linked by alternating single and double bonds. Such compounds have
even numbers of carbon atoms.
*Cyclo-octatetraene, C 8 H 8 , is the next
in the series following benzene.
Higher annulenes are usually referred to by the number of carbon
atoms in the ring, as in [10]-annulene, C 10 H 10 , [12]-annulene, C 12 H 12 ,
etc. The lower members are not stable as a result of the interactions between hydrogen atoms inside the
ring. This is true even for molecules
that have the necessary number of pi
electrons to be *aromatic compounds. Thus, [10]-annulene has 4n +
2 pi electrons with n = 2, but is not
aromatic because it is not planar.
[14]-annulene also has a suitable
number of pi electrons to be aromatic (n = 3) but is not planar because of interaction between the
inner hydrogens.
The compound [18]-annulene is
large enough to be planar and obeys
the Hückel rule (4n + 2 = 18, with n =
4). It is a brownish red fairly stable
reactive solid. NMR evidence shows
that it has aromatic character. The
annulene with n = 7, [30]-annulene,
can also exist in a planar form but is
highly unstable. See also pseudoaromatic.
anode A positive electrode. In
*electrolysis anions are attracted to
the anode. In an electronic vacuum
tube it attracts electrons from the
*cathode and it is therefore from the
anode that electrons Ûow out of the
device. In these instances the anode
is made positive by external means;
however in a *voltaic cell the anode
is the electrode that spontaneously
becomes positive and therefore attracts electrons to it from the external circuit.
anode sludge See electrolytic refining.
anodizing A method of coating objects made of aluminium with a protective oxide Ülm, by making them
the anode in an electrolytic bath containing an oxidizing electrolyte. Anodizing can also be used to produce
a decorative Ünish by formation of
an oxide layer that can absorb a
coloured dye.
anomeric effect If a pyranose ring
has an electronegative substituent at
the anomeric carbon then this substituent is more likely to take the
axial conÜguration than the equatorial conÜguration. This is a consequence of a general effect (the
generalized anomeric effect) that in a
chain of atoms X–C–Y–C, in which Y
and X are atoms with nonbonding
electron pairs (e.g. F, O, N), the synclinal conformation is more likely.
For example, in the compound
CH 3 –O–CH 2 Cl, rotation can occur
along the O–C bond. The conformation in which the chlorine atom is
closer to the methyl group is preferred (the lone pairs on the oxygen
atom act as groups in determining
the conformation).
anomers Diastereoisomers of cyclic
forms of sugars or similar molecules
differing in the conÜguration at the
C1 atom of an aldose or the C2 atom
of a ketose. This atom is called the
anomeric carbon. Anomers are designated α if the conÜguration at the
anomeric carbon is the same as that
at the reference asymmetric carbon
in a Fischer projection. If the conÜguration differs the anomer is designated β. The α- and β-forms of
glucose are examples of anomers (see
monosaccharide).
anoxic reactor A *bioreactor in
which the organisms being cultured
are anaerobes or in which the reaction being exploited does not require
oxygen.
39
anoxic reactor
a
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even numbers of carbon atoms.
*Cyclo-octatetraene, C 8 H 8 , is the next
in the series following benzene.
Higher annulenes are usually referred to by the number of carbon
atoms in the ring, as in [10]-annulene, C 10 H 10 , [12]-annulene, C 12 H 12 ,
etc. The lower members are not stable as a result of the interactions between hydrogen atoms inside the
ring. This is true even for molecules
that have the necessary number of pi
electrons to be *aromatic compounds. Thus, [10]-annulene has 4n +
2 pi electrons with n = 2, but is not
aromatic because it is not planar.
[14]-annulene also has a suitable
number of pi electrons to be aromatic (n = 3) but is not planar because of interaction between the
inner hydrogens.
The compound [18]-annulene is
large enough to be planar and obeys
the Hückel rule (4n + 2 = 18, with n =
4). It is a brownish red fairly stable
reactive solid. NMR evidence shows
that it has aromatic character. The
annulene with n = 7, [30]-annulene,
can also exist in a planar form but is
highly unstable. See also pseudoaromatic.
anode A positive electrode. In
*electrolysis anions are attracted to
the anode. In an electronic vacuum
tube it attracts electrons from the
*cathode and it is therefore from the
anode that electrons Ûow out of the
device. In these instances the anode
is made positive by external means;
however in a *voltaic cell the anode
is the electrode that spontaneously
becomes positive and therefore attracts electrons to it from the external circuit.
anode sludge See electrolytic refining.
anodizing A method of coating objects made of aluminium with a protective oxide Ülm, by making them
the anode in an electrolytic bath containing an oxidizing electrolyte. Anodizing can also be used to produce
a decorative Ünish by formation of
an oxide layer that can absorb a
coloured dye.
anomeric effect If a pyranose ring
has an electronegative substituent at
the anomeric carbon then this substituent is more likely to take the
axial conÜguration than the equatorial conÜguration. This is a consequence of a general effect (the
generalized anomeric effect) that in a
chain of atoms X–C–Y–C, in which Y
and X are atoms with nonbonding
electron pairs (e.g. F, O, N), the synclinal conformation is more likely.
For example, in the compound
CH 3 –O–CH 2 Cl, rotation can occur
along the O–C bond. The conformation in which the chlorine atom is
closer to the methyl group is preferred (the lone pairs on the oxygen
atom act as groups in determining
the conformation).
anomers Diastereoisomers of cyclic
forms of sugars or similar molecules
differing in the conÜguration at the
C1 atom of an aldose or the C2 atom
of a ketose. This atom is called the
anomeric carbon. Anomers are designated α if the conÜguration at the
anomeric carbon is the same as that
at the reference asymmetric carbon
in a Fischer projection. If the conÜguration differs the anomer is designated β. The α- and β-forms of
glucose are examples of anomers (see
monosaccharide).
anoxic reactor A *bioreactor in
which the organisms being cultured
are anaerobes or in which the reaction being exploited does not require
oxygen.
39
anoxic reactor
a
www.AzShimi.ir www.AzShimi.com