2020 marks 75 years since the end of the Second World War. A brutal global conflict that left millions dead, millions injured and dramatically altered and affected almost every country in some way. The final weeks of the war also saw the decimation and devastation caused by two atomic bombs.

On August 6th 1945, an aircraft bomber called Enola Gay, dropped the first atomic bomb, codenamed Little Boy, on the Japanese city of Hiroshima. 3 days later, a second atomic bomb was dropped, codenamed Fat Man. This time from the bomber Bockscar on a second Japanese city, Nagasaki. The impact of these two atomic bombs on their targets were catastrophic. The radiation they produced continued to seriously affect the areas for many, many years afterwards. Thankfully, these remain the only times atomic bombs have been used in conflict.

The development and use of atomic weapons overshadows the important scientific discoveries that were made during the process. However, the atomic bomb is just one example showing the best and worst of man’s use of science.

Not the first time…

The creation of atomic bombs was certainly not the first time physics, chemistry, molecules and atoms have been used to fight, injure or even kill. 30 years before the dropping of the atomic bombs, the world was once again suffering another world war. This time using chemical warfare. One of the most infamous molecules being hurled across the battlefield and causing horrendous injuries was mustard gas.

A yellow gas of fear

The name mustard gas can be a little misleading. We have already seen this for theobromine, which despite its name contained no bromine at all. The main issue with the name mustard gas, is the ‘gas’ part. For one thing, it is not actually a gas but a collection of tiny liquid droplets that form more of a mist. The name ‘gas’ also suggests that this molecule does most of its damage when breathed in. While inhaling this molecule can result in terrible injuries, it was contact with the skin and external organs that caused many soldiers (and unfortunately also civilians) a horrific amount of suffering.

For all its devastating impact, the structure of mustard gas is deceptively simple. The image below shows mustard gas to contain a sulfur atom right at the centre of the molecule. On opposite sides of this sulfur atom are two chains of two carbon atoms. An atom of chlorine caps both of those carbon chains. Despite its simple structure, the technical name for mustard gas is the rather complicated 1-Chloro-2-[(2-chloroethyl)sulfanyl]ethane.

The structure of a molecule of mustard gas. Note the symmetry in the structure, with a large atom of sulfur in the middle, with an atom of chlorine at the end of each chain.

In its pure form, mustard gas is a colourless liquid. The impure forms often used during war gives it a yellow-brown colour, though. As the name suggests, this molecule has a smell very similar to mustard or horseradish. Not that you want to be smelling this molecule.

What’s the first thing that comes to your mind for ‘mustard’?

The man-made route to suffering

Only humans have made mustard gas. It is not a naturally occuring compound. In 1860, long before the First World War, a British chemist called Frederick Gutherie, reacted a molecule called ethene with chlorine to produce mustard gas. He noted the irritating effect the molecule had on his skin, the first indication of the damaging health effects.

A few years later a German chemist, Viktor Meyer, also reported making mustard gas. Meyer’s mustard gas was much purer, but also a lot more harmful, with more serious effects on those in lab. Finally, in 1913 the English chemist Hans Thacher Clarke and German chemist Emil Fischer improved on Meyer’s method of making mustard gas. During this process, an accident in the lab caused Clarke to spend 2 months in hospital.

With the process of making mustard gas perfected and the horrible effects on humans obvious, it was only a matter of time before it was used as a weapon. The first major use of mustard gas during the First World War was by the Germans near Ypres, Belgium in 1917.

A note of caution

One of the main problems with mustard gas, or any type of chemical gas weapon, was the weather. Sudden changes in wind direction can rapidly blow the gas back towards those who have launched it. Mustard gas is also relatively dense, causing it to sink to lower ground and remain there, creating gas hotspots where unfortunate soldiers run into. This leads us to an obvious question and a very tough topic of discussion. What were the physical effects of this molecule on people?

The devastating impact

Let’s be clear from the start, the physical effects of mustard gas were truly awful. This was a molecule intended to severely damage the physical and mental strength of the people it was used on. In this sense the molecule was ruthlessly effective.

Symptoms of mustard gas exposure don’t always appear immediately. Sometimes it could take up to a day for any problems to occur. However, the first symptom was an intense irritation and itching of the skin. This would then lead to the formation of very large blisters filled with a horrible yellow liquid. For those of you who ever burnt yourself, you may have noticed similar blisters. This is because these blisters were a sign of the chemical burn caused by mustard gas. This makes the molecule a vesicant, or blistering agent.

It was also difficult to protect yourself as mustard gas could easily pass through cotton and woolen clothing, reaching the skin quickly. So while masks might prevent you from inhaling this gas, it did very little to stop it reaching your skin.

It was not just your skin that mustard gas could damage. Exposure to your eyes could also result in immense swelling and temporary blindness. Long exposure to high concentrations could also cause much more permanent injuries to the eyes.

If you were unlucky enough to be caught in a mustard gas attack without a mask, then you definitely did not want to inhale this molecule either. Much like contact with the skin, inhaling the gas caused blisters and bleeding to occur internally such as in the lungs. This can lead to a build-up fluid and in very severe cases lead to death.

The Reactivity of Mustard Gas

The effects of mustard gas on the skin result from the various reactions this molecule can undergo. These reactions can produce even more reactive molecules that are responsible for most of the damage.

