ence of impurities may tint it grey,
blue, green, or pink.
arene complex A complex in
which an aromatic ring is bound to a
metal atom by its pi-electrons. Exam-
ples of arene complexes are the
*sandwich compounds (C 6 H 6 ) 2 Cr and
(C 5 H 5 ) 2 Fe.
arenes Aromatic hydrocarbons,
such as benzene, toluene, and naph-
thalene.
argentic compoundsCompounds
of silver in its higher (+2) oxidation
state; e.g. argentic oxide is silver(II)
oxide (AgO).
argentiteA sulphide ore of silver,
Ag 2 S. It crystallizes in the cubic sys-
tem but most commonly occurs in
massive form. It is dull grey-black in
colour but bright whenÜrst cut and
occurs in veins associated with other
silver minerals. Important deposits
occur in Mexico, Peru, Chile, Bolivia,
and Norway.
argentous compoundsCom-
pounds of silver in its lower (+1) oxi-
dation state; e.g. argentous chloride
is silver(I) chloride.
arginineSee amino acid.
argonSymbol Ar. A monatomic
noble gas present in air (0.93%); a.n.
18; r.a.m. 39.948; d. 0.00178 g cm–3;
m.p. –189°C; b.p. –185°C. Argon is
separated from liquid air by frac-
tional distillation. It is slightly solu-
ble in water, colourless, and has no
smell. Its uses include inert atmos-
pheres in welding and special-metal
manufacture (Ti and Zr), and (when
mixed with 20% nitrogen) in gas-Ülled
electric-light bulbs. The element is
inert and has no true compounds.
Lord Rayleigh and Sir William
Ramsey identiÜed argon in 1894.
A
- Information from the WebElements site
Arndt–Eisert systhesis A method
of converting a carboxylic acid into
the next higher homologue acid (or
one of its derivatives). Diazomethane
is used to insert a CH 2 group.aromatic compound An organic
compound that contains a benzene
ring in its molecules or that has
chemical properties similar to ben-
zene. Aromatic compounds are un-
saturated compounds, yet they do
not easily partake in addition reac-
tions. Instead they undergo elec-
trophilic substitution.
Benzene, the archetypal aromatic
compound, has an hexagonal ring of
carbon atoms and the classical for-
mula (the Kekulé structure) would
have alternating double and single
bonds. In fact all the bonds in ben-
zene are the same length intermedi-
ate between double and single C–C
bonds. The properties arise because
the electrons in the π-orbitals are de-
localized over the ring, giving an
extra stabilization energy of
150 kJ mol–1over the energy of a
Kekulé structure. The condition for
such delocalization is that a com-
pound should have a planar ring
with (4n + 2) pi electrons – this is
known as the Hückel rule. Aromatic
behaviour is also found in hetero-
cyclic compounds such as pyridine.
Aromatic character can be detected
by the presence of a ring current
using NMR. See also annulenes; non-
benzenoid aromatic; pseudoaro-
matic.
A- Information about IUPAC nomenclature
aromaticity The property charac-
teristic of *aromatic compounds.ARPESSee angle-resolved photo-
electron spectroscopy.Arrhenius, Svante (August)
(1859–1927) Swedish physical
chemist whoÜrst demonstrated that43 Arrhenius, Svante (August)
a