Potassium hydride self-ignition

I had a rather bad fire last Friday. I was washing a large jacketed glass reaction vessel used for polymer scale-ups, after pouring the reaction mixture out, and a tiny particle of potassium hydride (from this poorly quenched reaction) that was adhering to the bottom of the reaction flask ignited just as I was giving the flask a proper acetone rinse. So I had a flaming flask in my hands + burning hands + flaming sink in front + a whole bunch of wash bottles ablaze next to me (plastic wash bottles peeing their burning solvents around…) A colleague promptly put the fire out with a mid-sized CO2 fire extinguisher before the flames spread any further. There was no damage to the lab, my fingers or the reaction mixture but it was a pretty scary situation – considering how fires in organic labs can get out of control so fast.

Potassium hydride pyrophoric nature is well documented in the literature; from my limited experience I would say KH is quite comparable to potassium metal in its tendency to flame up. But there are some aspects that make KH more treacherous than K metal: KH in paraffin or mineral oil is docile and only when the oil or wax is washed off the pyrophoric nature becomes apparent. Also, the KH appearance (a grayish-white powder) is less dramatic than shiny low-melting globules of K metal and one cannot easily guess whether KH is fully consumed or quenched by the sediment appearance if the reaction produces inorganic precipitate of its own. Also, I noticed that some alcohols react with KH in THF surprisingly sluggishly while reaction of other alcohols is prompt – I believe the solubility of the K-alkoxide in THF plays a role and the KH particles may get coated by a poorly soluble material and laze about the bottom – and then at some later point flame up when least expected.

Since K-alkoxides have significant reactivity advantages over Na and Li alkoxides in alkylation reactions[2], and since the easy-to-handle KH formulation in paraffin wax is now commercially available, it is likely that KH will get used increasingly more often in place of NaH. Despite its innocuous appearance KH is less tame than NaH;  having unreacted KH excess present in the reaction mix makes it prone to auto-ignition during the workup if the reaction was not quenched with care.

Note 1: I was impressed how good is CO2 extinguisher for large solvent fires – and it leaves no mess behind. I don’t think a dry powder extinguisher would have worked nearly as well.

Note 2: Taber et. al.: Tet. Letters 51 (2010), 3545-6

Expanding liquids break closed vessels

I had a dumb mishap today: A 100mL Schlenk storage flask with 1,5-cyclooctadiene shattered. When I distilled my COD by vacuum transfer this morning I filled the storage flask all the way to the top and then turned the teflon stopcock shut. There was no head space left in the flask; as the liquid warmed from about 10C up to room temperature it expanded enough to burst the glass.

Coincidentally, my colleague finished off a 20L jacketed glass reactor in a similar manner just yesterday – he was cleaning it after the experiment and the heating jacket was shut off, both the inlet and outlet valves were closed while the jacket was still filled with polysiloxane heat transfer fluid. When the reactor was rinsed with ambient water it suddenly shattered: a small temperature difference was apparently enough to cause the silicone fluid expansion in the jacket and there was no air bubble space nor a tubing attachment whereto the silicone liquid could expand. Looking back, this jacket over-pressurizing would not have happened if one of the valves was left open.

I suppose we proved that liquids are incompressible and expand with heat.

Link: The Great Boston Molasses Disaster

Dear ACS journal editors – please return from your vacation soon

credit: National Geographic ‘Photo of the day’

The spectacular “NaH catalytic” oxidation recently published in JACS has been thoroughly covered elsewhere. I would like to bring your attention to another jaw-dropping paper that just came out in Org Letters:

http://pubs.acs.org/doi/abs/10.1021/ol901250c

Acetophenones and 1-aryl-ethanols are oxidized to benzamides by heating the material with 3 – 4 equivs of iodine and ammonia in a pressure vessel. There is only a passing reference and footnote that “nitrogen triodide might form in the mix.”

In fact, NI3.NH3 readily precipitates upon mixing iodine with aqueous ammonia. (The products vary; a gradual iodine addition to a large excess of ammonia yields ammonium iodide and nitrogen.) Nitrogen triodide is a notoriously super-sensitive primary explosive. I spent some time hospitalized in eye clinic when I was ten years old – my corneas got burned with iodine and my eardrums ruptured because of playing with a spoonful of nitrogen triodide. (The window pane flew out and I was thrown to the ground by the blast;  it took me half a year to fully recover and this all was from few grams of dry material going poof, unconstrained). I cannot warn strongly enough against mixing iodine with ammonia in a pressure flask and then heating the stuff up!

The authors run these experiments on a 1 mmol scale and they give no details about the order of addition. Since the transformation is pretty useful – with a good substrate scope and it looks simple enough (its done in water) –  sooner or later some innocent person is bound to mix up a big batch in the wrong way – and as he screws on the the pressure vessel cap he is gonna blow himself up into a mauve cloud…

More lab disasters 2

When I was in high school, I got free run in a chemistry lab that belonged to a youth center. I was trying to synthesize papaverine, and this was completely above my ability (and the lab resources) but I was very persistent. The key building blocks for papaverine are homoveratric acid and homoveratryl amine, and I set out to make them by myself: I had several bottles of catechol to start from - my problem was how to methylate it, I could not just buy stuff – I had to go by with what was in the stockroom.

