Why are nitro compounds used in the manufacture of explosives?
The first thing we need to identify is what nitro compounds are. Nitro compounds contain the organic NO₂ group. This is likely in addition to carbon and hydrogen. One example of a nitro compound is trinitrotoluene, otherwise known as TNT. The second part of the question involves explosives. The first feature of an explosive is that a large amount of energy is released when it goes off. The second feature of explosives is that they often produce gas when they react. Now, we can start analyzing nitro compounds to see what about them makes them good explosives.
The first feature is that they already contain oxygen. Oxygen is necessary for the remainder of the compound to combust. Having it inbuilt means that oxygen doesn’t have to come from an outside source. Oxygen reacts with carbon and hydrogen in these organic compounds, producing carbon dioxide and water. Both carbon dioxide and water will be gases at the detonation temperature. The second feature of nitro compounds is that they contain nitrogen. When nitro compounds detonated, these nitrogens pair up to form nitrogen gas. So far, I’ve demonstrated that nitro compounds fulfill one of the requirements for a good explosive, generating gases upon combustion.
In order to demonstrate that nitro compounds release a lot of energy when they combust, I’m going to look at the bond energies involved. Let’s have a look at the energetics of one of the simplest nitro compounds, nitromethane. Here, all the bonds have been labelled with the respective energies. And the products are a mixture of carbon dioxide, carbon monoxide, hydrogen gas, water, and nitrogen gas. Let’s look at each element in turn and see what happens to the respective energies of the bonds it forms. In the reactants, oxygen is weakly bound to nitrogen, with an average bond enthalpy of about 400 kilojoules per mole. The average value in the products is significantly higher.
The same is true for nitrogen. The nitrogen atoms pair up, forming very, very strong nitrogen-nitrogen triple bonds. The same is true for the bonds formed by carbon and hydrogen. Since there are much stronger bonds in the products than in the reactants, there is a large energy release when nitro compounds are combusted. Of course, this is only looking at the special case, that of nitromethane. But we can write this generalizing it for all nitro compounds.
Organic nitro compounds contain everything they need to be excellent explosives. They contain oxygen, which can act as an oxidizing agent, as well as carbon and hydrogen. The oxygen is weakly bound to nitrogen in the nitro group. This bond can be broken easily and replaced with strong bonds between carbon and oxygen, C=O, in carbon dioxide or hydrogen and oxygen, O–H, in water. A strong bond, N≡N, also forms between nitrogen atoms in molecular nitrogen. These reactions produce a large amount of heat and large quantities of gas, producing an explosion.