The carbides (in which C reacts with another element to form a binary compound) can be broadly categorized depending on the nature of the bonding which exists within these compounds. Saline Carbides are those where the bonding is largely ionic; and Metallic Carbides are those which exhibit metallic conductivity.
Saline Carbides
These can be further subdivided into graphite intercalation compounds, dicarbides, and methides.
Graphite intercalation compounds As seen in the reactions of graphite, alkali metals form intercalated compounds with graphite, in which the bonding is formally ionic, with the presence of alkali metal ions. Examples include KC8 and KC16. Dicarbides Structure
These contain the C22- anion, often referred to as the acetylide anion, which is isoelectronic with CN– or N2, and it contains a CC triple bond. CaC2 has the rocksalt (NaCl) structure, but as the Cl– ion has been replaced by the dumbbell shaped acetylide anion, the crystal is tetragonally elongated the axis parallel to the dumbbells.
Preparation
These are produced by direct reaction of the elements (at high temperatures), reaction of carbon and a metal oxide (at high temperatures), and reaction of acetylene with a metal ammonia solution.
Direct reaction requires the breaking of the strong CC covalent bonds, and hence the high temperatures.
In the reaction of acetylene with the metal-ammonia solution, the mild conditions ensure that the CC triple bond of the starting material remains intact.
Reactions
the carbides are readily oxidized and protonated. Hydrolysis of CaC2 leaves the CC triple bond of the acetylide group intact, producing ethyne, C2H2. This is the transfer of a proton from a Bronsted acid (H20) to a weaker acid (acetylene).
Methines These formally contain the C4- ion, and are very rare. An example is Na4C, but this is readily hydrolyzed to give methane (CH4) and sodium hydroxide (NaOH).
Metallic Carbides
These are often called interstitial carbides, and are formed by the d-block metals. They are formed by insertion of C into the octahedral holes of the metal face centered cubic– or hexagonal close packed–lattices (which results in the rocksalt structure for carbides of stoichiometry MC, eg. TiC, or half occupation of the Oh holes for M2C, eg. V2C). These contain strong metal-carbon bonding, as demonstrated by the hardness of these compounds (eg. tungsten carbide, WC, which is one of the hardest substances known).