Org Prep Daily

January 30, 2012

Potassium hydride self-ignition

Filed under: lab destruction — milkshake @ 1:40 pm

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

January 18, 2012

Replacement process solvents

Filed under: procedures — milkshake @ 1:45 pm

A recent Organic Process R&D editorial (thanks Chemjobber for pointing it out) publicizes Pfizer Process Group green solvent replacement chart that discourages chemists from using solvents that are either known to be toxic, dangerous to use on large scale or are expensive to dispose as waste. OPR&D makes it now a submission policy that if you used a problematic solvent in your work you have to demonstrate in your paper that you tried (and failed) to find more process-friendly alternatives. I think it is a sensible policy for a chemical industry process journal (and it probably makes the editors job of rejecting marginal manuscripts easier).

Also, Innocentive challenge was recently promising an award (up to 8k) to a winning proposal for replacing dipolar aprotic solvents like DMF, DMAc, NMP with less enviro-problematic alternatives.

I have few comments on the recommended solvent replacements in the table:

1) Acetonitrile is a perfectly good replacement of other dipolar aprotic solvents for things that dissolve in it, unfortunately MeCN dissolving power is quite poor. On the other hand, DMSO is famously bio-innocuous and it dissolves almost anything organic, and quite a few inorganic salts as well. But DMSO properties can complicate the workup, and DMSO can participate in quite a few unwanted sidereaction. I think overall DMSO is a pretty good media for alkylations that involve a reactive nucleophile. If the alkylating agent is highly reactive one might end up with S-alkylated DMSO-derived sideproducts although for many reactions this is not really a problem. Boiling DMSO has oxidizing properties and gives off Me2S funk so the reactions run in DMSO should not be heated above 140C. For acylations (where DMSO would interfere badly) an inexpensive eco-friendly solvent to try is 1,2-propylene carbonate, perhaps diluted with MeCN or DCM to cut down on this high-boiling solvent and to lower the viscosity. Propylene carbonate stability is quite remarkable – it tolerates alkali metals – but I would not heat it with alkoxides and reactive amines, the same limitation as with DMF and NMP. Another possibility for acylations is sulfolane-MeCN mixture. Adventurous eco-fanatic types may even go for triethylphosphate, another cheap degradable goo.

2) A suitable alternative for replacing DCM and DCE in many reactions (but not for AlCl3-promoted Friedel-Crafts) is trifluoromethylbenzene, bp. 102C.

3) For pyridine replacement the chart recommends NEt3 but I think N-methylmorpholine would be a closer surrogate/better alternative – NMM it is much less basic than NEt3 thus less prone to cause ketene-related dark impurities and racemizations during acylations, and it is a better solvent also. A strong fishy reek of NMM is a bit put-off though. If one so desires, Grignard reagents can be prepared in NMM.

4) One relatively underused process solvent is di-n-butyl ether. Its odor is annoying, the boiling point is quite high (142 C) and the dissolving power of Bu2O is not great but this solvent is cheap to buy and easy to dry. Room temperature lithiations with BuLi that require an etheral co-solvent might be a good pick for Bu2O (THF gets cleaved with BuLi at room temperature at appreciable rate, MTBE is pretty inefficient for solvating Li)

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