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Pentaborane(9)

en.wikipedia.org

53 pointsrbanffy33 comments

I recall this being mentioned in Ignition!: An Informal History of Liquid Rocket Propellants [0] :

Boranes are unpleasant beasts. Diborane and pentaborane ignite spontaneously in the atmosphere, and the fires are remarkably difficult to extinguish. They react with water to form, eventually, hydrogen and boric acid, and the reaction is sometimes violent. Also, they not only are possessed of a peculiarly repulsive odor; they are extremely poisonous by just about any route. This collection of properties does not simplify the problem of handling them. They are also very expensive since their synthesis is neither easy nor simple. But they possess one property which attracted rocket people to them as hippies to a happening. They have an extremely high heat of combustion — gram for gram about 50 percent more than jet fuel.

[0] https://www.amazon.ca/Ignition-Informal-History-Liquid-Prope...

>Fifteen years ago, people used to ask me “What is an exotic fuel anyway?” and I would answer “It’s expensive, it’s got boron in it, and it probably doesn’t work.”

What a great book.

Excellent book even if one is not interested in anything in any way related to rocketry. I wish there was a corresponding, detailed account from the Soviet side, but as far as I know none exists. “Ignition!” does cover the Soviet program, but only very sporadically, to the effect of “they skipped most of this expensive foolishness, and went straight to hydrazine and kerosene”. Well, apparently not - pentaborane was tried in the engines as well.

As one of the compounds that have a NFPA 704 (fire diamond) rating of 4 for every category, it is naturally extremely dangerous. Above 30 °C it can form explosive concentration of vapors with air. Its vapors are heavier than air. It is pyrophoric—can ignite spontaneously in contact with air, when even slightly impure. It can also readily form shock sensitive explosive compounds, and reacts violently with some fire suppressants, notably with halocarbons and water. It is highly toxic and symptoms of lower-level exposure may occur with up to 48 hours delay. Its acute toxicity is comparable to some nerve agents.

Friendly

Fluorine gas seems safer by comparison

Friendly fluorine and its positively sociable cousin hydrofluoric acid, the lifes of the party compared to pentaborane.

It's like it's asking to be banned.

¿Porque no los dos?

https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Bipro...

edit: I found people that tested the pentaborane/fluorine combination specifically,

https://apps.dtic.mil/sti/trecms/pdf/AD0270941.pdf ("EXPLORATORY RESEARCH ON HIGH ENERGY PROPELLANT SYSTEMS" (1961))

- "An instantaneous hypergolic ignition occurred with a very long brilliant greenish-white flame and clouds of dense white smoke. Approximately four seconds after the start of the run, a leak developed in the fluorine valve stem. The lowered pressure in the fluorine line caused a flashback and the fluorine line ruptured. The very fuel rich combustion and decomposition of pentaborane which then resulted quickly plugged up the injector holes with the thick polymerized products of decomposition of pentaborane."

This made me think about the excellent "things I won't work with" series by Derek Lowe.

https://www.science.org/topic/blog-category/things-i-wont-wo...

He didn't write write an article about pentaborane, he left this one to Max Gergel https://www.science.org/content/blog-post/max-gergel-s-memoi...

From the book:

(Warning: Spoilers ahead)

> The next day I told Parry that I was flattered but would not make pentaborane. He was affable, showed no surprise, no disappointment, just produced a list of names, most of which had been crossed off; ours was close to the bottom. He crossed us off and drove off in his little auto leaving for Gittman's, or perhaps, another victim. Later I heard that he visited two more candidates who displayed equal lack of interest and the following Spring the Navy put up its own plant, which blew up with considerable loss of life. The story did not make the press.

Hahaha. Fuck. The history of pentaborane is littered with human tragedy. What an appropriate compound for this troubled age.

Definitely don’t read about the history of acetylene then.

Same as it’s always been.

Hahah. Oh gosh. As an aside: Your username checks out. Azides are nothing to be sneezed at either, IIRC.

Hah, first time someone noted that connection!

