I’m still working on my reply to the Matthew Holt article I mentioned yesterday, so I thought I’d do one of the awful reagents that I spoke of. I’ll kick things off with hydrogen fluoride.
The chemically inclined members of my audience might be saying “Hold it! You said yesterday that you’d used hydrofluoric acid!” And that’s true, and that stuff is certainly bad enough on its own merits. It gives terribly painful burns, and it eats through glass, to pick two of its fine qualities. But if you’re going to be precise, hydrofluoric acid is a water solution of hydrogen fluoride, HF. That’s a gas, and it’s a lot worse.
Actually, it’s just barely a gas. In a cool room it’ll condense out as a liquid (it boils at about 20 degrees C, which is 68 F.) The straight liquid must really be a treat, but I’ve never seen it in that form, and would only wish to through binoculars. It’s sold compressed in metal cylinders, like other gases, but it needs some care in packaging. The stuff is so corrosive that special alloys need to be used, usually ones high in nickel. If you stick an ordinary gas regulator on top of an HF cylinder, you’re in for trouble, and the complete destruction of the regulator is the least of your worries.
HF has actually been used right out of the cylinder for a long time in Merrifield peptide synthesizers. It’s the traditional way to cleave the peptide off the resin at the final step, so there are actually a lot of people who’ve used the stuff. But it’s in a dedicated apparatus that is (that had better be) well sealed, and people treat it with due respect. At a former employer of mine, there was an accident with one of these machines right before I joined the company. The shout “HF LEAK!” went out into the halls, and I’m told that the whole area set a never-to-be-equaled evacuation record. This was one of those drop-things-right-where-you-stand type evacuations, a real sauve qui peut moment.
I’ve caught some whiffs of HCl, like any chemist has, and it’ll wake you up for sure. And I was wrestling with a lecture bottle of HBr gas in grad school, only to have it start to hiss onto my shirt – which was never the same afterwards. But I’ve never smelled HF, and I hope I never will. As bad as it is on metals and glass, it’s much worse on living tissue, although (as I mentioned) a lot of synthetic peptides can stand up to it.
Oddly enough, it’s not that strong an acid in the traditional sense. The fluorine doesn’t want to let go of the proton enough. It’s strong enough to burn, but the big problem is how penetrating it is. As soon as it hits anything moist – like your lungs – it dissolves in the water and turns into hydrofluoric acid again. And that soaks into tissue very readily, with the acid part doing its damage along the way, and the fluoride merrily poisoning enzymes and wreaking havoc. The damage isn’t immediately apparent, and there are terrible cases of people who’ve been exposed and didn’t realize it for hours – by which time a lot of irreversible damage had been done.
Fortunately, I have very little cause to even think about using HF. I don’t do Merrifield peptide synthesis, and the only times I even use the solution forms of the reagent are on a very small scale and in weakened form (like its complex with pyridine.) Should some lunatic discover a wonderful reaction that requires the gas, I will respectfully pass. As will everyone else.