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Today’s Lab Prep

OK, this one’s a fairly easy procedure, which you will find way back in Volume 6 of the ever-entertaining Inorganic Syntheses. You’ll need a tube furnace, though, and you may or may not have one of those sitting around. You’ll also need to soften up the glass tube that goes into it, enough to make a couple of fairly deep indentations on the sides. Maybe you can buy tubes like that, but I believe that I had to make my own back in 1982 as an undergraduate, when I last did this preparation myself. I haven’t checked the catalogs, because I have absolutely no intention of ever doing this again.

At any rate, those shoulders in the tube are for holding a pile of magnesium turnings, because this is a prep of the magnesium analog of ferrocene, magnecene. Ferrocene is, of course, a perfectly well-behaved compound (and an interesting story in itself, which I included in The Chemistry Book), but its magnesium cousin is not so much at peace with itself. It reacts with oxygen, it reacts with moisture, it reacts with carbon dioxide – all kinds of stuff. I believe it’s a good starting material for preparing all sorts of other metal cyclopentadienyl sandwich compounds, because almost anything would rather be in there than the magnesium atom. In this decadent era you can buy it from Aldrich, but where’s the fun in that?

Now, turn your tube furnace on the side so it’s vertical. I would recommend using something non-combustible for the needed support rather than, say, old lab-supply catalogs or cans of acetone. You’ll need to rig up some sort of connection to a flask or receiver on the bottom end, with a venting stopcock or something, because there will be a steady flow of nitrogen through the whole shebang. That flow is coming in from the top, where you’ll need to rig up another connection to the head of a dicyclopentadiene cracking still. “Ah-hah!” say the chemists in the crowd, who have now grasped the Spartan simplicity of this reaction. Two cyclopentadienes, one magnesium – what could be more atom-efficient? Of course, cyclopentadiene doesn’t exist as such in the bottle – it Diels-Alders with itself to the dimer, but as any synthetic organic chemist knows, it reverts on heating and distillation, and the vapors at the top of the still are cyclopentadiene monomer. So’s the distillate, for that matter, but that’s a limited-time offer and you’d better work fast. In this case, though, a flow of nitrogen gas will sweep the pure vapor over the magnesium turnings, and there you have it.

Oh, there’s one other little detail. This doesn’t exactly happen at room temperature, thus all that fiddling with the tube furnace. The bolus of Mg metal needs to be up around 500 to 600 degrees. Make sure that your thermostat is working, because Pyrex will start to goop up on you if you go over 800, and the next person who wants to use the tube furnace, assuming there’s a next person, assuming that there’s still a tube furnace, will be very unhappy with you. Assuming that you’re still around as well. And make sure that your apparatus is reasonably gas-tight, because if oxygen gets in there with those five-hundred-degrees magnesium turnings you’re going to kick off a very bright fireworks display. If at any point a blinding white light starts shining out the top and bottom of the tube, probably to the accompaniment of interesting noises, something has gone wrong and it will indeed be one of those days.

But if you don’t set off a magnesium flare, then you should produce a pile of thin white flaky crystals in the receiver flask. The magnecene is still in the vapor phase in the hot zone of the tube, but sublimes back out once things return to what passes for normal. For this reason, you probably also want to have a short path out the bottom of the apparatus, for fear of clogging the tube with product. I remember thinking a bit about what to do in that case – I mean, it’s not like I was going to be able to ram a bent coathanger through the thing – and I decided that I should have a heat gun handy to resublime the product along if needed.

Not being stocked with fancy glassware, I held everything together in my setup with big honking pieces of Tygon tubing. The result did not exactly look like a glassware advertisement, although I suppose it could have been a Tygon advertisement, but it did the job. Actually, it did even more of a job than I was planning on. The first time I ran this reaction – I was only planning on one time, but you know how it goes – something happened to the gas flow, and while frantically messing around with the line, I seem to have caused things to go in reverse. My tip-off to that was looking up at the big Tygon elbow at the top of the apparatus, where the tube furnace part was connected over to the still. With some consternation, I noticed that this was no longer a big clear piece of plastic tubing, but was now, for some reason, black. And bubbling. It occurred to me, after a few seconds of intense puzzlement that I would enjoy having seen captured on video, that the wad of red-hot magnesium turnings had been propelled back up the tube and was now stuck inside the Tygon elbow part. I’m pretty sure that I said something vulgar and shameful at this point, as who the hell wouldn’t, because I had visions of the magnesium chewing its way through the obviously hard-pressed plastic tubing and giving me just the sort of Fourth of July experience I’d been trying to avoid.

