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Previous preparations of aluminum hydride have resulted either in a very low yield of impure product1 or in a solid solvated polymer from which it has been impossible to remove all the solvent without decomposition2. We have now succeeded in preparing non-solvated aluminum hydride as follows:
A solution of aluminum hydride in diethyl ether is prepared in the usual fashion2 and filtered promptly (before polymerization can occur) through sintered glass under nitrogen into an inert liquid, which is not a solvent. Pentane and ligroin have been found to be suitable. It is essential that the hydride solution be rapidly mixed with a relatively large volume of this inert liquid (at least 100 mL for each gram of aluminum hydride); a satisfactory method is to run the solution in a thin film down a wire while the precipitant is vigorously stirred by a magnetic stirrer. Precipitation is instantaneous and a very fluffy product results.
The bulk of the ether-precipitant mixture is removed by vacuum, and the apparently dry residne is subjected to high vacuum at room temperature. At least twelve hours are ordinarily required to remove all volatile material from the product. During evacuation, it is beneficial to grind and stir the product by means of the magnetic stirrer. Aluminum hydride samples prepared in the above manner were analyzed for aluminum by precipitation as the 8-hydroxyquinolinate and for hydrogen by measuring the gas evolution produced by a water-dioxane mixture.