carbides in which the carbon atoms
occupy interstitial positions in the
metal lattice. These substances are
generally hard materials with metal-
lic conductivity. Some transition met-
als (e.g. Cr, Mn, Fe, Co, and Ni) have
atomic radii that are too small to
allow individual carbon atoms in the
interstitial holes. These form car-
bides in which the metal lattice is
distorted and chains of carbon atoms
exist (e.g. Cr 3 C 2 , Fe 3 C). Such com-
pounds are intermediate in character
between interstitial carbides and
ionic carbides. They give mixtures of
hydrocarbons on hydrolysis with
water or acids.
carbocationAn ion with a positive
charge that is mostly localized on a
carbon atom. There are two types:
Carbonium ions haveÜve bonds to
the carbon atom and a complete
outer shell of eight electrons. A sim-
ple example is the ion CH 5 +, which
has a trigonal bipyramidal shape.
Ions of this type are transient
species. They can be produced by
electron impact and detected by
mass spectroscopy.
Carbenium ions have three bonds
to the carbon atom and are planar,
with six outer electrons and a vacant
p-orbital. Ions of this type are inter-
mediates in a number of organic re-
actions (for example, in the SN 1
mechanism of nucleophilic substitu-
tion). Certain carbenium ions are sta-
bilized by delocalization of the
charge. An example is the orange-red
salt (C 6 H 5 ) 3 C+Cl-. Carbenium ions can
be produced by superacids.
carbocyclic See cyclic.
carbohydrateOne of a group of
organic compounds based on the
general formula Cx(H 2 O)y. The sim-
plest carbohydrates are the sugars
(saccharides), including glucose and
sucrose. Polysaccharides are carbo-
hydrates of much greater molecular
weight and complexity; examples are
starch, glycogen, and cellulose. Car-
bohydrates perform many vital roles
in living organisms. Sugars, notably
glucose, and their derivatives are es-
sential intermediates in the conver-
sion of food to energy. Starch and
other polysaccharides serve as energy
stores in plants. Cellulose, lignin, and
others form the supporting cell walls
and woody tissue of plants. Chitin is
a structural polysaccharide found in
the body shells of many invertebrate
animals.
A- Information about IUPAC nomenclature
carbolic acidSee phenol.
carbonSymbol C. A nonmetallic el-
ement belonging to *group 14 (for-
merly IVB) of the periodic table; a.n.
6; r.a.m. 12.011; m.p. ∼ 3550 °C; b.p.
∼ 4827 °C. Carbon has three main al-
lotropic forms (see allotropy).
*Diamond (r.d. 3.52) occurs natu-
rally and can be produced syntheti-
cally. It is extremely hard and has
highly refractive crystals. The hard-
ness of diamond results from the co-
valent crystal structure, in which
each carbon atom is linked by cova-
lent bonds to four others situated at
the corners of a tetrahedron. The
C–C bond length is 0.154 nm and the
bond angle is 109.5°.
Graphite (r.d. 2.25) is a soft black
slippery substance (sometimes called
black lead or plumbago). It occurs
naturally and can also be made by
the *Acheson process. In graphite
the carbon atoms are arranged in lay-
ers, in which each carbon atom is
surrounded by three others to which
it is bound by single or double bonds.
The layers are held together by much
weaker van der Waals’ forces. The
carbon–carbon bond length in the
layers is 0.142 nm and the layers are
0.34 nm apart. Graphite is a good99 carbon
c