CR Scientific: Chemistry Articles: The Dangers of Nitrogen Triiodide, etc.

The Dangers of Nitrogen Halides

WARNING! This article discusses compounds which are so unstable that they will decompose energetically at the slightest perturbation. Do not attempt to make them.

Background

Internet message boards, often the source of entertaining dialogue as well as the occasional bit of useful knowledge, can also contain some very unreliable information. In particular, there is unfortunately much talk of energetic materials by people obviously unschooled in any aspect of chemistry, let alone in compliance with the regulations attending the manufacture of things that dissociate energetically. *

There is a video floating around on the internet, showing a couple youths mixing chlorine and another compound in a plastic bottle. One of them shakes the bottle, which promptly causes it to explode in his hand. We can only hope he escaped unhurt, but from the looks of it, it didn't turn out well.

If he'd mixed something with the chlorine that could produce NCl₃, he'd probably have lost his hand.

Etymology & Definitions:

Posters on internet message boards commonly refer with derision to people known as "kewls". This author does not know offhand who coined the term. The word "kewl" in its noun form is slang for someone who lacks competence but is so bent on producing an exciting, dangerous, and sometimes malicious result that they proceed anyway.

The word seems to be pronounced "kyool", but the "y" is only mildly enunciated-- almost silent.

Kewls are usually males under 25, but this is not always so; ignorance and recklessness subside at different ages for different individuals, and the male gender is far from having a monopoly on doing stupid things.

As for the origin of the term itself: the word "cool" at some point became corrupted into "kewl", whether by design or by accident. It seems that, before their luck runs out and they get maimed or killed, these individuals do stupid things and exclaim "Cool! Let's do that again!". Worse yet, "Kewl! Let's do that again!"... or, when speaking in hypothetical terms, "That would be kewl!"

On the message boards it also appears that numbers can substitute for certain letters within words. This can occur in combination with abbreviations, deliberate (?) misspellings, and juxtapositions; for example: "d00d taht wud B kew1". Sometimes the word "kewl" itself appears as "k3w1" or some equivalent.

These careless and ignorant people (kewls) have done immeasurable harm to the science hobbies, and indirectly they've hindered professional science as well by causing many an amateur to become discouraged at the poor availability of lab reagents before ever making it into the professional realm. (Many of us have done k3wlish things at some point or another, but the true k3wl is a habitual offender... until some tragic, irreversible episode finally happens).

Kewls, it seems, have a special fondness for easy-to-make but ridiculously dangerous materials such as organic peroxides (containing the -O-O- functional group) and nitrogen trihalides (NCl₃ and NI₃ in particular). Let's discuss the properties and dangers of the latter.

Chemical Formulae:

Although some sources claim the nitrogen trihalides can exist as pure compounds, the iodide (at least) usually exists as a complex with aqueous NH₃ molecules. Thus, this writer would suspect the triodide to be NI₃ • (NH₃)_n. It is not certain how many ammonia molecules provide the optimal 'stability' for each complexed NI₃ molecule, nor whether a predictable ratio even exists, but such 'stability' the molecules might have would be exceedingly precarious and therefore fleeting. The compound doesn't hold together long enough to do much analysis on it.

For convenience, however, we'll refer to the compounds in question simply as NCl₃ and NI₃.

N-Trihalides: Dangers, and some Science:

It is unfortunate that two of the nitrogen trihalides (the chloride and the iodide) are trivially easy to make, because that puts them in the reach of "kewls". We won't detail the methods here; any readers advanced enough to use the N-trihalides as actual reagents would certainly be familiar with their preparation.

The chemistry of these unstable nitrogen trihalides is, however, very interesting-- at least as far as what's known about them.

NCl₃ is extremely dangerous. It has caused many experimenters to lose fingers, eyes, hands, and probably lives- so don't make it. P.L. Dulong, the chemist who first prepared it in 1811, became grievously injured when some NCl₃ violently exploded (Thenard and Berthollet, 1813). Warmth or sunlight can cause NCl₃ to explode. Sometimes it can explode for no apparent reason.

Raising the pH can also cause NCl₃ to explode; before examining the literature on possible mechanisms, let's speculate on why this happens:

Theory 1 supposes that each NCl₃ is complexed with one or more aqueous NH₃ molecules. Since changes in pH shift the equilibrium between aqueous NH₃, NH₄OH, and NH₄⁺ / OH^-, this could also affect the equilibrium with complexed NH₃ molecules. In Theory 1, changing the pH therefore means the equilibrium between bound and unbound NH₃ molecules shifts to where the whole thing just comes apart once and for all.

Theory 2: NCl₃ exists in solution with chloramine (NH₂Cl) and dichloramine (NHCl₂). Chloramine is favored over NCl₃ at high pH (ca. 9 or higher), while dichloramine is favored at acidic pH. When the pH is elevated and there is a great deal of NCl₃ present, the compound doesn't give up a chlorine and simply become one of the lower chloramines; it explodes! We can observe that addition of alkali will cause NCl₃ to explode immediately and with great violence. Actually, let's not observe it; we'll satisfy ourselves with the chronicles of past experimenters in this case.

