Lithium Aluminum Hydride LiAIH4 LAH

[147]~Everything here must be performed in the hood and everything must be as absolutely water-free as possible. The apparatus to use is the one in fig. 15. In the reaction flask is placed 30mL Et20 (ether) and 23.5g lithium hydride which is stirred for a few minutes. In the separatory funnel is placed a mixture of 71.2g anhydrous aluminum chloride (AICI3) and 300mL ether which is dripped in at such a rate that the reaction produces enough generated heat to cause a sustained reflux but not so much that the reaction gets out of control. When the reaction has visibly ceased, the chemist filters the white particulates from the solution by vacuum filtration (the LiAIH4 is in the filtrate solution). The ethereal filtrate is distilled with no vacuum until the residue that remains is syrupy then the rest of the ether is removed under vacuum to give a residue in the flask that is LiAIH4 (86%).

There are a few points to remember about making this catalyst. When scientists were first synthesizing LiAIH4 they found that it

was necessary to have a tiny piece of LiAIH4 already in the reaction vessel to facilitate the start of the reaction between the LiH and AICI3. If the LiAIH4 was not present then the AICI3 would keep being added and added until the solution would suddenly burst into an uncontrolled reaction. It was determined later that what caused this 'induction' period of no activity was the time it was taking for the protective coating of lithium hydroxide to dissolve away from the lithium hydride. Apparently all commercial lithium hydride has such a coating. Some people found that a drop of iodine would negate such a phenomenon but it was finally shown that if absolutely, 100% dry ether was used then everything went smoothly. This means that the chemist needs to dry her ether through Na2S04 and distill it before use. The last thing to say is that it is probably a good idea to order lithium hydride and aluminum chloride separately.

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