On the original topic of the thread, check out Chemical Forces video on boranes - [https://youtu.be/8hrYlhTYl5U?si=4SDJq4MxAEu714iY]

I’m not a chemist, but used to read my copy of ‘chemistry of powders and explosives’ to get to sleep, and synthesized a few out of curiosity over the years. There are some real fun wiki holes in the topic too.

The azides do tend to be a bit unstable as well, same as the fulminates.

Most are still more stable than the organic peroxides, at least if they’re uncontaminated,

Energetics chemists tend to be the Leroy Jenkins of scientists.

Lead(ii) azide has mostly been replaced by lead styphnate or other compounds in commercial use, safer to synthesize [https://en.m.wikipedia.org/wiki/Lead_styphnate]

In the analytical lab we had been using dinitrophenylhydrazine, in very low concentrations, in the determination of trace aldehydes. When the previous bottle was almost empty, I found out it could not be reordered from our established supplier. One chemist showed me how little there was left, he had been banging the bottle against the bench to get the last gram out. I was about in shock, apparently less so than the compound itself, and advised don't do that again because it's like a cross between TNT and rocket fuel.

Then found out the DNPH was no longer available in dry form, packed under water now under a different part number and with a revised SDS.

When Gergel was writing this I was working for one of the similar companies, a research chemicals company fairly close to the bottom of that list also.

We made lots of different unique chemicals ourselves but distributed many more.

Quite a few from Columbia Organics, I remember their isopropyl bromide well.

Hexanitrohexaazaisowurtzitane is pretty funny and very similar.

"Hexanitro? Say what? I'd call for all the chemists who've ever worked with a hexanitro compound to raise their hands, but that might be assuming too much about the limb-to-chemist ratio." (classic chemist joke)

"make a more stable form of it, by mixing it with TNT. Yes, this is an example of something that becomes less explosive as a one-to-one cocrystal with TNT.."

And then hexaaza, which is six more nitrogens…

Wacky structure! Not just a cage, but also has 2 B-H-B bonds (https://en.wikipedia.org/wiki/Three-center_two-electron_bond) - no wonder it is so reactive...

Yeah, amazing that something so relatively unstable can actually exist long enough to be dangerous. I suppose the exothermic output is not helped by how stable H2 is.

Do other molecules fit inside the cage?

no, far too small i would think. not sure what the smallest cage is that can act as a host-guest complex

https://en.wikipedia.org/wiki/Host%E2%80%93guest_chemistry

A fun one is Prof Rebek et al's 'tennis ball' - bisglycoluril that forms two halves that cup around a guest:

https://www.scripps.edu/newsandviews/e_20011203/rebek1.html

Hey, you’re not giving pentaborane enough credit - it’s got 4 of those bridging hydrides.

We often see green flames shooting across the sky without descending in SA near the Woomera test facility. I put it down to boronated fuel, but most people put it down to meteors with high nickel iron or copper content.

Naah - she‘ll be right mate ;-)

I was confused because I read the 9 as a manpage section.

    9   Kernel routines [Non standard]

    syscall(SYS_PENTABORANE, 0x42);

Yup, that's non-standard alright.

> The propellant mix that would produce the greatest specific impulse for a rocket motor is sometimes given as oxygen difluoride and pentaborane.

Spicy! How does that compare to metallic hydrogen and metastable helium?

https://www.projectrho.com/public_html/rocket/enginelist.php...

An interesting property of pentaborane (and other boranes) is it reacts with hydrazine to make boron nitride and hydrogen, with passable Isp (around 250 s). The hydrogen is then available for further combustion with air in combined rocket/ramjet engines.

As I remember, Russians once even launched "Soyuz" (R-7) on Pentaborane, achieved some additional kilograms of load. Fortunately , all finished ok, but they don't repeat these experiment "because of expensive".

"Problems with this fuel include its toxicity and its characteristic of bursting into flame on contact with the air. Furthermore, its exhaust (when used in a jet engine) would also be toxic."

These do seem like downsides for sure.

Is it as much fun to work with as Azidoazidolazide?

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