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Odd Peroxides Indeed

You know, normally when you start combining interesting or reactive functional groups in the same molecule, you end up with something that’s worse than before. Would I pick up a flask containing a compound that has both a perchloryl ester and a geminal di-azide? I would not, and neither should you, should someone ever be foolhardy enough to prepare such a thing. Those are only for lobbing at hostile monsters in video games. So I was interested to see a recent paper on perfluoroalkyl peroxides. Changing the electronics of that group would surely have an effect, but of what kind? (And perfluoroalkyls want all the electron density, so you know that something is going to change).

Well, to my surprise, the result is the friendliest batch of alkyl peroxides that you’re ever going to see. There was some foreshadowing, though. It turns out that the only member of this compound class that had really been characterized in the literature to date was the bis-trifluoromethylperoxide, which dates back to 1933. It was found to be weirdly stable, as in up to 200 degrees C. Do not, under any circumstances, try heating up a regular dialkyl peroxide in such a manner – you’re not going to get very far, although the pieces of the reaction vessel will.

Preparing these things is worse than handling them once they’re made, by far. As this paper shows, the previous syntheses relied on rather spirited reagents like chlorine trifluoride, and when your prep calls for that one, it’s time to re-examine the life choices that have brought you to that point. That’s what this Berlin group did, clearly, coming up with a route that involves some generally unappealing chemistry, such the treatment of silver wool with elemental fluorine gas at 100 degrees. OK, you don’t start at that temperature, no matter how much of a buckaroo you are – they introduce “small portions” of fluorine to the silver in a sealed metal apparatus at RT “until the pressure remains constant”, that is to say, until it stops whooping and hollering in there, and then they raise the temperature. You also have to prepare hypofluorites of the alkyl groups, which you do from the alcohol and cesium fluoride, introducing (yet again) elemental fluorine in portions until all the overpressures stop spiking, and then do trap-to-trap distillation at -150C. Treating the hypofluorite with that silver fluoride/silver wool, followed by another trap-to-trap purification, gives you the desired perfluoroalkyl peroxide.

None of this am I lining up to perform. Straight fluorine has long been on my “Things I Won’t Work With” list, although if I had to choose, I would go for that over using chlorine trifluoride, albeit with profound unhappiness and thoughts of chucking the whole business and opening a taco truck instead.But at the end of all this, the compounds you get out are, as mentioned, surprisingly tame. The bis(perfluoro-t-butyl) peroxide, for example, melts at about 18C, so at most room temperatures it’s colorless liquid that just sits there, bizarrely. It’s insensitive to shock, insensitive to friction, and you can even get a boiling point on it (99C), albeit with decomposition.

You cannot say any of that about the plain bis-alkylperoxides, that’s for damned sure, except for the decomposition part, which unfortunately can happen with fearsome speed when you so much as look at them funny. So what’s going on? The paper notes that the O-O bond energy isn’t that different in the two sorts of compounds. What changes, though, is the energetics of the alkoxy radicals that form when that bond cleaves. The reactions of the perfluoroalkoxy species are slow and endothermic; the reactions of the regular alkyls, definitely not. The paper goes into a number of structural and computational details – there have been some longstanding arguments about peroxide conformations and bond angles versus theoretical predictions, and these compounds have some bearing on that.

So there you have it: if you’re willing to walk through the valley of elemental fluorine, you can arrive at these odd compounds. It’s be interesting to see what sort of reactivity they have as selective oxidants. Who knows, they may turn into familiar reagents if they do something useful, or they may just remain as chemical curiosities. They’ll always be that!

35 comments on “Odd Peroxides Indeed”

  1. Chad Irby says:

    Now I want to see what it looks like when you hit silver wool with a puff of fluorine.

    In a video, not in real life.

    1. haarp says:

      over at periodic videos they react F2 with Fe wool, in the fluorine video.

  2. Uncle Al says:

    Consider a bis-peroxy ester of acetylenedicarboxylic acid, ROOC(=O)-C#C-C(=O)OOR. The capping group may be cumyl-, t-Bu-, Me3Si-, F3C-, etc. Entrain vapor in argon or helium. Flow it through a flash pyrolysis or flash photolysis tube. Homolytically scission and (transiently) obtain cap oxy-radical – CO2 – C2 diradical – CO2 – cap oxy-radical. The fun part is definitely not ·C#C· electronically, re the turquoise Swann band comet cloud or tail.

