Magnesium Carbide

Moissan thought that magnesium carbide could not exist at electric-furnace temperature, but Lebeau obtained acetylene by the action of water on his product. Similar products have been obtained by heating magnesium in benzol vapour or in a current of acetylene. Lebeau thought that the vapours of carbon and magnesium reacted, but carbon reduces magnesia at 1700° C., if reoxidation is prevented, and, since hydrogen from water is usually present in furnaces, magnesium carbide probably results from an interaction between acetylene (produced by a union between carbon and hydrogen) and magnesium.

When acetylene is passed over heated magnesium, MgC₂, Mg₂C₃, and C are produced in varying proportions according to the temperature. The decomposition

2MgC₂ = Mg₂C₃+C

is rapid between 570° C. and 610° C. Above 610° C. the reaction

Mg₂C₃ = 2Mg+C

predominates. Mg₂C₃ predominates over MgC₂ when paraffin or aromatic hydrocarbons are passed over the heated metal.

Both carbides are decomposed by water -

MgC₂+2H₂O = Mg(OH)₂+C₂H₂;
Mg₂C₃+4H₂O = 2Mg(OH)₂+C₃H₄ (allylene).

The products of the acetylene reaction between 465° C. and 515° C. are hard, compact, steel-grey masses which decompose water slowly; those obtained above 550° C. are grey, brittle, and rapidly decomposed by water.