Question

LiAlH₄ is obtained by reacting an excess of ........ with an ethereal solution of AlCl₃.
A. LiCl
B. LiH
C. Li
D. LiOH

Answer 1

Hint: In this question we will use the reaction of formation of LiAlH₄ (Lithium aluminium hydride) also known as LAH. It is an excellent reagent for the reduction and hydrolysis of certain polar groups.

Complete Step by Step Answer:
LiAlH₄ (Lithium aluminium hydride) is obtained by reacting an excess of LiH (lithium hydride) with an ethereal solution of AlCl₃ (aluminium chloride).
The reaction is:
4LiH + AlCl₃ → LiAlH₄ + 3LiCl

The correct answer is A.

Additional Information:
In addition to the given method, the industrial synthesis necessitates the initial heating and high-pressure preparation of sodium aluminium hydride:
Na + Al + H₂ → NaAlH₄

After that, LiAlH4 is created via the following salt metathesis reaction:
NaAlH₄ + LiCl → LiAlH₄ + NaCl

LiAlH₄ must be used as the reducing agent in anhydrous non-protic solvents such diethyl ether, THF, and others for the reduction reaction to occur.

In diethyl ether, it is very soluble. However, it might spontaneously break down if there are catalytic impurities present. THF is the chosen solvent for LAH despite having a low level of solubility.
The reactions are typically conducted with too much LiAlH4. A tiny amount of the reagent is added to the solvent to eliminate any moisture that may be present.

Note: The reaction of lithium hydride and aluminium chloride produces lithium aluminium hydride. The reaction progresses with a high yield of LAH (97% w/w). Filtration is used to remove LiCl from an ethereal LAH solution, and LAH is then precipitated to produce a product containing around 1% w/w LiCl.