Aluminium amalgam

Aluminium can form an amalgam in solution with mercury. Aluminium amalgam may be prepared by either grinding aluminium pellets or wire in mercury, or by allowing aluminium wire to react with a solution of mercury(II) chloride in water.

This amalgam is used as a chemical reagent to reduce compounds, such as the reduction of imines to amines. The aluminium is the ultimate electron donor, and the mercury serves to mediate the electron transfer.

The reaction itself and the waste from it contain mercury, so special safety precautions and disposal methods are needed. As an environmentally friendlier alternative, hydrides or other reducing agents can often be used to accomplish the same synthetic result. Another environmentally friendly alternative is an alloy of aluminium and gallium which similarly renders the aluminium more reactive by preventing it from forming an oxide layer.

Aluminium in air is ordinarily protected by a molecule-thin layer of its own oxide. This aluminium oxide layer serves as a protective barrier to the underlying aluminium itself and preventing chemical reactions with the metal. Mercury coming into contact with it does no harm. However, if any elemental aluminium is exposed (even by a recent scratch), the mercury may combine with it to form the amalgam. This process can continue well beyond the immediately exposed metal surface, potentially reacting with a large part of the bulk aluminium before it finally ends.

The net result is similar to the mercury electrodes often used in electrochemistry, except instead of providing electrons from an electrical supply they are provided by the aluminium which becomes oxidized in the process. The reaction that occurs at the surface of the amalgam may actually be a hydrogenation rather than a reduction.

The presence of water in the solution is reportedly helpful—even necessary; the electron rich amalgam will oxidize aluminium and reduce H⁺ from water, creating aluminium hydroxide (Al(OH)₃) and hydrogen gas (H₂). The electrons from the aluminium reduce mercuric Hg²⁺ ion to metallic mercury. The metallic mercury can then form an amalgam with the exposed aluminium metal. The amalgamated aluminium then is oxidized by water, converting the aluminium to aluminium hydroxide and releasing free metallic mercury. The generated mercury then cycles through these last two steps until the aluminium metal supply is exhausted.

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