chemrox
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reduction f an imine wih NaBH4
I have done this reduction with regular apparatus; flask, condenser, powder addition funnel and it works fine but I was thinking I could put in all
the hydride in a cold solution and heat it up in an Erlenmeyer and stir it hot for an hour. I have 0.05 moles of imine which would take about 2g of
the hydride so why mess around with all the gear? Another question: The latest batch of NaBH4 from TCI is a very fine powder. This is not ideal for
the conditions I use. The problem being the stuff is so hygroscopic it clings to the inside of my funnels and i have to wash the stuff down with MeOH.
I may as well be dropping it in in dollops. I would prefer to have the hydride in coarser form. I'm thinking I could recryx it but have never done. A
few of you use this hydride to generate H2. Is recryx it feasible?
BTW- incase you think of it, I have done this with a Ar atmosphere run through a drying tube just in case the welding supply is tainted. A little
better but same issue.
[Edited on 18-3-2016 by chemrox]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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zed
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What kind of solution? Seems to me, NaBH4 is stable in NaOH/ H2O solutions. Not so good in Methanol, as you can see little bubbles of H2 forming, at
STP, right before your very eyes....very expensive bubbles! Dammit!
Better in Ethanol, but I wouldn't warrant long term stability in that solvent either.
I've dissolved NaBH4 in anhydrous ethanol, as needed, at room temp, and then injected the solution, via hypodermic needle, through a septum. Nice
clean way to get it into your reaction. Whether hydrogenating, or not.
Well, that Hive archive returning to the net has proved useful. Got some good info. on Borohydride.
http://chemistry.mdma.ch/hiveboard/methods/000448578.html
Recrystallizing? Hmmmm. Dunno. Isopropyl-amine?
[Edited on 22-3-2016 by zed]
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chemrox
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*All of the pubs I have mentioning NaBH4 reduction also use MeOH. Long term stability? Why? I wouldn't make a hydride stock solution for any reason I
can think of. Ether is hygroscopic and most other ethers are easily wetted. Wet ether is tried and true for quenching LAH. The question I've asked a
couple of times is, "what is the reax with MeOH?? I haven't checked but I doubt those bubbles are CH4. I do think remixing the NaBH4 with MeOH is an
option but it's easier to tell what's going on when adding solid. Major (relatively) bubbling while the reduction is taking place. Not so much when
adding solid to a refluxing solution. Still best to rely on stoichiometry. The visual clues being helpful but not determinative. BTW this post
resulted in six additions to the dictionary. Could we collectively edit it? So that "stoichiometry"as an example doesn't get a red underline?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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ziqquratu
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Chemrox, I've done a lot of NaBH4 reductions in my time, on tens of grams of material, and I can't say I've ever used the type of
apparatus you've described for the job!
Sure, if I were reducing hunderds of grams of something, I might like a (preferably automated) solid addition funnel, but for such a small scale I
would just slowly add the solid NaBH4 over 5-15 mins (a small portion every 20-30 seconds, say, ensuring you add it slowly enough to avoid
exothermic runaway - use a thermometer to keep an eye on it if you're not confident) then let it go for as long as required from there. In other word,
adding "dollops" is precisely what I'd be doing on such a scale!
Aldehydes, ketones and imines are typically readily reduced using NaBH4, no heating required. You might want to use an ice bath whilst
adding the NaBH4 (as security against adding it a little too fast), but then allow the solution to react at room temperature once you've
added it all.
Whilst hygroscopic, NaBH4 is reasonably water tolerant, and most procedures call for a reasonable excess of the reagent as well, so I'd
argue that there's not really a need for an inert atmosphere as far as the reagent is concerned.
---
As far as purification goes, allow me to copy-paste (with possible formatting errors - apologies if I missed any) from Amarego and Chai, "Purification
of Laboratory Chemicals, 5th Edition", p466
| Quote: | Sodium borohydride [16940-66-2] M 37.8, m ~400°(dec), d 1.07. After adding NaBH4 (10g) to freshly distilled diglyme (120mL) in a dry three-necked
flask fitted with a stirrer, nitrogen inlet and outlet, the mixture was stirred for 30min at 50° until almost all of the solid had dissolved.
Stirring was stopped, and, after the solid had settled, the supernatant liquid was forced under N2 pressure through a sintered-glass filter
into a dry flask. [The residue was centrifuged to obtain more of the solution which was added to the bulk.] The solution was cooled slowly to 0° and
then decanted from the white needles that separated. The crystals were dried by pumping for 4h to give anhydrous NaBH4. Alternatively,
after the filtration at 50° the solution was heated at 80° for 2h to give a white ppte of substantially anhydrous NaBH4 which was
collected on a sintered-glass filter under N2, then pumped at 60° for 2h [Brown, Mead and Subba Rao J Am Chem Soc 77 6209 19551.
NaBH4 has also been crystd from isopropylamine by dissolving it in the solvent at reflux, cooling, filtering and allowing the solution to
stand in a filter flask connected to a dry-ice/acetone trap. After most of the solvent was passed over into the cold trap, crystals were removed with
forceps, washed with dry diethyl ether and dried under vacuum. [Kim and Itoh J Phys Chem 91 126 1987]. Somewhat less pure crystals were obtained more
rapidly by using Soxhlet extraction with only a small amount of solvent and extracting for about 8h. The crystals that formed in the flask were
filtered off, then washed and dried as before. [Stockmayer, Rice and Stephenson J Am Chem Soc 77 1980 1955.1 Other solvents used for crystallisation
include water and liquid ammonia. |
I'll note that I've never needed to purify my NaBH4 for synthesis - and if those are the options, I think that buying a new bottle (or,
better, adapting your technique to deal with the apparently perfectly good material you already have) would be both more practical and, in fact,
cheaper than the solvents required for said purification!
---
Finally, the reaction is NaBH4 + 4 MeOH --> 4 H2 + NaB(OMe)4 (with, of course, other borate intermediates along
the way!). According to Wikipedia (ref. 8 therein), complete decomposition takes some 90 min at room temperature in MeOH.
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