Mercury-aluminum amalgam
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A mercury-aluminum amalgam is a chemical reagent that uses aluminum coated with mercury to reduce compounds, such as the reduction of imines to amines. Since this reaction produces waste mercury metal, it is best avoided in favor of more environmentally friendly reagents such as hydrides. The reaction tailings must be properly disposed of by a hazardous waste management company.
This reaction was popularized by Alexander Shulgin in his book PiHKAL, but has few virtues beyond being inexpensive and using chemicals that are readily available.
Normally pieces of aluminum aren't very reactive because they are covered with a thin layer of inert aluminum oxide (Al2O3). The mercury in the solution allows this protective layer to be removed, then prevents its (otherwise very rapid) re-formation by creating a thin layer of mercury over the bare aluminum. 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 aluminum (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 reduce water to hydroxide, creating aluminum hydroxide (Al(OH)3) and hydrogen gas (H2).
[edit] Example reactions using Al-Hg amalgams
From PiHKAL:
"To 40 g of thin aluminum foil cut in 1 inch squares (in a 2 L wide mouth Erlenmeyer flask) there was added 1400 mL H2O containing 1 g mercuric chloride. Amalgamation was allowed to proceed until there was the evolution of fine bubbles, the formation of a light grey precipitate, and the appearance of occasional silvery spots on the surface of the aluminum. This takes between 15 and 30 min depending on the freshness of the surfaces, the temperature of the H2O, and the thickness of the aluminum foil. (Aluminum foil thickness varies from country to country.) The H2O was removed by decantation, and the aluminum was washed with 2x1400 mL of fresh H2O. The residual H2O from the final washing was removed as thoroughly as possible by shaking, and there was added, in succession and with swirling, 60 g methylamine hydrochloride dissolved in 60 mL warm H2O, 180 mL IPA, 145 mL 25% NaOH, 53 g 3,4-methylenedioxyphenylacetone, and finally 350 mL IPA. If the available form of methylamine is the aqueous solution of the free base, the following sequence can be substituted: add, in succession, 76 mL 40% aqueous methylamine, 180 mL IPA, a suspension of 50 g NaCl in 140 mL H2O that contains 25 mL 25% NaOH, 53 g 3,4-methylenedioxyphenylacetone, and finally 350 mL IPA. The exothermic reaction was kept below 60 ° C with occasional immersion into cold water and, when it was thermally stable, it was allowed to stand until it had returned to room temperature with all the insolubles settled to the bottom as a grey sludge. The clear yellow overhead was decanted and the sludge removed by filtration and washed with MeOH."
The amount of mercury salt (HgCl2) used in this particular example is quite high; much less can be used to produce the same results. Considerably more refined versions of this reaction were developed by contributors to the Rhodium.ws archive, which can be viewed at Erowid.