The tubing held, though, and should the makers of Tygon wish me to provide a testimonial, I’m more than happy to deliver. After waiting for a bit to make sure of things, I decided that this was a good time to let the whole rig come back to room temperature under a healing stream of nitrogen, at which point I would remount and reload. I adjourned for a few hours, but I still remember the blackened, crackling lump of fused magnesio-gunk that I cleaned out in preparation for another (successful) run.

15 comments on “Today’s Lab Prep”

  1. Vader says:

    Obviously something you *will* work with, notwithstanding. Perhaps this should be tagged “Things I’ll Work With Even Though I Should Know Better”?

  2. Slurpy says:

    In grad school, I did some nanoparticle synthesis via ultrasonic spray pyrolysis. Quartz tube furnace running about 800C, if I remember correctly. You know it’s a good experiment when you are killing metal keck clips.

  3. Old Timer says:

    As an undergrad!?!? I am positive that none of the undergrads (grads?) at our institute currently possess that level of equanimity! Hats off to a different era!

  4. exGlaxoid says:

    More like “Things I worked with before I knew better.” I have done a few of those myself. And know people who did even worse ones.

  5. Dr. Lloyd T J Evans says:

    Reminds me of the synthesis of thorium tetrachloride. This uses a similar setup, though the tube furnace remains horizontal. You start with thorium dioxide in the tube, heat it to a similarly high temperature and pass carbon tetrachloride vapour through. Yes, the by-product is phosgene. So this is best done on a small scale, in an extremely efficient fumehood, at night or over the weekend.

    1. Derek Lowe says:

      I’ve seen that one done with chromium, to make metallic-purple chromium (III) chloride. And yeah, phosgene. Some of my classmates did that one, actually. We had a pretty lively group.

  6. Andy says:

    When I saw a furnace setup and the reaction Se + CH2Cl2 -> CSe2 displayed, I knew to keep on walking. Ugh.

  7. Anonymous Researcher snaw says:

    The fact that Derek actually has worked with this prep gives me a renewed appreciation for his “Things I won’t work with” posts!

  8. milkshake says:

    I knew a fierce carbohydrate chemist in Prague who was making his own ZrBr4 because he could not afford to buy it. You take a very long quartz glass tube, fill it with Zr metal shavings and pour neat bromine on top. Zirconium self-ignites and burns like a flare in vapors of boiling bromine. What comes out at the bottom is crude ZrBr4, to be re-distilled. You can imagine his fume hood when he was doing this – an alchemist would pale

    1. Derek Lowe says:

      Oh boy. Clouds of rolling orange vapor spewing out the tube and eating whatever’s in the fume hood – sounds wonderful! I wouldn’t be all that keen on redistilling the bromide, either, because that sounds like a great way to fuse some glass joints (among other difficulties).

  9. Kaleberg says:

    Being a decadent sort I followed your “buy it” link to the Aldrich site and could not help noticing the “Personalized Product Recommendations” and “Customers Also Viewed” sections. I assume these are like the “Customers Who Bought This Item Also Bought” on Amazon. This is kind of fascinating. Do these suggestions provide any insight into chemical thinking today? Does anyone actually find these recommendations useful?

    Is it like: “I bought a gram of Bis(cyclopentadienyl)magnesium(II), so I might as well pick up a gram or two of (S)-(−)-(6,6′-Dimethoxybiphenyl-2,2′-diyl)bis(diphenylphosphine) and some 2-Fluoro-3-(trifluoromethyl)aniline. Hey, you never know.”?

    1. loupgarous says:

      I’m sure that the US Department of Homeland Security has a chemist whose only job is to carefully note who’s ordering things that have been publicly identified as precursors of Novichok agents. If they don’t, perhaps they should….

  10. newnickname says:

    I did vertical FVP to reproduce a Corey prep of 2-methylene-1,3-propanediol. H2C=C(CH2OH)2. First, Cp monomer + acrolein to make the bicyclo-aldehyde adduct. Then a crossed Cannizaro to the bicyclo bis-methanol. I think the diol was first protected as the acetonide. Then, it was dissolved in hexane and syringe pumped into the FVPd under N2 flow to crack out Cp and product. It worked very well. (I borrowed the tube furnace and a quartz tube, so I had no worries about melting pyrex.)

    1. newnickname says:

      Correcting my own post: I needed the ethylidene propanediol, so I adapted the Corey prep using crotonaldehyde instead of acrolein. That prep also worked well. It was the new chemistry I tried after it that didn’t work!

  11. ppjm says:

    Why does the glass furnace need to be upright? Surely the flow of nitrogen gas will take the vapour product out of the furnace into the receiver?

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