What chemical do scientists fear the most?

I think a strong argument can be made for my least favorite chemical: dimethylmercury.

There are other things that are highly reactive. Dioxygen difluoride, FOOF, is probably among the most unstable, reactive chemicals allowed by the laws of physics. But nobody’s really afraid of it. It’s so unstable you can never make more than a handful of molecules at a time. It’s a curiosity, but not something that’s feared.

Chlorine trifluoride is legit terrifying stuff. It burns violently on contact with nearly anything, there is no known way to control a chlorine trifluoride fire, and the combustion byproducts are hot hydrofluoric acid and chlorine gas. The MSDS for chlorine trifluoride reads like the plot of a horror movie.

But Silicon Valley uses it by the ton in the production of semiconductors. You can buy it in bulk from chemical supply houses.

Speaking of hydrofluoric acid, that’s also legit terrifying stuff. The things it does on contact with flesh would frighten Quentin Tarantino. But it, too, is sold in bulk. It’s used to make etched glass.

There are some exotic nitrogen compounds that are so vigorously explosive they go bang if you look at them funny. And that makes them, perhaps paradoxically, less scary; you can’t manufacture them in large enough quantities to kill you, though they do tend to damage sensitive lab equipment.

But dimethylmercury?

This shit is easy to make, totally stable, and so toxic that one drop spilled on a glove in a laboratory glove box will kill you. Slowly, in agony, over a period of months.

Nobody will work with it any more. You can’t buy it from any chemical supply house. Every chemist knows what happened to Karen Wetterhahn, the scientist who spilled some drops on her glove.

As far as I know, no lab on earth will use it any more. No research facility will have anything to do with it. It’s simply too toxic, even to folks for whom chlorine trifluoride might be an “okay, I suppose I have to be a bit careful.” Dimethylmercury is the thing that goes bump in the night, the chemical that chemists who joke about using FOOF to light the candles on a colleague’s birthday cake speak of only in hushed whispers with the doors and windows bolted. Dimethylmercury is manifestly incompatible with the hypothesis of a benevolent creator who loves his creation.

Edited to add: You can find out more about this and other unlovely compounds here: Things I Won’t Work With. Among other horrors, you’ll find things like this:

And let me just say, I am gobsmacked that’s a real molecule someone really made (a synthesis pathway which, I can’t help thinking, must involve a crowbar).

Warning: this chemical may cause acid indigestion, watery eyes, shortness of breath, seizures, cardiac arrest, convulsions, hallucinations, lung damage, liver failure, brain damage, weakness, blindness, loss of balance and coordination, short and long term memory loss, kidney failure, digestive and reproductive system damage, nausea, vomiting, pulmonary collapse, brittle bones, deafness, slurred speech, aphasia, alexia, loss of motor control, muscle weakness, and irritability. Use only as directed.