    Aside from spectroscopy (via photolysis in a dilute frozen inert gas matrix), will it polymerize into capped polycarbynes (-C#C-)n or (=C=C=)n? Fullerenes? Grant funding would enjoy a (room temperature) superconducting deposit or organic magnet. Don’t disappoint by absence of mention. Perpendicular [2 + 2] dimerization of resultant acetylene bis-ethers, adjust oxidation state, hydrolyze…squaric acid. If the waste stream is more valuable than the inputs, is it still Green Chemistry?

  3. Kent G. Budge says:

    There are days I want to go start a taco wagon, too. I think it’s just part of working in science.

  4. oldnuke says:

    I worked for my Dad at the duPont Experimental Station for one summer between high school and college (Chemistry and Microbiology). Second summer, I asked my Dad what he thought about a summer job at Chambers Works in fluorocarbons. He told me that he thought working with insecticides was safer than playing with fluorine. 🙂

    1. Some idiot says:

      You have a wise father…! (-:

      1. oldnuke says:

        I spent summers working in biological and chemical warfare labs (Army) back in the days when we were working a lot with live agent. As I told my Dad, pesticides for people.

  5. Some idiot says:

    “walk through the valley of elemental fluorine”

    Thank you, that one made my day…!

  6. Tim says:

    Just a minor typo: “should som eone ever be foolhardy enough”

    Will this be filed under Things I Won’t Work With?

    1. Derek Lowe says:

      It’s a borderline case!

  7. Christophe Verlinde says:

    At least these Berlin daredevils added the following note to their paper:
    “CAUTION! Safety note: Although the peroxides described were found to be insensitive to shock and friction[18,37] according to the U. N. Recommendations on the Transport of Dangerous goods, and we have not observed any explosive decomposition during their handling, we cannot rule out that these compounds may react explosively when mixed with other substances.”
    You have been warned!

  8. Glassveins says:

    I wonder if they’ve tried reacting these with MMH, UDMH, or various hydrocarbons to see if they’re hypergolic. These could be useful!

  9. David Edwards says:

    While looking up alkyl peroxides, I found this page devoted to dimethyl peroxide, which is claimed on that page to be present in Malabar Spinach (Basella alba, listed on the page in question under the synonym of B. rubra).

    Now what is this plant doing, making a compound like this? Especially as it appears no one has tried determining its melting or boiling point, presumably on the basis that doing so would result in the determiner being spread across the surfaces of the laboratory, when the peroxide decided it was decomposition party time.

    So, discovering that perfluorinated versions of these alkyl peroxides are much more tame, is quite a surprise. Though I’ll take Derek’s advice, and try not to involve myself with the rather lively reagents used in their synthesis.

    1. wildfyr says:

      Seems like preparing it on the microscale (say, 5 mg) wouldn’t be too hazardous, just to get the properties.

  10. Matthew K says:

    If you’re a naïve individual such as myself and wondered, “how bad is a hydrogen fluoride burn?”, I would NOT recommend a Google image search.

    1. David Edwards says:

      I’ll take your advice and NOT go there …

      Though that video clip Derek linked to above, on the lively nature of chlorine trifluoride, illustrates graphically why this is on his TIWWW list, without horrific depictions of injury. Enjoy seeing various artefacts destroyed, all accompanied by some incongruously tranquil French avant-garde jazz backing music.

    2. Ian Malone says:

      The burns are horrific (and exacerbated by nerve damage that means they may not be initially noticed), but the burns aren’t even your main concern. “Once absorbed into blood through the skin, it reacts with blood calcium and may cause cardiac arrest. Burns with areas larger than 160 cm2 (25 square inches) have the potential to cause serious systemic toxicity from interference with blood and tissue calcium levels.” https://en.wikipedia.org/wiki/Hydrofluoric_acid

  11. loupgarous says:

    From the “Acknowledgements” section of the paper:

    “…The authors are very grateful to Prof. Dr. Thomas M. Klapötke for the sensitivity studies…. “

    But, of course. It wouldn’t be an Angewandte Chemie paper on organic peroxides with fluorine in them without mention of the redoubtable Dr. Klapotke.

