linked by alternating single and dou-
ble 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 re-
ferred to by the number of carbon
atoms in the ring, as in [10]-annu-
lene, C 10 H 10 , [12]-annulene, C 12 H 12 ,
etc. The lower members are not sta-
ble as a result of the interactions be-
tween hydrogen atoms inside the
ring. This is true even for molecules
that have the necessary number of pi
electrons to be aromatic com-
pounds. 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 aro-
matic (n = 3) but is not planar be-
cause 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 pseudoaro-
matic.
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 electronsÛow 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 at-
tracts electrons to it from the exter-
nal circuit.
anode sludge See electrolytic re-
fining.
anodizingA method of coating ob-
jects made of aluminium with a pro-
tective oxideÜlm, by making them
the anode in an electrolytic bath con-
taining an oxidizing electrolyte. An-
odizing can also be used to produce
a decorativeÜnish 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 sub-
stituent is more likely to take the
axial conÜguration than the equator-
ial conÜguration. This is a conse-
quence 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 syn-
clinal conformation is more likely.
For example, in the compound
CH 3 –O–CH 2 Cl, rotation can occur
along the O–C bond. The conforma-
tion in which the chlorine atom is
closer to the methyl group is pre-
ferred (the lone pairs on the oxygen
atom act as groups in determining
the conformation).anomersDiastereoisomers 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 desig-
nated αif the conÜguration at the
anomeric carbon is the same as that
at the reference asymmetric carbon
in a Fischer projection. If the conÜgu-
ration differs the anomer is desig-
nated 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 reac-
tion being exploited does not require
oxygen.39 anoxic reactor
a