Aluminium and its Compounds

Aluminium is the third member of Group 13 and, unlike boron, is a genuine metal — the most abundant metal in the Earth's crust. Its chemistry contrasts sharply with boron's exclusively covalent, acidic behaviour: aluminium is amphoteric, reacting with both acids and bases. With dilute hydrochloric acid, aluminium dissolves with the liberation of hydrogen gas, forming aluminium chloride:

With aqueous sodium hydroxide, aluminium likewise dissolves and liberates hydrogen, but here the aluminium-containing product is sodium aluminate (often written as the tetrahydroxoaluminate complex):

In ordinary use, aluminium metal appears far less reactive than these equations suggest, because a thin, extremely adherent, self-healing layer of aluminium oxide forms instantly on any freshly exposed surface and passivates it against further attack; only once this protective oxide film is breached, or is prevented from forming (as by concentrated NaOH, which dissolves it), does the underlying metal react readily. Because of its light weight, good electrical conductivity, corrosion resistance and ease of fabrication, aluminium is used extensively in aircraft and vehicle bodies, structural alloys, electrical transmission cables, cooking utensils and packaging foil.

Two aluminium compounds have particular importance. Lithium aluminium hydride, $\text{LiAlH}_4$, is prepared under strictly anhydrous conditions (typically in dry diethyl ether) by the reaction of lithium hydride with aluminium chloride:

$\text{LiAlH}_4$ is one of the most powerful and widely used reducing agents in synthetic organic chemistry, readily reducing carbonyl compounds (aldehydes, ketones, esters, carboxylic acids) and many other functional groups to alcohols or amines by delivering hydride ($\text{H}^-$) to the electrophilic carbon. It must be handled under anhydrous conditions because it reacts violently — even explosively on a large scale — with water and other protic solvents, liberating hydrogen gas and generating considerable heat.

Aluminium oxide, $\text{Al}_2\text{O}_3$ (also known in its crystalline mineral form as corundum), is likewise amphoteric, dissolving in both acid and base:

$\text{Al}_2\text{O}_3 + 6\text{HCl} \longrightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$ …