Things I Won't Work With

Every so often, chemist Derek Lowe writes about especially "interesting" chemicals - today it is Bromine:

The Higher States of Bromine
Chemists have a familiarity with many elements and many compounds, from having worked with them or studied them in the literature. You get a feel for what’s “normal” and for what’s unusual, and there are quite a few degrees of the latter. Take compounds of bromine, for example. Most any working chemist will immediately recognize bromine (there are exceptions) because we don’t commonly encounter too many opaque red liquids with a fog of corrosive orange fumes above them in the container. Which is good. That’s bromine in oxidation state zero, elemental, and then you have bromide (oxidation state -1), one of the most common anions around. “Chlorides are rabble”, said Primo Levi in one of my favorite lines from The Periodic Table, and he was right about that, but bromides are not of much higher social standing. Every cation has a bromide salt, and it’s usually one of the cheaper ones in the catalog.

So far, so good. But bromine can also go up to +3 and +5 oxidation states, and there things start to get interesting. You can have various mixed-halogen things, all of which are reactive and toxic and are distinguished by their various degrees of vileness. And you can get all sorts of bromine-oxygen species, ranging from the pretty well-known ones like bromate ion (BrO3–) all the way up to. . .well, to the stuff described in this new paper., from Konrad Seppelt at the Freie Universität Berlin. It contains a whole list of new compounds that send my chemical intuition completely off the rails.

I have no “feel” for them whatsoever except a strong desire never to prepare any of them. Prepare any of them? I don’t even want to make the starting material. You know you’re in for a bumpy ride when your work needs something like bromine fluorine dioxide (bromyl fluoride, BrO2F); no one can claim that they weren’t warned. There hasn’t even been a reliable synthesis of that stuff until now – Seppelt describes a new one, from the aforementioned sodium bromate, which is fine, and bromine pentafluoride, which is not fine, because it’s a hideous oxidizing and fluorinating agent fit to fluorinate you right into the afterlife and whose attempted use in liquid rocket propellent mixtures was abandoned because it was too foul to work with, and, oh yeah, redistilled pure hydrogen fluoride, which is also about as far from “fine” as you can get. The SI of the paper casually mentions that you can use double vacuum distillation in a metal line to get your HF sufficiently anhydrous for the reaction, and you can go ahead and get cranking on that without waiting for me to show up.

Heh - fun writing and great chemistry. More on the synthesis at the site.