Well, it’s been a bit too serious around here this week. So I thought today I’d step back to a period when men were men and chlorine azide was a reactive, toxic, and unstable compound that was only good for finding out what sort of explosion it would set off next. What’s that? You say that that’s still about all it’s good for? Staying power, that’s what I call it. If you work with the halogen azides, you work with things whose essential character time does not alter.

“Until they blow up”, you say. Ah, but that is their essential character. It’s the things around them that alter. Make sure you don’t put anything next to them that you’re not comfortable seeing altered – you know, all sudden-like.

A reader forwarded this 1943 JACS article, the first comprehensive study of chlorine azide, and it’s a joy to read. Part of the fun is, of course, watching these folks set off the fireworks. (The challenge with a substance like chlorine azide is finding something that it won’t react with violently):

Owing to the extreme instablity of the compound accurate determinations of the bioing and melting points have not been made as yet. Numerous explosions, often without assignable cause, have occurred during the experiments. . .”

Another thing I always enjoy in these papers is the list of recommended protective gear. No leather suits this time and (interestingly) no earplugs. Nope, it’s straight to the Iron Man look. These azidonauts endorse:

“. . .masks and breast-plates of sheet iron worn by observers during times of danger. Each mask is provided with a rectangular pane (7 x 3 inches) of shatter-proof glass. Although scores of violent detonations have occurred, with resultant demolition of much apparatus, no personal injury has been suffered.”

That last part is sort of a “no graduate students were maimed during the course of this research” statement, which really is good to know. But another nice thing about this paper is the way some parts of it are written, in a style which was a bit formal and archaic even for 1943. A sample:

“If small pieces of yellow phosphorus be added, with stirring, to a solution of chlorine azide in carbon tetrachloride at 0C, the solution gradually becomes turbid, and a succession of slight explosions takes place beneath the liquid. If stirring be omitted until the maximum turbidity is attained, the slightest agitation results in a detonation that demolishes the apparatus. . .”

Do not be omitting the stirring, then. I have to say, not being used to this sort of chemistry, that if I saw these events going on in my fume hood that a series of slight explosions might well take place beneath my iron breastplate. What else doth chlorine azide detonate with? Well, in case you had any doubt, the gaseous reagent “reacts violently” with sodium metal. They had four explosions at -78C, while the fifth run (persistence!) yielded a mixture of sodium chloride and sodium azide. (Actually, the other runs probably yielded that, too, albeit as a fine haze). I really have to salute the dedication involved in finding that out, though – after two or three violent explosions, you or I might be tempted to just say that we couldn’t determine the products of the reaction. But they were made of sterner stuff back in 1943.

The date does make me wonder if there was war research money involved; I wouldn’t be surprised. But chlorine azide has not been weaponized, nor will it be. It remains, with its chemical relatives, off in a part of chemical science that’s safe from human exploitation. It’s a spacious game preserve, that territory, and over the gate is the ornate motto Noli me tangere. Take heed.

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