Carbides

Carbon will form binary compounds, called carbides, with most elements. These generally fall into three classes; ionic (or salt-like), covalent and interstitial.

Ionic carbides are those formed with highly electropositive metals i.e. the alkali and alkaline earth metals and also the lanthanides, and metals like aluminium.

The ionic carbides are easily hydrolysed by water and can be classified according to the aliphatic hydrocarbon which they yield. The ionic carbides are formed mainly by elements from Groups 1, 2 and 3 of the Periodic Table.

1(a) The methanides:

For example Be₂C; Al₄C₃

These are presumably ionised, containing metal cations and anionic carbon in the form C^4-

These are exceedingly reactive with water, giving methane. For example:

Al₄C₃ + 12 H₂O → 4 Al(OH)₃ + 3 CH₄

Both these carbides are made by direct combination of the elements at about 1800K, and are much harder materials than the acetylides (ethenides) (see below)

1(b) Acetylides (Ethynides):

This is the large group of acetylides, which have the carbon atoms in pairs, forming the anion [C≡C]^2-.

These are formed by the elements in Groups 1 to 6 of the Periodic Table; especially Groups 1, 2 and 3, e.g:

Na₂C₂, Cu₂C₂, BeC₂, CaC₂, Al₂C₆

These compounds are also exceedingly reactive with water, giving ethyne (acetylene), for example

CaC₂ + 2 H₂O → Ca(OH)₂ + HC≡CH

This reaction was utilized in the acetylene (carbide) lamps, once used in mines, vintage cars and for bicycle lamps.

Calcium carbide, therefore, is an important raw material for the manufacture of many organic compounds, eg ethanal, and ethanoic acid, where ethyne is an intermediate.

Calcium carbide itself is made by heating coke and calcium oxide (lime) in an electric furnace. These reactions provide us with one of the few methods available for making organic compounds directly from carbon:

1(c) Propynides:

The only common one is Mg₂C₃, which has the anion [C=C=C]^2-

This yields propyne on hydrolysis:

Mg₂C₃ + 4H₂O > 2Mg(OH)₂ + CH₃-C-C≡CH