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. Examples 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 naphthalene.
argentic compounds Compounds
of silver in its higher (+2) oxidation
state; e.g. argentic oxide is silver(II)
oxide (AgO).
argentite A sulphide ore of silver,
Ag 2 S. It crystallizes in the cubic system but most commonly occurs in
massive form. It is dull grey-black in
colour but bright when Ürst cut and
occurs in veins associated with other
silver minerals. Important deposits
occur in Mexico, Peru, Chile, Bolivia,
and Norway.
argentous compounds Compounds of silver in its lower (+1) oxidation state; e.g. argentous chloride
is silver(I) chloride.
arginine See amino acid.
argon Symbol 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 fractional distillation. It is slightly soluble in water, colourless, and has no
smell. Its uses include inert atmospheres 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 benzene. Aromatic compounds are unsaturated compounds, yet they do
not easily partake in addition reactions. Instead they undergo electrophilic substitution.
Benzene, the archetypal aromatic
compound, has an hexagonal ring of
carbon atoms and the classical formula (the Kekulé structure) would
have alternating double and single
bonds. In fact all the bonds in benzene are the same length intermediate between double and single C–C
bonds. The properties arise because
the electrons in the π-orbitals are delocalized over the ring, giving an
extra stabilization energy of
150 kJ mol
–1 over the energy of a
Kekulé structure. The condition for
such delocalization is that a compound should have a planar ring
with (4n + 2) pi electrons – this is
known as the Hückel rule. Aromatic
behaviour is also found in heterocyclic compounds such as pyridine.
Aromatic character can be detected
by the presence of a ring current
using NMR. See also annulenes; nonbenzenoid aromatic; pseudoaromatic.
A
• Information about IUPAC nomenclature
aromaticity The property characteristic of *aromatic compounds.
ARPES See angle-resolved photoelectron spectroscopy.
Arrhenius, Svante (August)
(1859–1927) Swedish physical
chemist who Ürst demonstrated that
43
Arrhenius, Svante (August)
a
www.AzShimi.ir www.AzShimi.com
blue, green, or pink.
arene complex A complex in
which an aromatic ring is bound to a
metal atom by its pi-electrons. Examples 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 naphthalene.
argentic compounds Compounds
of silver in its higher (+2) oxidation
state; e.g. argentic oxide is silver(II)
oxide (AgO).
argentite A sulphide ore of silver,
Ag 2 S. It crystallizes in the cubic system but most commonly occurs in
massive form. It is dull grey-black in
colour but bright when Ürst cut and
occurs in veins associated with other
silver minerals. Important deposits
occur in Mexico, Peru, Chile, Bolivia,
and Norway.
argentous compounds Compounds of silver in its lower (+1) oxidation state; e.g. argentous chloride
is silver(I) chloride.
arginine See amino acid.
argon Symbol 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 fractional distillation. It is slightly soluble in water, colourless, and has no
smell. Its uses include inert atmospheres 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 benzene. Aromatic compounds are unsaturated compounds, yet they do
not easily partake in addition reactions. Instead they undergo electrophilic substitution.
Benzene, the archetypal aromatic
compound, has an hexagonal ring of
carbon atoms and the classical formula (the Kekulé structure) would
have alternating double and single
bonds. In fact all the bonds in benzene are the same length intermediate between double and single C–C
bonds. The properties arise because
the electrons in the π-orbitals are delocalized over the ring, giving an
extra stabilization energy of
150 kJ mol
–1 over the energy of a
Kekulé structure. The condition for
such delocalization is that a compound should have a planar ring
with (4n + 2) pi electrons – this is
known as the Hückel rule. Aromatic
behaviour is also found in heterocyclic compounds such as pyridine.
Aromatic character can be detected
by the presence of a ring current
using NMR. See also annulenes; nonbenzenoid aromatic; pseudoaromatic.
A
• Information about IUPAC nomenclature
aromaticity The property characteristic of *aromatic compounds.
ARPES See angle-resolved photoelectron spectroscopy.
Arrhenius, Svante (August)
(1859–1927) Swedish physical
chemist who Ürst demonstrated that
43
Arrhenius, Svante (August)
a
www.AzShimi.ir www.AzShimi.com