Figure 35.1 Structure of oxygen and reactive oxygen species (ROS).
Oxygen is shown at the far left as the incorrect double-bonded diatomic
form. This form, known as singlet oxygen, exists to a significant
extent only at high temperature or in response to irradiation. The
diradical is the natural, ground-state form of O2 at body
temperature. ROS are partially reduced, reactive forms of oxygen. The
first product is the anion radical, superoxide (O2•),
which is in equilibrium with the weak acid, hydroperoxyl radical (pKa ≈
4.5). Reduction of superoxide yields hydroperoxide O2-2,
in the form of H2O2. Reduction of H2O2
causes a hemolytic cleavage reaction that releases hydroxyl radical
(OH•)
and hydroxide ion (OH-). Water is the end product of
complete reduction of O2.
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35 Oxygen and Life
>
THE
INERTNESS OF OXYGEN
In most textbooks, oxygen is shown as a diatomic molecule
with two
bonds between the oxygen atoms. This is an attractive presentation,
from the viewpoint of the electron dot structures and electron pairing
to form chemical bonds, but it is incorrect. In fact, at body
temperature, O2 is a biradical, a molecule with two unpaired
electrons (
Fig. 35.1).
These electrons have parallel spins and are unpaired. Since most
organic oxidation reactions, e.g. the oxidation of an alkane to an
alcohol or an aldehyde to an acid, are two-electron oxidation
reactions, O2 is generally not very reactive. In fact, O2
is completely stable in the presence of H2, a strong
reducing agent, until enough heat is provided to flip an O2
electron and initiate the combustion reaction. Once it is started, the
combustion provides the heat needed to propagate the reaction,
sometimes explosively.
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