Lithium aluminium hydride

Lithium aluminium hydride, or LAH, is a powerful inorganic compound. It is used primarily as a reducing agent in organic synthesis. A reducing agent is a substance that changes other molecules by adding electrons or hydrogen.

This makes LAH a vital tool for chemists. It can transform many different types of organic structures. Because it is so effective, it is a staple in many chemistry laboratories.

Chemists use LAH to change how atoms are connected in various molecules. It is more powerful than sodium borohydride. This extra strength comes from its chemical structure. The aluminium-hydrogen (Al-H) bond is weaker than the boron-hydrogen (B-H) bond.

This weaker bond allows LAH to react more vigorously. For example, it can convert esters and carboxylic acids into primary alcohols. It also turns amides, nitriles, and oximes into amines. It can even reduce quaternary ammonium cations into tertiary amines.

In its pure form, LAH is a colorless solid. However, most commercial samples appear gray due to contamination. These impurities are often harmless and easily separated from the final products.

To get a pure sample, scientists use a process called recrystallization from diethyl ether. Large-scale purification often uses a Soxhlet extractor. If a sample is old and has been exposed to air, it might look white. This happens because it has absorbed moisture to form lithium hydroxide and aluminium hydroxide.

Researchers discovered LAH in 1947. The discovery was made by Finholt, Bond, and Schlesinger. Since then, several ways to create the compound have been developed. One common method reacts lithium hydride (LiH) with aluminium chloride (AlCl3).

An industrial method involves first making sodium aluminium hydride (NaAlH4) using high pressure and temperature. Another way uses metallic aluminium and lithium hydride. This alternative method uses a small amount of TiCl3 as a catalyst to help the reaction. This specific method is useful because it avoids creating salt as a byproduct.

LAH is a highly reactive and energetic substance. It reacts violently with water to release hydrogen gas (H2).

This reactivity is why the powder is called pyrophoric. Pyrophoric means it can catch fire spontaneously in air. To stay safe, it is often packed in moisture-proof plastic sacks. Some versions contain mineral oil to stop reactions with moisture in the air. Because of these dangers, scientists must handle it with extreme care in fume hoods or dry boxes.

Beyond organic chemistry, LAH has potential uses in energy science. It contains 10.6 wt% hydrogen by weight. This high hydrogen content makes it a candidate for hydrogen storage. Scientists are studying if it could store fuel for future hydrogen-powered vehicles.

However, there are challenges. To get all the hydrogen out, you must heat it above 400 °C. Also, recycling the material back into LAH requires extremely high pressure, over 10,000 bar. These factors make its use in transportation difficult for now.

Finally, it is important to understand how LAH breaks down. It is metastable at room temperature. This means it is stable for a while, but will eventually decompose. Over long periods, it slowly turns into lithium hexahydridoaluminate and LiH. This breakdown can happen faster if titanium, iron, or vanadium are present. When heated, it follows a three-step decomposition process. It begins by melting between 150 and 170 °C. Eventually, at very high temperatures, it converts into lithium aluminium (LiAl).