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Arenes Chemistry Organic chemistry

Nitration of toluene

OK we are onto the final part of the lesson on aromatics, we need to consider the synthesis of TNT. It is interesting to note that non military and peaceful uses of explosives result in greater consumption / use. There are some explosives which are more suitable for military use rather than for mining. Years ago I visited a slate mine in England (Honister Pass) which was reopened by the late Mark Weir.

He was new to mining, and he attempted to mine a block of slate from the ine. He used plenty of black powder (gun powder) and he managed to break off a big lump of slate. The only problem was by using too much explosives he managed to reduce the slate block to small lumps which are close to worthless. At the mine he and his staff soon learnt that an Italian rock saw (nicknamed “the Italian job“) was a better method of freeing big blocks of slate.

I reason that TNT would be a dire explosive for slate mining, it is a rather fast explosive (detonation velocity 6900 ms-1) which has a high shattering ability (brisance). While TNT is not quite as shattering as RDX (8650 ms-1) would be it is far too shattering for slate mine use. For mining I think things like ANFO (4200 ms-1) or black powder would be more suitable when used in moderation. If I was a running a slate mine I would go for the black powder as the ANFO requires a rather strong boost charge to trigger it. If only a small amount of ANFO was to be used to loosen some rock then the boost charge will contribute a lot to the bang.

A word of advice do not play with explosives or experiment at home with them, the sentences for violations can include death or a serious injury resulting in permanent disability. These two harsh penalties can be dispensed by the energetic material, there is no appeal process. But after the energetic material has imposed its penalty then the criminal justice system may choose to impose a penalty.

This is no joke, some years ago some friends of mine at another university (University of Bleep) had a horrible event. A foolish student stole some chemicals from a teaching lab and illegally made a batch of an explosive. The student smuggled it out of the teaching lab and took it home. The student was later killed in an accidental explosion which destroyed the kitchen of the flat they were living in. Please do not bother asking me for more details as I will not answer that question.

The industrial synthesis of TNT is based on the reaction of a mixture of nitric and sulfuric acids with toluene. The reaction will occur in three stages, the first stage is the reaction of toluene with the nitration mixture to form the nitrotoluenes. From what we know already we should be able to understand how and why the main products of this reaction are 2-nitro-1-methyl benzene and 4-nitro-1-methyl benzene.

If we now consider the reaction of the 2-nitro-1-methyl benzene with the nitration mix. Before we start there are a couple of things to consider, there is something which is on my dirty washing list. Something which needs to slither back the swamp and crawl under a rock. Something which needs to never show its face again. I will let you look at it for a moment.

Bad bad bad, never let me see you drawing such a nitro group !

Now consider what is wrong, what is wrong is that we have a nitrogen to which we have five bonds. Now while things like sulfur and phosphorus (and more heavy p block atoms) can expand above and over the octet. Nitrogen can not do so, structure has a nitrogen which has five bonding pairs around it. This will not work well ! I know that the symmetry of this wrong nitro group might appeal to some people but it is still wrong.

Now we have seen it we should make it go away again, we should replace it with something far better. If we consider only the nitro group then this group of three resonance forms explains things better.

If we consider more of the resonance forms of nitrobenzene we can see that partial positive charges appear because of resonance on the carbons ortho and para to the nitro group.

Now lets look at the ortho-nitrotoluene. I calculated the partial charges on the carbons in the molecule, it is clear that those carbons which are ortho and para to the nitro group have partial positive charges while the carbons at the meta positions have slight partial negative charges.

This result suggests to me that the following resonance form makes a large contribution to the overall reality.

Now we should consider the reaction of the electrophile from the nitration mixture with 2-nitrotoluene. If we get it to react at the three position then we will go via the cation with the following resonance forms. The one on the left will have a very high energy, the nitro group will also be trying to remove electron density from the same carbons as were the positive charges in these three resonance forms are.

The formation of the other isomers of dinitrotoluene from 2-nitrotoluene will occur via the cations with the following resonance forms.

The last thing to do is to consider the order of reactivity. The reaction at the sites ortho and para to the nitro group will be slowed down by two things. These two things are the inductive pull of the nitro group on the electron density in the benzene ring and also the resonance withdrawal of electron density from both the benzene ring and the cationic intermediate by the nitro group.

The reaction at the positions meta to the nitro group will only be slowed by the inductive effect, so as the reaction. These positions are ortho and para to the methyl group. As a result the product of the second stage of the nitration of toluene will be a mixture of 2,4-dinitrotoluene and 2,6-dinitrotoluene.

By Mark Foreman

I am a Swedish / British chemist who works in Sweden at Chalmers University of Technology as an Associate Professor. I am based in the Chemistry and Chemical Engineering Department and I work in the Nuclear Chemistry / Industrial Materials Recycling units. My academic interests include things from both nuclear chemistry, recycling and some other unrelated things.

I originally come from the UK (England). I have degrees from Imperial College (BSc + ARCS) and the University of Loughborough (PhD).

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