  12. Jim Hartley says:

    From the Wikipedia article on chlorine trifluoride, attributed to John Drury Clark:

    “It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals—steel, copper, aluminum, etc.—because of the formation of a thin film of insoluble metal fluoride that protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

    1. mjs says:

      John Drury Clark as in the author of “Ignition! An Informal History of Liquid Rocket Propellants.” A highly entertaining book. Educational when you try to fill in the scientific gaps. And the book shows that there are chemists at least one sigma beyond most of the people here.

      1. Rhenium says:

        Also,” Ignition” was recently just republished and is available at reputable vendors (likely disreputable vendors as well).
        I’m thinking of making it part of the Inorganic Chemistry award at my institution.

  13. Fred says:

    hhmm… from CDC Facts About Hydrogen Flouride (https://emergency.cdc.gov/agent/hydrofluoricacid/basics/facts.asp):

    “You may be exposed to hydrogen fluoride as part of a hobby.”

    …really?

    1. Derek Lowe says:

      Glass etching creams, gels, etc. You can buy them at craft stores (!)

      1. Fred says:

        Yikes… I think I’d resist (;-) that hobby…

    2. Ryan says:

      HF is also available in a 1.0-2.5% solution as a “rust remover”. As a matter of fact, a local big box hardware retailer has it on the shelf for sale in 1L and 0.5L bottles.

      I think it’s safe to say that, if misused, that is more than enough to kill someone.

      1. metaphysician says:

        I once watched an episode of ‘House’, and laughed in disbelieving mockery when the interns discovered “hydrofluoric acid carpet cleaner” while investigating the cause of the disease of the week. Talk about non-sense.

        . . . then I looked it up, and found such things actually existed. :-O

        1. Hap says:

          It’ll clean your carpet, and your skin, and your bones. It’s multipurpose!

  14. milkshake says:

    perfluoroalkyl azides are remarkably stable. And they are far more reactive in copper-free click chemistry with strained acetylenes (like BCN or DAIBO) than regular azides. Whats more, you can introduce 2-azidotetrafluoroethyl group on protein SH groups in water, by using modified Togni reagent chemistry.

    Hope I did not give up a secret here/

  15. Fluorine Chemist says:

    Perfluoroalkyl compounds have their own reactivities. I have worked with a few of them starting from perfluoropropyl to perfluorohexyl iodides. The primary halides just do not under the classic SN2 substitution reactions. However, you can insert a zinc or cadmium and generate the corresponding organometallic, which can then be coupled with stannyl or silyl halides to yield the corresponding stannane. Yields are really good, the lower molecular weight stannanes have the annoying property of decomposing in a silica gel column.

    One-pot reaction involving perfluoroalkyl iodide with aromatic aldehyde in the presence of Zinc gave the alcohols in good yields. Surprisingly, we didn’t notice loss of yield due to fluoride elimination!

    We also did 2D NMR (Fluorine – fluorine COSY) on them and weirdly, found very strong correlation between alternate fluorines!

    Interesting beasts, perfluoroalkyls!

    1. Fluorine Chemist says:

      The primary halides just do not under the classic SN2 substitution reactions.

      Should read as The primary halides just do not react under the classic SN2 substitution reactions.

      My bad!

  16. AC says:

    “Fluorinated silver wool” is a reagent I hope I never have to prepare and use.

  17. Scott says:

    A *taco* truck, Derek?

    Shouldn’t a self-respecting Arkansan go for the *pulled pork sandwich* truck?

    1. Derek Lowe says:

      Or a catfish and hushpuppies truck! I would happily be a customer for any of them. . .

  18. Nick says:

    As a side bar to diabolical chemistry reactions, I found the following video ” Making Sodium 5,5′-AzoTetrazole From 5-Atz”. https://www.youtube.com/watch?v=_m52RquJi_M . I got to 1:07 and thought of you. Would definitely make your “Things I Won’t Work With” list…

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