Sunday, June 17, 2012

Hydrogen and Oxygen Are More Than Just Water - The Synthesis of Hydrogen Peroxide

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.


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