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 1st, 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.