When we were in Cinque Terre
swimming, some hard fought battles with the Ligurian Sea required some
over-the-counter medical correction in the form of hydrogen peroxide. Now,
everyone’s familiar with this substance, it is very versatile and has many
practical household and industrial applications, the most familiar to us are as
a cleaning agent and as an antiseptic. Well, a few of us had to use the latter
property after said fights with the Ligurian Sea, and I was amazed at how well
it worked. However, I had many questions about this compound.
The “signature” of hydrogen
peroxide that makes it easily identifiable in the medicine cabinet is its brown
bottle (the ones we bought were in a white bottle, but it was also opaque). The
reason for this is that hydrogen peroxide is not very stable, and in the
presence of light will easily convert into water and oxygen gas (2H2O2
à 2H2O + O2).
Therefore, if you let a bottle of hydrogen peroxide sit out too long
(especially with the cap open), you end up with a bottle of water, which is not
very useful when you actually need to use it. When hydrogen peroxide is placed
on a cut, the same mechanism is catalyzed by the enzyme catalase (which is an
enzyme that breaks down peroxides to help limit the damages of oxidative
stress).
My question was that this reaction
is exothermic and favorable, and it occurs rather spontaneously. So, how do
they make hydrogen peroxide? Why doesn’t the process that creates it always
just create water, because that’s more thermodynamically favorable? How do they
make large quantities of this compound that is ubiquitous in pharmacies all
over the world? Well, turns out the answer involves a rather cool bit of
chemistry.
Now, it’s clear that this reaction
takes place in a different way than the reverse of the reaction of water in
hydrolysis (2H2O à
2H2 + O2). Well, that may be true, but you wouldn’t get
that from the net chemical formula of the reaction (H2 + O2
à H2O2).
Hmmm…seems simple enough, right? Well, why isn’t it? Because, as I was
pondering, water is so much more stable! If that reaction were to be done,
wouldn’t water just be formed? Well, turns out that throughout the years, many
ways have been devised to chemically produce hydrogen peroxide. Most of them
involved acids to provide the protons and barium oxide to provide the oxygen.
However, a newer method put into effect by E.I. Dupont de Nemours (yes, one of those Duponts) takes the cake.
This process involves what is known as a 2-alkylanthraquinone.
A hydrogenated version of the anthraquinone is exposed to compressed oxygen gas (usually through compressed air),
and the oxygen reacts with the hydrogen on the compound to form hydrogen
peroxide, which is purified away. Then, the anthraquinone is flooded with hydrogen
gas with a metal catalyst to rehydrogenate the resultant ketones. This process repeats itself as long as the anthraquinone remains stable, and allows for an effective way to create hydrogen
peroxide from hydrogen gas and oxygen gas, while catalyzing a reaction that
allows for this compound to be created in bulk and used around the world, from
at home to in a Vernazza pharmacy.
Sources:
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