smaerd - 27-1-2015 at 07:34
Now I have worked with sodium hydride many times in the past but always with fresh solvents from a still. My question is, are mol sieves proficient
enough to produce solvent (THF) dry enough to 'safely' use sodium hydride dispersions?
Meaning, if I say dry THF over mol sieves for a week with stirring, in inert atmosphere. Could I use sodium hydride in it (small quantities 1x10-2
mol) without it going it going exo and puffing a mol equivalent of hydrogen gas? Unfortunately the lab I am working in now doesn't have solvent
stills.
The reason I am asking is this. The literature on drying solvents seems to support that yes mol sieves can dry THF to the desired amount. However,
there are contradictory procedures still requiring the sodium metal/benzophenone distillation prior to the dessication process. Any advice would be
appreciated. I believe my RA expects me to do this irregardless, but having a little corroborating information would ease me about the process.
Nicodem - 27-1-2015 at 08:05
Molecular sieves dried THF is just fine for sodium hydride. I used to use only THF from sodium ketyl in my previous job. Currently, I only use MS
dried solvents, even for organolithiums. Where I work now, reactions properly working strictly only with sodium ketyl dried THF are considered
useless.
Dr.Bob - 27-1-2015 at 11:35
I will second that. We often use either a fresh bottle of anhydrous solvent (aldrich or Acros brand mostly) or MS dried solvents for NaH and similar
experiments. The quality of the NaH is the bigger problem that I see, as once a can of it has been opened, it goes bad fairly quickly, if not keep
sealed tightly and handled carefully. But even then, often using a larger excess of base will work up to a point. Once you get too much water in
the solvent or NaH, then it becomes much less effective in some reactions. But for most of what I have done, both will keep 6-12 months OK if well
sealed between uses. And THF is easier to dry than DMF, which is even harder to keep dry.
The only reactions that I remember being consistantly tough to do without a still are many variations of Sharpless epoxidations, especially the
kinetic chiral ones, complex claisen reactions, and oxygen sensitive reactions, which are hard to do anywhere/anyhow.
[Edited on 27-1-2015 by Dr.Bob]
smaerd - 27-1-2015 at 14:24
Thanks for the advice.
I got the THF stirring with some sieves and I'm gonna let it go for maybe a week or so just to be sure. The reaction is nothing fancy. All I'm doing
is generating an alkoxide and hopefully SN2'ing that with a tosylate to make the ether. Cross my fingers to ensure elimination doesn't occur because I
don't want to re-synthesize these targets. Really only have one crack at it too and unfortunately there's no proper means of analysis to monitor the
progress hah. So it'll be interesting. The bottle is old but it's all that's available there. Nothing like unscrewing old bottles of THF after reading
those horror stories about epoxides lurking on the cap threads.
We purchased about a 1/4 mole of NaH for this series of experiments. So if that goes bad it's not as big of deal. Once I open it I'll be keeping it in
a dessicator for re-use. At my temporary job, we were pretty careless with NaH dispersions but we stored it in a dessicator in a can and inside a bag.
Last time I used it I think it was on a half mole scale or something ridiculous.
AvBaeyer - 27-1-2015 at 20:09
Both Nicodem and Dr Bob are right on regarding using mol sieves as effective drying agents. One thing they will not do is remove any peroxides from
your THF, one of the "benefits" of sodium/benzophenone. However, if your THF has been handled with care and stored under an inert atmosphere (N2 or
Ar), there is not much to worry about.
One final thing, there is no need to stir your THF with the sieves. In fact, you will create quite a mess as the sieves will break down into a fine
powder with excessive agitation - experience speaks. Just let your solvent stand over the sieves for a couple of days.
AvB
smaerd - 27-1-2015 at 20:59
Thanks for the heads up I'll turn the stirrer off tomorrow. The bottle was supposedly stabilized but I really don't trust it because it was old. I'll
just be as careful as I can be in regards to peroxides as always and handle it at the work-up.
Dr.Bob - 28-1-2015 at 11:39
You can always test for peroxides with starch iodide paper or other tests. As long as it is not many years old, and you don't do a large scale and
then concentrate it, there is usually not a big issue for THF and diethyl ether. Some other ethers are much more prone to it, like dibutyl.
And when you run the reaction, the reaction conditions may well destroy peroxides, you can always wash the organics with sulfite or thiosulfite before
concentrating, which will also reduce them some. The main issue is if you concentrate a large amount of solvent with peroxides and then end up with
mostly peroxides in your flask. That is bad, but even a little water or other solvent will keep them dilute.
smaerd - 28-1-2015 at 12:09
I did a bit of research on peroxides over the summer to assess whether a colleague was trying to kill me or lacked safety knowledge. Considering he
was working with palladium catalsyts I'm aiken to believe it was the former and not the latter. He tasked me with drying some sieves with a bunsen
burner on the schlenk and didn't want me to use a kugelrohr. Maybe 2 feet away from where I was working he clipped an air-line to a liter beaker about
80% full of diethyl ether and was letting it evaporate in the hood I was working in. Rather then taking it to the rotovap? As soon as I grabbed the
bunsen burner he went to "lay down in the office". Hah... Needless to say I unclipped the air line, covered the beaker and waited a few minutes before
lighting the bunsen burner...
I'm not 100% sure how I am going to do the work-up yet which isn't a good sign. I think what I'll end up doing is quenching any remaining alkoxide
with a weak acid. My material will be quite large in MW(1500-1800g/mol) and should be practically insoluble in THF and/or water, so I may be lucky
enough to do a filter -> rinse -> dry -> collect work up. If not I'll get the starch iodide paper out. Really appreciate the information!