Moin!
Kann man im Labor Natriumborhydrid herstellen?
Wenn ja, wie?
Ja, wobei es auch für Hobbychemiker ökonomischer ist, es zu bestellen. Bei Kremer Pigmente ging das, soweit ich informiert bin, noch vor einiger Zeit völlig problemlos – jetzt steht etwas zu erforderlichen Nachweisen auf der Website.
Eine Vorschrift findest Du im englischen Brauer (Handbook of Preparative Inorganic Chemistry), dessen Copyright bereits abgelaufen ist (
www.sciencemadness.org/library/books/brauer_ocr.pdf).
Sodium Borohydride NaBH4
4NaH + B(OCH3)3 → NaBH4 + 3 NaOCH3
The reaction is carried out in a round-bottom, three-neck cylindrical flask. A mercury-seal Monel stirrer is placed in the central neck. The stirrer is equipped with five blades, arranged one over the other. The blade dimensions should be such that the stirrer can fit through the neck, but still fit the wall of the flask as closely as possible. A thermometer is placed in the second neck and a condenser on the third. The top of the condenser is equipped with a wye-tube adapter, one side of which connects to a dropping funnel and the other to a soda-lime drying tube. The flask is placed in an electric furnace, the top of which is covered with glass wool and an asbestos lid. The thermometer is removed, and the flask flushed with N3 through this neck. Then 50 g. of NaH is rapidly added and 50 g. of B(OCH3)3 is placed in the dropping funnel. The stirrer and the furnace are then turned on. As soon as the thermometer indicates a temperature of 200°C in the flask, the ether is added dropwise at a uniform rate. The addition should require 20-40 minutes, during which the temperature is kept at 230-270°C. The stirring is continued at this temperature for another hour. The flask is then allowed to cool and thoroughly dried isopropylamine or liquid NH3 is used to extract the NaBH4 from the now solid mixture, which, however, should have become well pulverized as a result of the constant stirring. The extraction with isopropylamine is carried out by refluxing for a few minutes; the extraction with NH3 is done by stirring the reaction product for a few minutes. In either case the extract is filtered through a fritted glass filter and the solvent is evaporated. The NaBH4 remains as a fine white powder, with a purity of 90-96%. The yield is 86-94%. The NaBH4 is purified by recrystallization from either isopropylamine or water (it forms a dihydrate). For unknown reasons the nature of the NaH used exerts a great influence on the yield and purity of the final product. Therefore, it is best to check the suitability of the starting material by making a small-scale preparation first. If the product is unsatisfactory, the NaH is preheated to 250°C and a small quantity of impure NaBH4 from a preceding run is added to start the reaction. The yield can also be improved by a more uniform rate of addition of the B(OCH3)3.
PROPERTIES:
Fine, white crystals (cubic system). Decomposes slightly in neutral aqueous solutions, from which it can be partially recrystallized as a dihydrate. Rapidly hydrolyzes in acid solution. Stable up to 400°C.
REFERENCES:
H. I. Schlesinger.H. C. Brown and A. E. Finholt. J. Amer. Chem. Soc. 77, 205 (1953).