Rowena Fletcher-Wood discovers the many uses for a toxic compound
Hydrazine is a molecule of two singly-bonded nitrogen atoms and four peripheral hydrogen atoms. In its anhydrous form, it is a colourless, toxic irritant and sensitiser, which damages the central nervous system, producing symptoms as extreme as tumours and seizures. The pungent smell of hydrazine is not unlike that of ammonia, and it is so powerful a reducing agent that it is highly explosive.
Considering this, it seems strange that around 100,000 metric tonnes of the stuff are manufactured worldwide every year. But hydrazine does influence our everyday lives. It keeps us warm, clothes and feeds us, can save our lives and even take us to the moon. In a way, it can even turn back time.
So how does such a simple molecule do all that? The answer lies in the industries that use it.
The most common use of hydrazine is to make foaming agents like azodicarbonamide. When azodicarbonamide is bubbled through a liquid polymer precursor, it thermally decomposes to nitrogen, carbon dioxide, carbon monoxide and ammonia. These gases form bubbles in the liquid, which then polymerises to leave a lightweight, foamy plastic.
But hydrazine gives us more than just disposable cups and packaging: polymer foams are used to make insulation panels for our homes, and even spandex.
Hydrazine is also used to make pesticides and pharmaceuticals. Although it is toxic to us directly, hydrazine rapidly breaks down in oxygen, making release into the environment low risk.
It could even be an environmentally friendly fuel. In some types of hydrogen fuel cell, hydrazine breaks down to make nitrogen and water and gives out energy in an exothermic reaction. It actually produces a little more energy than hydrogen, is easier to store because it’s a liquid at room temperature, and avoids the need for an expensive platinum catalyst. But it also requires more manufacturing – it all depends on the balance.
Hydrazine can even be found in fossil fuels and nuclear power stations. As a powerful reducing agent, it acts as an oxygen scavenger, reducing metal oxides back into metals, and literally turning the chemical clock back on corrosion processes.
The explosive nature of hydrazine has been put to good use, too. Sodium azide, the compound that activates car air bags, is manufactured directly from hydrazine and sodium nitrate. Sodium azide decomposes rapidly when it gets warm, creating huge volumes of nitrogen gas.
Hydrazine is also used as rocket fuel propellant. Mixing it with oxidising agent dinitrogen tetroxide, N2O4, creates a hypergolic mixture – a mixture so explosive, no ignition is required. As the fuel burns, three reactions take place, decomposing hydrazine into ammonia, nitrogen and hydrogen gases. Within milliseconds, the reaction chamber can exceed 800°C due to these extremely exothermic reactions. Ammonia is also broken down – this is endothermic and takes away some of the heat energy, but produces more nitrogen and hydrogen gases that are forced out of the rocket through a tight nozzle to create thrust.
The first hydrazine fuel was used with methanol during world war two in the Messerschmitt Me 163B; the world’s first – and last – rocket-powered fighter plane. As you can imagine, these planes had a tendency to explode. This, along with fuel leaks, killed more pilots than enemy fire.
Today, some military vehicles use hydrazine, but it is employed mostly in extraterrestrial exploration. The Curiosity Rover, which has been resident on Mars since 2012, used hydrazine to land on the red planet. And, in July this year, the NASA New Frontiers mission ‘Juno’ made its incredible insertion into the orbit of Jupiter powered by hydrazine and dinitrogen tetroxide.
So there you have it, hydrazine. The small, explosive molecule at the forefront of future science, bringing planetary exploration, spandex and air bags together.