen thermometer). The great majority of Streng’s reactions have surely never been run again. The paper goes on to react FOOF with everything else you wouldn’t react it with: ammonia (“vigorous”, this at 100K), water ice (explosion, natch), chlorine (“violent explosion”, so he added it more slowly the second time), red phosphorus (not good), bromine fluoride, chlorine trifluoride (say what?), perchloryl fluoride (!), tetrafluorohydrazine (how on Earth. . .), and on, and on. If the paper weren’t laid out in complete grammatical sentences and published in JACS, you’d swear it was the work of a violent lunatic. I ran out of vulgar expletives after the second page. A. G. Streng, folks, absolutely takes the corrosive exploding cake, and I have to tip my asbestos-lined titanium hat to him.
Even Streng had to give up on some of the planned experiments, though (bonus dormitat Strengus?). Sulfur compounds defeated him, because the thermodynamics were just too titanic. Hydrogen sulfide, for example, reacts with four molecules of FOOF to give sulfur hexafluoride, 2 molecules of HF and four oxygens. . .and 433 kcal, which is the kind of every-man-for-himself exotherm that you want to avoid at all cost. The sulfur chemistry of FOOF remains unexplored, so if you feel like whipping up a batch of Satan’s kimchi, go right ahead.
Update: note that this is 433 kcal per mole, not per molecule (which would be impossible for even nuclear fission and fusion reaction (see here for the figures). Chemists almost always think in energetics in terms of moles, thus the confusion. It’s still a ridiculous amount of energy to shed, and you don’t want to be around when it happens.
So does anyone use dioxygen difluoride for anything? Not as far as I can see. Most of the recent work with the stuff has come from groups at Los Alamos, where it’s been used to prepare national-security substances such as plutonium and neptunium hexafluoride. But I do note that if you run the structure through SciFinder, it comes out with a most unexpected icon that indicates a commercial supplier. That would be the Hangzhou Sage Chemical Company. They offer it in 100g, 500g, and 1 kilo amounts, which is interesting, because I don’t think a kilo of dioxygen difluoride has ever existed. Someone should call them on this – ask for the free shipping, and if they object, tell them Amazon offers it on this item. Serves ’em right. Morons.