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. |