# Why is picric acid more explosive than TNT?

I've recently read that trinitrophenol (picric acid - relative effectiveness factor $1.20$) is more explosive than trinitrotoluene (TNT - relative effectiveness factor $1.00$).

Personally, I think that the mesomeric effect of the oxygen's lone pairs in picric acid should keep the nitro groups 'happy', because of the increase in electron density on them. This should enhance the overall stability of the molecule, and reduce the relative effectiveness factor. However, this is not the case. Why is this? My guess is that the electron withdrawing nature of oxygen somehow causes this.

• I believe you're overthinking this. When it comes to explosives, what usually matters most is simply the atomic ratios of carbon, nitrogen, hydrogen and oxygen in a molecule or mixture. As a rule of thumb, the more oxygen and nitogen, the more powerful the explosive. May 15, 2015 at 14:55
• @NicolauSakerNeto this is true for oxygen poor explosives like, picric acid and TNT but not in all cases: the most powerful explosives have a balanced mixture of carbon/hydrogen to oxygen. For example ammomium nitrate NH4NO3 is oxygen rich, decomposing to N2 + 2H2O + 0.5O2. Grind it up and mix it with diesel and it becomes a lot more powerful. In general you are are right: even the infamous nitroglycerin C3H5N3O6 is very slightly oxygen poor: combustion products (2 equivalents) 3N2 + 5H2O +5CO + CO2. Aug 8, 2015 at 16:03

You're trying to define the stronger explosive by comparing the reagents, but in this case comparing the products is far more important.

Trinitrotoluene is a rather oxygen-poor explosive, to the point that when detonated as a pure substance, it produces carbon soot and hydrogen gas:

$$\ce{2C7H5N3O6 -> 3N2 + 5H2 + 12CO + 2C}$$

The production of $\ce{CO}$, $\ce{CO2}$ and $\ce{H2O}$ is highly exothermic, so a lot of extra energy could be obtained if there were more oxygen to burn the soot and hydrogen. However, in detonations there's not nearly enough time for atmospheric oxygen to partake extensively in the reaction. This means that a fair amount of the explosive potential of TNT is wasted, decreasing its detonation yield per gram of substance. One way to compensate this yield loss in oxygen-poor explosives is to mix them with an oxidiser, usually an oxygen-rich substance such as $\ce{KClO3}$.

Now, on to picric acid. The structure and molar mass are very similar to TNT, but replacing the methyl group with a hydroxyl means the oxygen content of picric acid is higher. While still overall oxygen-poor, the detonation proceeds with much less soot and hydrogen generation, meaning a more complete burn, a higher energy release, and consequently a greater explosive yield.

$$\ce{2C6H3N3O7 -> 3N2 + H2 + 2H2O + 12 CO}$$

In reality, the decomposition reactions are more complex. The balanced equations above follow a rule of thumb presented here. However, a more thorough analysis should agree with this simplified result.