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DBL%20Hendrix%20small.png College chemistry, 1983

Derek Lowe The 2002 Model

Dbl%20new%20portrait%20B%26W.png After 10 years of blogging. . .

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: Twitter: Dereklowe

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April 6, 2004

Osmium Tetroxide, Of All Things

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Posted by Derek

This morning brings the news, via ABC, that the recently discovered bomb plot in London involved a quantity of osmium tetroxide. That's a surprise.

I know the reagent well, but it's not what anyone would call a common chemical, despite the news story above that calls it "easily obtained." It's quite odd that someone could accumulate a significant amount of it, and it's significant that anyone would have thought of it in the first place. It's found in small amounts in histology labs, particularly for staining in electron microscopy, but that's generally in very dilute solution. If these people had the pure stuff, well, someone's had some chemical education, and probably in my specialty, damn it all.

The reagent is used in organic synthesis for a specific (and not particularly common) reaction, the oxidation of carbon-carbon double bonds to diols. I've done that one myself once or twice. OsO4 comes in and turns the alkene into a matched pair of alcohols, one on each carbon, and it stops there. Other strong oxidizing reagents can't help themselves - they find the diol easier to attack than the double bond was, and go on to tear it up further. There was a recent paper in the literature on the mechanistic details, actually, going into just why the osmium reagent stops where it does.

Unfortunately, the alkenes it could attack are unsaturated fatty acids and such, as found in lipoproteins and cell membranes. Exposed tissue is vulnerable. Breathing a large amount of the vapor can kill a person through irritation of the lungs, but it's not as bad that way as the better-known agents like phosgene. A bigger problem is the cornea of the eyes, and the reagent is mostly feared for its ability to bring on temporary (and in some cases, permanent) blindness.

There's no doubt in my mind that any terrorist with the stuff was going for that effect. Could it have worked? Well, it's a solid at room temperature, but a hot day will melt it. The stuff sublimes easily; it has a high vapor pressure. Just being around the solid crystals is enough to get you overexposed to the vapors. I don't know how much of the reagent these people had, but I tend to think (again, contrary to the ABC story) that an explosion would have dispersed it to the point that it was just down to irritant levels. I wouldn't want to find out, though.

If they were planning to use it in a non-explosive gas attack, that's another matter. But the vapors are said to be very irritating, with a distinctive chlorine-like smell - which I cannot verify, thank God. It's not like no one would have noticed that there was some nasty chemical in the air. I think that they could have done some damage, certainly. But what disturbs me more than the reagent itself is the thinking behind it. . .

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