First I made lots of methyl iodide, from red P + iodine and methanol – refluxing and distilling it on the bench so I know how methyl iodide smells - but the methylation was messy. Next I tried to make dimethyl sulfate from sulfuryl chloride and methoxide in situ, and it worked to some degree – the vanilla smell of guaiacol was everywhere – but again I could not isolate anything from the mess. So my next idea was to use diazomethane made from nitrosomethyl urea.

So I was cooking and then distilling AcNH2 on a grand scale, from AcOH and urea - and this went quite well (apart from the all-pervasive mice urine-like smell of acetamide) and then I was to carry out the Hoffman degradation.  When one uses concentrated aq KOH and bromine, the in-situ generated MeNCO reacts with a second molecule of acetamide to produce MeNHCONHAc, an intermediate for preparing nitroso methylurea; I needed it for a large-scale diazomethane reaction so I did it on a mol scale on the first run.

The procedure called for Br2 to be added into AcNH2 + 30% aq KOH mix in a 1L flask, then gently heating the mix until a rapid gas evolution commenced. Since I scaled up the preparation by a factor of twenty on the first run, and I did not have a 20L flask, I used the biggest flask I could find, a 4L Erlenmeyer, and loaded the stuff up; it all fit in there. But then, the mix did not wait to be gently heated and instead jumped out at me all at once.

I usually did my work without glasses, on the bench - but this one time I put goggles on and it was well worth it. The hot KOH + KOBr solution rained all over the place and bleached my hair blond; also my T-shirt and jeans ended up with white vertical stripes. A colleague stood nearby and saw the whole thing and dragged me into shower.

Eventually I did make some nitrosomethyl urea and diazomethane but never finished the papaverine project. No explosion, poisoning or other injuriy happened during all these crazy experiments. But there was another like-minded highschool kid, repeatedly working on some chemistry involving acetone cyanohydrine, and he was making it from acetone and NaCN, in our little fume hood with a lousy fan-driven exhaust. I think he never finished his project either but I remember once we were going down the stairway and he was saying “I don’t know if this exhaust really works ’cause I was smelling hydrogen cyanide this time a lot” and then we got outside there were two dead pigeons on the grass and we looked up and the exhaust from our fume hood was looming right above us…

More lab disasters

Ψ*Ψ has a new post on undergrad lab disasters, I would like to add few more of my own making, from two decades ago:

In the junk-room of our chemistry department I found an ancient belt-driven vacuum pump that operated on 380V three-phase AC. I brought it from the basement into our freshly-renovated lab, put it on the bench, plugged it in to see if it worked – and it did. Unfortunately the 380V AC wiring must have been wrong in our lab (we never used 380V plug in there before) or the phase order in the pump itself was switched. At any rate, the motor started spinning backward and the pump pumped out its oil from the inlet hole at once -  a gallon of black muck that hasn’t been changed for eons. The intlet had a short piece of rubber hose on and it worked like a nozzle - directing the stream of goodness at the high ceiling, right in the middle of the room whence it rained down all over the place. This mishap actually shut down our lab for two weeks as the oil-soaked plaster had to be knocked off down to the brick and concrete in order to make the new plaster stick.

Some months later I was working as a guest student in another lab (in another building, at another school) – and they had a gas-powered water heaters installed above the sinks because their building lacked central hot water. It was a strange and dangerous thing to have in the lab (right next to the  organic solvent bottles); and the last person in the lab always made sure to turn off the pilot lights before leaving.  One early morning I was washing my hands and water was coming out freezing cold - the  pilot light was off -  so I grabbed matches and without turning off the running water (and the stream of gas), l lit the pilot.  A yellow fireball shot up and the casing flew off from the heater infront of me, with an impressive bang. When my ears stopped ringing  I could hear a calm voice from the opposite corner of the lab where a colleague sat at her desk: “Mr. Borivoj, I promise that the next time I’ll pour my coffee over you.”

The same colleague a week later decided to clean up and inventorize all her glassware - she emptied her drawers and put it all on a long bench. Meanwhile I was distilling 0.5L of old and nasty-looking  N-methylmorpholine to which I added lots of calcium hydride - and when I finished I had CaH2-rich leftover sludge in the distillation flask. I was asking around if it was OK to quench it with ethanol (I had quenched NaH and BuLi before) and a faculty dude said I should go right ahead; I did not realize they probably did not work much with CaH2 in that lab.  So I was carefully adding ethanol with cooling, the bubbles were coming out, calm and nice.  One hour later (back from lunch) I added some more ethanol - no bubbling anymore – so I was certain I could pour that into the waste, and I went to the sink to dilute the sludge with water a bit so that I could pour it out. Suddenly it became clear that ethanol does not really quench CaH2, and water does. The  mud volcano in my hands erupted away, spewing the hot lime and fishy amine on the nearest bench - all over that clean and inventoried glassware. The owner again took it  calmly - she just muttered ”We have to assign you a working space in the hallway”…

