Chemistry Reference
In-Depth Information
human contribution by the increasing burning of non-renewable fossil fuels (all based
on carbon, namely coal, natural gas, and petroleum) has outpaced volcanic emissions
hundredfold. The estimated amount of carbon in accessible fossil fuels is about 5 Tt.
1.3
Carbon Chemical Bonding
Atomic orbitals 2s and 2p host the four valence electrons of carbon in the ground
state configuration 1s
2
2s
2
2p
2
, but the covalent bonds have different energies and
geometries (length and bond angle) according to the hybridization. Having an elec-
tronegativity situated midway between the extremes (fluorine and cesium), similarly
to hydrogen, carbon is able to form stable long chains or rings with itself. Divalent
sulfur also has this property, but this leads only to sulfur element allotropes. Alternat-
ing SiO bonds can also lead to long chains, and the resulting mineral silicates form
a large part of the Earth's crust. However, only carbon's bonds were selected by the
evolution of life on our planet. The huge diversity of carbon compounds makes this
unique element the central constituent of the 7
10
7
substances indexed till now in
the
Chemical Abstracts Registry
database, and this fact justifies the study of Organic
Chemistry in a distinct class of chemical compounds. Indeed, more than 95 % of the
70 million substances are organic. Unlike the combinatorial large but finite number
of inorganic substances (excluding silicates), the number of possible hydrocarbons
is infinite. Along with carbon and hydrogen, most of the known organic substances
contain only a small number of other elements: O, S, N, P, F, Cl, Br, I. The element
carbon appears as a tetravalent anion only in few methides (Al
4
C
3
and Be
2
C) that
produce methane with water or acids; other carbides contain C
2
−
×
2
or C
4
3
and afford
acetylene or allene/propyne, respectively (Cotton et al.
1999
). Silicon and boron
form with carbon hard covalent tridimensional lattices. Although stable carboca-
tions, free radicals, or carbenes exist when electronic delocalization is present, the
overwhelming majority of carbon compounds form molecular covalent bonds.
Although it is impossible to enumerate all the uses and applications of organic
compounds, one should mention that the marked increase of life expectancy during
the last centuries is mainly due to the progress of medicinal chemistry and deter-
gents; local and general anesthetics allow surgeons to perform operations and organ
replacements that were unimaginable a century ago; nowadays plastics tend to be-
come the preferred materials; dyestuffs and synthetic textiles surround us; Alfred
Nobel's fortune was due to the invention of dynamite, an organic explosive; pes-
ticides and herbicides contribute to increasing productivity of agriculture; organic
compounds contribute to the electronic industry, liquid crystals for color screens,
heat transfer agents for air conditioners and refrigerators, etc.
Living cells produce powerful catalysts, enzymes, which are proteins. In many
cases biosynthetic reactions proceed on the basis of effects caused by carbonyl
groups. The chemical reactions allowing chemists to synthesize new organic com-
pounds involve, however, bonds with other types of atoms. As an illustration, the
following paragraph contains a list of such atoms (in brackets) associated with the
