Top 10 Toxic Hydrides

So I wanted to write a post for the Toxic Carnival but found myself not entirely sure what to write about. I mean… I don’t know a huge amount about cool biogenic toxins like tetrodotoxin. Or unicorns, for that matter. But, back in my comfort zone, if we’re talking about astronomical molecules then I have poisons-a-plenty to choose from. Some of them in unspeakably huge quantities. So I mused on this for a while… And I realised that, at least in some ways, I’m a physical organic chemist. And while organic things (like ethanol and benzene and people) are based on carbon hydrides, well… A lot of hydrides are rather toxic. So I bring you, without further ado, my 10 favourite toxic hydrides!

Of course, everyone knows the lighter hydrides. We get taught about methane and ammonia pretty early on in school, and water covers a significant fraction of the planet. But most elements can form hydrides. In some ways these hydrides act much the same as their lightweight brethren, while in other ways they’re remarkably different.

10. Hydrogen Sulfide (H2S)

Hydrogen sulfide is one hydride most of us will have encountered, and it’s a safe bet none of us will have enjoyed it. H2S has the repugnant odour of rotten eggs and is produced by all sorts of bacteria in the world as they go about their daily lives. Notably it’s used by purple sulfur bacteria which, instead of metabolising water into oxygen, metabolise H2S into sulfur granules!

9. Arsine (AsH3)

Arsenic compounds are always good for the lulz. They almost always have hilarious names. They’re also made with arsenic, which makes them inherently lethal. Arsine here, as well as being deadly, is also pyrophoric — which means it spontaneously ignites in air. When not on fire, it apparently has a faint garlic-like scent, but chances are if you can smell it you won’t live long enough to tell anyone that!

8. Beryllium Hydride (BeH2)

Beryllium hydride is quite interesting because it’s covalently bonded. That isn’t normally interesting, but beryllium is in group 2 of the periodic table. All of the other members of that group (like magnesium and calcium) are too electropositive to form covalent bonds, preferring to be ionic. This only happens because both Be and H atoms are so tiny! Beryllium is poisonous because it can displace magnesium from enzymes. It’s quite effective too, effective enough that professional “poisoners” in history used to frequently use beryllium compounds to perform their assassinations! BeH2 is sometimes used in rocket fuels. Presumably it has a good energy density, being made up of such light elements.

7. Germane (GeH4)

No, not that guy from Flight of the Conchords. Germane is the heavier brother of methane, and it acts in almost exactly the same way. It’s happy to play nice with carbon and make organo-germanium compounds, and even burns in air to give GeO2 and water. Germane has also been spotted in the atmosphere of planet Jupiter.

6. Uranium Hydride (UH3)

Containing uranium instantly makes a compound cool. Even though, unfortunately, uranium compounds also have comical names. I suppose, if it followed the same naming convention as group 15, it could possibly be called urine. But that would be particularly unfortunate. As a compound, it’s a pyrophoric black powder which, peculiarly, has metallic conductivity. As with most things containing heavy metals, it’s also scarily toxic. Hydrogen apparently has quite an affinity for uranium. Expose the metal to hydrogen gas, and hydrogen atoms will diffuse into it rapidly, forming hydrides at the grain boundaries and causing the metal to become brittle. UH3 was also used in the so called “tickling the dragon’s tail” experiments when the critical mass of uranium was discovered.

5. Diborane (B2H6)

Diborane is a colourless gas with a repulsively sweet scent (probably the sort which an old chemistry lecturer of mine would have referred to as “really nice and very toxic”). The scent alone, as with arsine, is noted as not being sufficient warning of the presence of diborane, due to its high toxicity. Diborane is chemically rather interesting. In the non-space-filling model here, you can see that the central hydrogen atoms are bonded to two boron atoms each. Boron atoms, due to their combination of size and electron deficiency, are capable of 3-centre-2-electron bonding, which is essentially exactly what it sounds like (just like in H3+). Thanks to having 3 regular valencies and 3c2e bonding, boranes can form all kinds of interesting cage-type structures.

4. Hydrogen Telluride (H2Te)

Planet Earth is deficient in tellurium, making this heavy analog of water quite a rare molecule. The reason why Earth has a lower abundance of tellurium than the rest of the Universe is that when Earth was still forming, most of the tellurium formed into H2Te. Because H2Te is surprisingly volatile for its mass, Earth’s gravity couldn’t hold on to much, losing most of that tellurium into space.

3. Phosphine (PH3)

Many chemists will tell you that ammonia’s big brother here has a foul odour somewhere between garlic and rotten fish (yum yum!), but actually, pure phosphine has no scent. The odour comes from substituted phosphines and diphosphenes mixed in with impure samples of the chemical. Diphosphine molecules are also the reason why phosphine gas can occasionally combust spontaneously. The two molecules are often found mixed in with swamp gas, and the spontaneous ignition of the phosphines in that gas is thought to be what causes Will o’ the Wisps in marshes. Phosphine has also been detected in the atmosphere of a red giant star.

2. Hydrazine (N2H4)

Many of the more complex nitrogen compounds tend to have a habit of exploding, and hydrazine is not an exception, being listed as “dangerously unstable” when not kept in solution. Seeing as hydrazine loves nothing more than to combust viciously, it’s used as a very effective fuel for both rockets and orbiting spacecraft. It’s also very toxic even in relatively low doses. You may remember a couple of years ago when the US government decided to shoot down a defective military satellite, causing much furore over the possibility of spilling a scary amount of hydrazine into Earth’s atmosphere…

1. Hydrogen Fluoride (HF)

The number one toxic hydride on my list has to be hydrogen fluoride. And unlike the others, the main reason this is here is due to its toxicity. HF, quite frankly, scares the hell out of me. HF boils just below room temperature, diffuses rapidly through virtually any porous material, and on contact with any kind of water, it instantly hydrolyses into hydrofluoric acid. I’ve only had the joy of using hydrofluoric acid once in my life, to etch metal samples. I used about 1 ml, for 5 minutes. To do that, I had to sign a couple of forms and have a short safety briefing. Any safety briefing which involves phrases like “immediate medical attention” is always… concerning. Basically, it’s a contact poison. It can cause deep burns and tissue necrosis (initially painless because it also dissolves your nerve endings). Then it interferes with calcium metabolism and causes systemic toxicity and cardiac arrest. You see? There aren’t many things I’m actually scared of, but HF is one of them. High powered Nd-YAG lasers are possibly another, but I’m digressing now.

Interestingly, there’s a species of plant somewhere in Africa which uses hydrofluoric acid as a natural defense. Lovely. A bit further away, HF is easily detectable as an interstellar molecule (as you’d expect from a molecule with an obvious dipole moment, it’s easy to pick out with rotational spectroscopy). I’m not sure offhand how abundant it is though. Fluorine is formed readily by stars, but tends to fuse further by catching neutrons and transmuting into neon, so it’s not all that abundant in the cosmos.

So there you have my favourite 10 toxic hydrides. There are many hydrides out there, but these are the ones I find most intriguing! (What’s that, you say? Silicon…?)

Image: “gas mask” by RJetti @ deviantART

All other images courtesy ofWikimedia Commons.

What? I’m a chemist, I’m allowed to put the words favourite, toxic, and hydrides together in the same sentence.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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