Part One: Hemimustard

Contact between mustard gas and moisture, such as wet skin or inside our bodies, was a particular problem. Under these moist conditions, mustard gas will react with water, starting off a process of many reactions. These reactions produce two main products that also cause problems. One of these is called hemi-mustard, a molecule that has the same structure as mustard gas, but with one of the chlorine atoms replaced by OH.

Slide to see the difference between mustard and hemimustard.

Hemimustard is also a blistering agent and an irritant. The other product is HCl, hydrogen chloride, or when dissolved in water – hydrochloric acid. Again, this acts as an irritant and can cause further skin damage, and is even worse when formed inside the lungs.

Mustard Mechanics

Sulfur has two lone pairs of electrons – shown in the pink clouds above the sulfur atom. These electrons can be used to start reactions off. For most reactions you need to find some electrons that ‘want’ to leave where they are and go exploring, and an atom in the molecule that can take a few more electrons and go off on that adventure. The atom that can take those electrons here is chlorine.

We can ‘push’ electrons from the sulfur lone pair through the molecule to the chlorine atom, which leaves as a chloride ion with a negative charge. To keep the number of bonds to the carbon atom at a nice stable 4, the sulfur has formed a bond using the lone pair of electrons.

Another good rule for chemistry – small rings are tricky to make. We have a ring of three atoms – two carbons and the sulfur – and if we can bust that open the molecule will thank us. It’s like trying to do the Hokey Kokey – you need to have a big enough circle to put your arms in and out without turning it into the Hokey Karate. We all need space at times, and atoms in molecules are no exception.

Again, we need a molecule that can donate some electrons and somewhere for them to go. Water donates the electrons and they get pushed through the molecule to the sulfur lone pair again. We have 4 bonds to carbon – great – and the molecule is basically back to how it started but with -OH and -Cl swapped over. Notice though that the water molecule lost a hydrogen? So we’ve lost H+ and Cl – otherwise known as hydrochloric acid.

Only half the problem…

This mechanism is a fairly typical type of reaction in chemistry. We have a molecule with a good leaving group – that chlorine atom. We use water to donate the electrons that restore the right number of bonds and charges and so on at the end. There is one more thing to notice though with this molecule – we’ve drawn the mechanism and all the changes on the right side of the molecule but there’s no reason why it couldn’t occur on the left first. Or after… This molecule will go on to react again with another molecule of water to add an -OH to the other side of the molecule too. Which, you guessed it, does more damage and produces more hydrochloric acid. It’s a really nasty molecule.

A weapon for all ages

Unfortunately, the use of mustard gas did not stop following the end of the First War World. Almost 100 years later, mustard gas is still being used as a weapon and not just against soldiers. There have been many reported uses of mustard gas against civilians and people who were not even fighting, and definitely had no means to fight back.

The use of mustard gas and other chemical weapons was prohibited by the Geneva Convention of 1925 and the chemical weapons convention of 1993. Both of these make the use, creation and storing of mustard gas highly illegal under international law. This has not, however, stopped countries and terrorist groups from using it.

Mustard gas was not the only chemical weapon used during the First World War. Others, such as phosgene, were widely used and had an equally devastating impact. In fact, there are many, many types of chemical weapons that were, and still are, being used.


What about treating or removing mustard gas? Nowadays, there are a range of molecules that can be used to treat those exposed to mustard gas. These include a molecule called chloramine-T, a magnesium compound and peroxy acids. Unfortunately, during the First World War the only way to remove and deal with the toxic effects was to use hypochlorite bleaches, a method that does not sound too appealing. Like any type of burn, great care was needed in treating those exposed to mustard gas. Not only was this extremely painful, but the healing process was very slow. It was also essential to prevent the wound from becoming infected.

Getting inside the body

Skin is usually a great barrier that protects us from our old enemy, The Outside. This molecule is particularly dangerous to us because it can get through that barrier. The other problem with mustard gas is that it dissolves very well in fat, helping it to absorb into our skin very easily. Make no mistake, the damage caused by exposure to mustard gas was sheer agony.

The strong reactivity of mustard gas means it can attack and permanantely change the structure of one of our DNA bases called guanine. This change leads to cell death or the development of cancer. This means that people exposed to mustard gas are more likely to have cancer, even 10 or 20 years later.

A small hope

Despite the incredible amount of pain and suffering cause by mustard gas, there is one, small silver-lining. The fact that this molecule can target and cause chemical changes to DNA means that it also has potential to treat diseases that impact DNA. Research has shown that this molecule might be useful to help in the fight against cancer. However, the preferred choice is a very similar molecule to mustard gas, but with nitrogen instead of a sulfur atom.

In fact, the very first cancer therapy drugs we developed from mustard agents. The general idea is that cancer cells are less able to recover from damage to their DNA than normal, healthy cells. DNA is usually really well protected by being coiled and wrapped up to keep it tidy. It is only exposed while it is being copied and replicated so the faster a cell is dividing the more likely it is that these mustard agents will damage the DNA in that cell. The molecules have been developed over the years to be more targeted, less damaging, more helpful and less harmful to give us the modern day arsenal we have against cancers.

Lest we forget

The discovery, development and use of science throughout history have provided many benefits, but also many methods of chaos, destruction and death. The discoveries made during both World Wars helped our deeper understanding of the world, the Universe and science, but at the cost of weapons and explosives.

What is right or wrong, good or bad is far beyond the scope of this article. It would be naïve to ignore the creation and use of molecules purely for negative uses; however the vast majority of science is for the benefit of the world and society.


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