Sodium Borohydride Purification
Borohydride purification?
My supply of Sodium Borohydride has been exposed to the atmosphere for a time and swelled nicely. What used to be a granular solid is now a solid
mess.
Looking for alternatives to diglyme / isopropylamine (thanks to ziqquratu):
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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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Again thanks to ziqquratu for the above.
Dumb question: Is sodium metaborate (and any other possible decomposition products of borohydride and air) soluble in isopropyl alcohol (as NaBH4 is
not)?
My understanding is that the metaborate and borax is insoluable in ethanol. Could my mess be added to ethanol, filtered, and quickly vac distilled
(low temp to avoid decomposition).
My apologies if all this is too newbie ish…I U(ed)TFSI
If anyone has any sodium borohydride acquisition leads shoot me a PM, I would appreciate it.
Appreciate your time!
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