2012: Advent Chemistry: TNT

by on December 2, 2012

The structure of TNT in ball-and-stick format

This is trinitrotoluene. It’s an explosive, as I’m sure you know. Pure TNT is yellow and the chap who invented it, one Joseph Wilbrand, used it as a dye. While it gives a powerful explosion it’s quite hard to set off and easy to work with – much safer than, say, nitroglycerine, which will explode if you nudge it too hard.

The picture above bears some explanation. It’s a ball-and-stick model. The balls are atoms and the sticks are bonds. Where you see dotted lines, those are double bonds in their shuffled-around state, shared between several places.

All those N–O bonds can be shuffled around, and so can all the C–C bonds. That’s another aromatic ring in the middle, so that part of the molecule is flat. The bottommost NO2 group is in the same plane as the carbon ring. That tells us that double bonds can also be shuffled around between the ring and that NO2, which – you guessed it – makes it more stable.

We can’t do that with the other two NO2‘s. They don’t sit in the right plane. They’re tilted to avoid hitting the CH3 group. This sort of problem comes up a lot, and is called ‘steric hindrance’ or ‘steric distortion’. ‘Steric’ means, roughly speaking, that the effect is caused by the atoms taking up space. Two atoms can’t be in the same place at once, so one of them has to move out of the way.
We can only shuffle bonds around when things are in the same plane, so those two NO2s are out in the cold.

NO2 has some very efficient shuffling going on all by itself, though. There’s actually no way to tell which of the oxygens is N–O and which is N=O, because the shuffling means they’re both about a one-and-a-half bond. Nice and stable. The shuffling is how we get around what looks like a single bond to an oxygen – oxygen hates just having one bond, but one-and-a-half is okay. Nitrogen can maintain this arrangement because, while it really likes to have three bonds, it can cope with four by becoming positively charged. We ought to draw it ought with a ‘+’ on the nitrogen and a ‘-‘ shared between the oxygens

The explosiveness of TNT comes from all those oxygens, which speed up burning, and from the fact that when you burn this molecule you go from two solid molecules to fifteen gas molecules, which take up much more space. All that hot gas pushes outwards and give you an explosion.

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