Our lab you must not enter

When I was fired from my first lab in college for the phosphine stinkup, no-one volunteered to have me so for awhile I had a bench and hood in the teaching labs. I was there alone, enthusiastic and without supervision. The glycidol story was mentioned already; I will describe two more memorable experiences:

Burning acrolein: We couldn’t buy things from Aldrich under communism and acrolein was unavailable from domestic supliers. Eventually I decided to make my own; there was an ancient lab procedure from glycerol by pyrolytic dehydration over KHSO4. (The yield is dreadful but we had a drum of glycerol in the stockroom).  I did it on a grand scale and I ended up with about 0.5L of crude acrolein containing lots of water. To remove the water I got the idea of using magnesium perchlorate as a drying agent - we had a big bottle of that stuff on the shelf and I was reading somewhere that Mg(ClO4)2 was a potent desiccant comparable to P2O5 in its dehydrating proves. No wimpy sodium sulfate for me.

So I was spooning perchlorate into my acrolein and it kept dissolving – I ended up adding a whole lot of it and it was still dissolving and the mix was getting alarmingly hot and yellow. “Oh no, the perchlorate is all soluble and my acrolein is now polymerizing because of it, I will lose it all – quick, I must distill it all at once to rescue it!”. So I put the mix onto a heating mantle, added a distillation adapter and condenser and turned the heat on.

The hood sash was down and I was few meters away when a brilliant orange light from within the flask illuminated the lab and the entire hood turned black in an eyeblink. With a tremendous “wroooommrrr” that rattled the windows, the mix instantly burned away like a rocket engine and then the flame died out – before I realized what I have just done. The flask was still in one piece (only the condenser flew out) and a foot-high layer of soot was now filling the hood. The black tongues got painted on the wall behind the hood, emanating from the few places where the hood leaked. I turned around and saw a black cloud hanging by the ceiling and slowly settling down like a pillow. The entire  lab got dusted with the greasy soot - a notebook lifted from the bench left a light rectangle behind…

It is inconcievable how much fluffy black dust can be produced from a half-liter of this mixture – I remember scooping out several buckets of soot that I had to smuggle out of the building. About six hours later (and a bottle of detergent) I was looking like a chimney sweep but the lab was all scrubbed clean and nobody found out about my perchlorate+acrolein jet propulsion experiment.

Milling KOH: I was about to reproduce some old-fashioned procedure that used a slurry of powdered KOH in toluene as a base. Now KOH is very hygroscopic – I tried to powder it with a mortar and pestle at first and the pellets were flying all over as I pounded on it while the stuff was melting into a puddle of lye. I realized the grinding had to be done very fast. Asking around, I found out that one faculty man owned a fancy electric grinder: The machine looked like a giant coffee-grinder on a blender; the container was made of heavy glass and the oversized motor had a beautiful aluminum casing.

“It’s the only power-grinder we have – You are not going to use it on anything corrosive, right?” – the owner asked. I assured him I wouldn’t.

The grinder worked amazingly well and in no time I had lots of free-flowing KOH dust (which I immediately bottled to keep it from getting soggy) – but I noticed as I was taking the grinder apart that I spilled some KOH dust onto the motor casing and the aluminum was getting pitted by the hydroxide. More worryingly still it looked like the caustic dust has gotten into the electric motor itself through the vent holes in the casing. It gotta be cleaned promptly.

I was not familiar with the Mr. Bean skit character back then, but with the same kind of single-mindedness I proceeded to wash up the motor casing in the sink. I was not careful and some water splashed through the vent holes  – and since the KOH dust got in there and the electric motor was  already wet, I decided that the motor deserved a proper rinse as well – I would dry it afterwards. So I had the water flowing in and through the motor.

The owner of the grinder dropped by later that afternoon to find out how it worked (and if I was ready to return it). I said I needed to clean it up a bit more – the motor was still very soggy and I prefered not having to explain how it got that way.

Wet glassware dries pretty fast when rinsed with acetone. Being in hurry I reached for a squeeze-bottle and washed the motor with acetone too – and surprise – the acetone coming out from the motor was dark brown and smelling just like shellac resin that is used to insulate the fine copper winding in  electric motors…

I was horrified, I realized I just ruined it completely and I better try to cover it up. I dried the motor with a heat gun and assembled the grinder. I gave it a good final polish and then I waited patiently until I saw its owner walking away from his lab – and then I sneaked in and put the grinder back in the cabinet as if nothing bad was done to it (I turned the pitted aluminum part away from the sight). Just as I was closing the door the owner returned. “Thank you – it worked great. I put your grinder back and it’s all clean now…”

But the luck was not with me. “OK - let me see if it still works” the guy says – and he takes this thing out and plugs it in the wall. A loud bang and a green-and-white lightning, the sparks flying all across the room and rolling on the floor before gradually dying out like embers. (I saw a street transformer once, blowing up like that but from a safer distance). We were standing there in silence for few long seconds – the only sound was the “klip-klop-klop” from the hallway as the circuit breakers gave up one by one and plunged the chemistry building into darkness.
The faculty guy then turns to me and says: “Thank you. Don’t hesitate if you need my help again.”