A short history of explosives
Sat, Jan 31, 2015This is a short article which was published in Oxford’s student popular science magazine, Bang!. See it in its illustrated glory here (page 21).
Explosives
Ask the average ten-year-old what chemists do, and “make explosions” would probably be the first answer. As explosions tend to be inconvenient if you are concerned about the structural integrity of your working environment, most chemists try to avoid them. But what is an explosion?
Simply put, it’s the very rapid expansion or release of gas. Any kind of rapid expansion can cause an explosion, whether from a chemical explosive, the intense heat of a nuclear weapon or from steam, e.g. microwaving an unpierced potato for too long. Explosives, though, are by far the useful way to generate an explosion no matter if you want to fire a gun, demolish a building, or mine. The power of explosives, for example one gram of RDX, a modern explosive, produces 24000 times its own volume in gas, is down to their chemical structure.
The first explosive however, long predates chemistry. Gunpowder was created by Chinese alchemists over a thousand years ago and was the only explosive used until the middle of the 19th century. It consists of a mixture of three chemicals: potassium nitrate, charcoal and sulphur. The charcoal is the fuel, potassium nitrate contains the oxygen to allow for combustion, and the sulphur lowers the temperature needed to trigger the reaction. The key to making good gunpowder is grinding the mixture up very finely allowing the components to be in close contact and so speeds up the reactions. And the faster the reaction, the bigger the bang.
Any chemical reaction that can be sped up sufficiently can become explosive – even burning flour. We know that bakeries have been exploding since 1785, when a Turin bakery storeroom exploded. Flour isn’t highly flammable, but the carbohydrates do burn, slowly. When the flour is dispersed in a dust cloud, however, it has an enormous surface area and the reaction speeds up to such a degree the flour explodes.
The dangers of explosives have been understood ever since their creation - one memorable instruction to gunpowder mill workers from 1776 ‘earnestly beseech[s] them not to let emane from their mouths or oaths swearwords or other light or obscene language’, for fear of detonation. The desire for explosives that only explode when needed had driven research for centuries, but only with Alfred Nobel was it achieved.
The chemical revolution in the middle of the 19th century produced many new explosives, including nitroglycerine, a much more powerful and more sensitive explosive than gunpowder. It was so dangerous that the British government even banned nitroglycerine in the 1860s. Their concern was well founded. Alfred Nobel, owner of one of the largest nitroglycerine factories lost his brother to a nitroglycerine explosion. Perhaps it was this loss that drove him to create ‘dynamite’, the first stable explosive. He realised that liquid nitroglycerine could be tamed if it was absorbed by an inert material, in particular diatomaceous earth. The wealth that followed from this discovery provided the funds for his eponymous prizes.
Other explosives soon followed nitroglycerine, and many of these were safer, in particular picric acid and TNT. These new explosive have one chemical feature in common – the nitro ‘functional group’ in common. The nitro group consists of a nitrogen bonded to two oxygens, and explains why trinitrotoluene is explosive, but toluene merely flammable. The nitrogen-nitrogen bond in N2 is one of the strongest bonds known, so when nitrogen atoms in an explosive combine, they give out a lot of energy. The two oxygens are also key, as they can combine with carbons and hydrogens in the rest of the explosive to form CO or H2O, giving out even more energy. And finally, the products, N2, CO and H2O are all gases.
Research into ‘High Energy Materials’ aims to make safer and more powerful explosives, using chemical knowledge. To increase safety, molecules are engineered to bond more strongly to each other (for example, NH2 groups are added to form intermolecular H-bonds) to raise their melting points and so their temperature stability. To increase the power, the nitro groups and nitrogen atoms can be added – earlier this year a molecule was made that had 10 contiguous nitrogen atoms– each ready when jolted to break free and form N2. More powerful explosives can also be from strained molecules, for example, octanitrocubane an extremely powerful explosive with a carbon cube at its core. The eight carbon atoms in the cube are bonded tightly to each other, but the are bent, so when detonated, the cube springs open, releasing all the pent up energy.
It is inevitable as more ingenious techniques are devised that ever more unlikely-looking molecules will be crafted. However, this frontier of chemistry will probably remain the preserve of specialists, as even hardened chemists shy away from a reaction where you need to wear body armour. High Energy Materials Chemists- we salute you. At a safe distance.