Theory 3: We could imagine that NCl₃ requires a certain level of either Cl₂ (aq.) or HOCl (aq.) to stabilize it. Shifting the pH alters the equilibrium between HOCl (aq.) / H⁺ (aq.) / Cl^- (aq.) on the one side and Cl₂ (aq.) / H₂O on the other. In turn, HOCl itself is in equilibrium with OCl^- and H⁺. Higher pH shifts the equilibrium to favor the ionization of HOCl, lowering [HOCl]_aq. and destabilizing the NCl₃ molecule.

Finally:

What the Literature Suggests:

Saguinsin and Morris (1975) propose that aqueous NCl₃ reacts with H₂O in the presence of OH^- to give NHCl₂ and HOCl. The NHCl₂ then rapidly hydrolyses at high pH to give HOCl, H⁺, Cl^-, and gaseous N₂. (Note that while nitrogen gas is produced, this is FAR from being a safe way to prepare nitrogen.)

Krishan, Shinness, and Margerum (1987) suggest a different mechanism, in which some NHCl₂ is present and reacts with NCl₃ and OH^-, decomposing both itself and NCl₃. As in the mechanism previously suggested by Saguinsin and Morris (1975), this one (1987) also yields HOCl, Cl^-, and N₂. Krishan et al. also suggest the reactive intermediate Cl₂NClOH^- may be common to all breakdown pathways of NCl₃ (1987).

It is not clear if the mechanism by which NCl₃ decomposes explosively is the same mechanism as that by which the compound decomposes more slowly in dilute solutions. In the paper of Krishan et al. (1987) there is mention of autocatalytic breakdown, especially with regard to NHCl₂. Unstable compounds may start off with the most miniscule concentrations of breakdown products existing alongside them, but these products then catalyze further decomposition to make more breakdown products... which catalyze even more decomposition. The reaction thus goes out of control. If a "critical mass" is reached, the decomposition obliterates its container.

One mustn't get the impression from the above observations that there's some way to make NCl₃ behave. It won't.

NI₃ is also highly unstable. These extremely sensitive, purplish-black grains or crystals can explode from air currents passing over them, from an insect landing on them, or even just by themselves for no reason at all!

NI₃ cannot be stored. One generally shouldn't make it, let alone attempt to handle it. When it's wet it can explode, but when it's dry it WILL explode. As for the equilibrium with bound and unbound ammonia molecules, it makes sense that evaporation would upset any sort of equilibrium between these. Such tenuous 'stability' that NI₃ may have seems to depend on its being wet with ammonia solution.

Discussion:

Nitrogen trihalides should not be prepared by anyone except experienced chemists who are familiar with their properties, and only then in the tiniest quantities. Chemistry teachers occasionally prepare the iodide to demonstrate its instability to students, but they must be careful to keep the amounts extremely small. There is the now-infamous story of the chem students who prepared NI₃ and were letting it dry on top of the fume hood. A student came along and tossed a notebook up there, perhaps not knowing about the NI₃; the explosion reduced the notebook to confetti (Stanley, 2003).

NCl₃ in particular is better left to discussion than to hands-on experimentation (which can rapidly become no-handed experimentation).

Though the chemistry of the nitrogen trihalides is very interesting, we must again attempt to discourage the reader from making these compounds. Not only are they are unstable, but their breakdown products can also poison you.

Notes:

* This is not to say that everyone who discusses energetic materials is ignorant or incompetent, since there are a great many professionals and advanced amateurs who are capable of safe experimentation. It's probably tedious for such people to be admonished ad nauseam by laypersons-- people who don't know e.g. the difference between a chlorate and a perchlorate.

This author believes that amateur scientists should be free to explore the many aspects of chemistry on a small scale, as long as they know what they're doing and they don't hurt anyone. Amateur experimenters need to police their ranks and are usually quite aware of this fact, since accidents can be so damaging to the hobby. It is up to the amateur scientist to set a good example so that uneducated observers won't come away with bad impressions.
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References:

Kumar, Krishan, Roger W. Shinness, and Dale W. Margerum. "Kinetics and Mechanisms of the Base Decomposition of Nitrogen Trichloride in Aqueous Solution". Inorganic Chemistry 26: 3430-3434 (1987).

Saguinsin, J., and Morris, J., in Disinfection Water and Waste Water (Johnson, J.D., ed.). Ann Arbor, Michigan: Ann Arbor Science, pp. 277-299 (1975).

Stanley, Norm. "Don't Scale These Up!" [Online Article]. Society for Amateur Scientists, 2003.

Thenard, L.J., and Berthollet, C.L. Ann. Chim. (Paris) 86: 37-43 (1813).

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