outer-sphere electron transfer An outer-sphere elec-
tron transfer is a reaction in which the electron transfer
takes place with no or very weak (4–16 kJ mol–1) elec-
tronic interaction between the reactants in the transi-
tion state. If instead the donor and the acceptor exhibit
a strong electronic coupling, the reaction is described
as INNER-SPHERE ELECTRON TRANSFER. The two terms
derive from studies concerning metal complexes, and it
has been suggested that for organic reactions, the terms
nonbonded and bonded electron transfer should be
used.
overlap Interaction of orbitals on different atoms in
the same region of space.
See alsoATOMIC ORBITAL.
overpotential(overvoltage) Extra energy required in
addition to the reduction potential in order for a reaction
to proceed. In electrochemistry, it is the deviation of an
electrode potential from its equilibrium value required to
produce a net flow of current across an electrode/solution
interface. It is the voltage in excess of the voltaic cell
potential that is needed to cause electrolysis to occur.ovotransferrin An iron-binding protein from eggs,
structurally similar to the TRANSFERRINs.oxidase An ENZYMEthat catalyzes the oxidation of
SUBSTRATEs by O 2.oxidation (1) The complete, net removal of one or
more electrons from a MOLECULAR ENTITY(also called
“de-electronation”); (2) an increase in the OXIDATION
NUMBERof any atom within any substrate; (3) gain of
oxygen and/or loss of hydrogen of an organic SUB-
STRATE.
All oxidations meet criteria (1) and (2), and many
meet criterion (3), but this is not always easy to demon-
strate. Alternatively, an oxidation can be described as a
TRANSFORMATIONof an organic substrate that can be
rationally dissected into steps or PRIMITIVE CHANGEs.
The latter consist in removal of one or several electrons
from the substrate followed or preceded by gain or loss
of water and/or HYDRONs or hydroxide ions, or by
NUCLEOPHILIC substitution by water or its reverse
and/or by an INTRAMOLECULAR MOLECULAR REAR-
RANGEMENT.
This formal definition allows the original idea of
oxidation (combination with oxygen), together with
its extension to removal of hydrogen, as well as pro-
cesses closely akin to this type of transformation (and
generally regarded in current usage of the term in
organic chemistry to be oxidations and to be effected
by “oxidizing agents”) to be descriptively related to
definition (1). For example, the oxidation of methane
to chloromethane can be considered as follows:
CH 4 –2e––H++ OH–= CH 3 OH (oxidation) →CH 3 Cl
(reversal of hydrolysis)oxidation number The oxidation number of an ele-
ment in any chemical entity is the number of charges204 outer-sphere electron transfer
Ostwald process. An industrial process that produces oxide and
nitric acid from ammonia and oxygen