3096.     ---------------.  [SCHMIDT, J.]  [Decomposition of Carbides by Water or Dilute Acids.]  Ztscher. Elektrochem., vol. 40, 1934, pp. 170-174; Chem. Abs., vol. 28, 1934, p. 4675.

        Carbides that are decomposed by H₂O by dilute acids may be divided into 3 groups:  (1) Carbides of Fe, Co, Ni, and Mn having the formula M₃C.  Al₄C₃ probably is to be included in this group.  These carbides have a lattice in which the C atoms are separated, so that the products formed on treatment with H₂O or dilute acid result from the action of the H₂ set free by the decomposition upon each C atom separately.  The reaction proceeds in several steps;  =CH₂ radicals are formed 1^st, and these are either converted to CH₄ by hydrogenation or polymerized to C₂H₄.  From C₂H₄, by hydrogenation, polymerization, and further addition of =CH₂, hydrocarbons are formed with a larger number of C atoms.  C₂H₂ is not formed by the decomposition of these carbides.  (2) Carbides that give C₂H₂ on decomposition with H₂O or dilute acid.  To this group belong the carbides of the elements of groups I and II of the periodic table; also Al₂C:C)₃ and Ce₂(C:C)₃.  The formation of C₂H₂ is due to the presence of C-C linkages in the crystal lattices of these carbides.  UC₂ and the carbides of the rare earths give not only C₂H₂ but other unsaturated as well as saturated hydrocarbons, the composition of the gaseous product varying with the conditions of the decomposition.  These carbides have the lattice of CaC₂ or one very similar.  Their irregular behavior is due to the conversion of the metal, upon decomposition of the carbide, from the bivalent to the trivalent or quadrivalent form.  (3) Carbides that give allylene upon decomposition.  Only Mg₂C₃ is known here.  Since the structure of its crystal lattice has not been investigated, no definite statement can be made in regard to the mechanism of the decomposition, but the production of pure allylene would indicate the presence of C-C-C linkages in the lattice.