Stoichiometry of Reaction of Sodium Borohydride with Lithium Halide in Diethyl Ether Using Mechanical Stirring with Glass Beads.
In a typical experiment, a three-necked, 2000 mL, round-bottom flask with sidearm containing glass beads (6 mm diameter, 180g, ~1/4 volume of the solvent), a mechanically operated Teflon paddle (9 cm length), and an inlet for nitrogen was equipped with a reflux condenser leading to a bubbler outlet for the nitrogen gas. In the predried flask was placed 19.65g (500 mmol) of NaBH4 and 45.6 g (525 mmol) of LiBr under a nitrogen atmosphere. The flask was again flushed with nitrogen, and 500 mL of diethyl ether was introduced with a double-ended needle. The reaction was carried out at 25°C with overhead stirring for 18 h and at 35°C for 0.5h. Insoluble sodium bromide is formed as a white solid and precipitated out completely. The clear solution of LiBH4 in diethyl ether was transferred by a double-ended needle into a 500 mL volumetric standard flask. Approximately 350 mL were collected. An additional 50 mL of ether was added to the flask by a double-ended needle, and the flask was then maintained at reflux conditions with stirring for 15 min. On cooling, a clear solution was observed above the NaBr precipitate. This solution was also transferred to the volumetric flask. The ether extraction of LiBH4 from the precipitate was repeated twice, and the solutions were collected in the standard flask. The solution was made up to the mark, and the concentration of borohydride was determined to be 0.849 M by hydrolysis of a clear aliquot using a glycerine/H20/THF mixture for the hydrolysis. The remaining slurry was filtered and the volume of the filtrate made up to 50 mL. The concentration of borohydride in this filtrate was found to be 0.53 M. Hence, the total recovery of LiBH4 corresponds to 451 mmol (0.849x500 + 50x0.53) out of 500 mmol, a yield of 90%. Unsolvated LiBH4 can be obtained as described above.