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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.
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| 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ion has been replaced by the dumbbell shaped acetylide anion,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. |
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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).
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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).
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