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Reasoned runaway
Reasoned runaway Utter Urban
I
P. G. Urben is an industrial chemist currently working with fine chemicals. He is also the editor of Bretherick’s Handbook of Reactive Chemical Hazards.
1074-9098/02/$22.00 PII S1074-9098(01)00295-7
am often surprised to see reports of explosion that appear to regard explosions as events beyond chemical reason. Yet they certainly do not escape the laws of thermodynamics. Every explosion has to start somewhere, at low energy and temperature, the sort of conditions we study chemical kinetics and reaction mechanisms under, before it reaches such temperatures that all chemical bonds are torn asunder and the freed atoms reassemble themselves in lower energy configurations. In the lab, perhaps even on a small industrial scale, we are a little concerned with explosives that demand a booster containing hundreds of grams of tetryl for initiation. These kinds of materials we can handle safely, even when we ignore their explosive nature unless, that is, we add a catalyst that sensitizes them. It is the sensitive materials, that run away from mild initiation, that we must fear and those at least start their misbehavior with reactions having intelligible mechanisms. Though we should never regard mechanistic theory as the truth, it is a guide to thought that may allow us to predict risk. If you can draw a plausible intramolecular mechanism leading to weakening or rupture of the energy containing bonds, you are more likely to face a risk than if not. Considerable investigation of reaction mechanisms has been made where commercial, or even more, military explosives are concerned and it behooves us to know something of them. They are applicable more widely. A recent report that also shows the value of safety testing, inspired these reflections.1 During scale-up of a pharmaceutical synthesis, it was found that a mononitronaphthalene, 4-methy-8-nitronaphalene-1-sulfonyl chloride was shock sensitive and also capable of runaway to deflagration from around 40°C. Even mononitrobenzenes, with far less padding, are not that sensitive; indeed we generally assume
that a mono-nitroaromatic is safe. Also, the system is too deactivated to form polysulfones, so heating itself initially. I freely admit I would not have predicted this sensitivity, yet I really should have been able to do so. Draw it out. The name does not tell you, but the nitro and sulfonyl groups are close in space and displacement of chloride by one of the oxygens of the nitro group looks plausible. That gives you a salt, but this can lose a proton from the methyl group to give a species where the concealed nitro group has a quinimino structure, with a C ⫽ N⫹ unit. Now it is strongly suspected that such species are on the road to explosion for most nitro-explosives. Thus, TNT may well deflagrate if washed incautiously with alkalis, that are strong enough to deprotonate its methyl group, allowing a quinimine structure. Nitromethane salts, that have lost a proton and can have the aci-nitro structure, are dangerous; until we reach tankcar quantities, nitromethane itself is not. Both TNT and nitromethane are dangerously destabilized by contamination with the more basic amines. However, we should never assume that molecules have read our textbooks. It is difficult to see how a nitrocubane can have an aci-structure, yet heptanitrocubane still has a very acid proton and deflagrates on contact with pyridine, to which the unprotonated, but even higher energy, octanitrocubane is immune.2 References 1. Miller, R. A., et al., J. Org. Chem., 2000, 65(5), 1399. 2. Eaton, P. et al., Angewand. (Int.), 2000, 39, 401.
© Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Science Inc.
35
I
P. G. Urben is an industrial chemist currently working with fine chemicals. He is also the editor of Bretherick’s Handbook of Reactive Chemical Hazards.
1074-9098/02/$22.00 PII S1074-9098(01)00295-7
am often surprised to see reports of explosion that appear to regard explosions as events beyond chemical reason. Yet they certainly do not escape the laws of thermodynamics. Every explosion has to start somewhere, at low energy and temperature, the sort of conditions we study chemical kinetics and reaction mechanisms under, before it reaches such temperatures that all chemical bonds are torn asunder and the freed atoms reassemble themselves in lower energy configurations. In the lab, perhaps even on a small industrial scale, we are a little concerned with explosives that demand a booster containing hundreds of grams of tetryl for initiation. These kinds of materials we can handle safely, even when we ignore their explosive nature unless, that is, we add a catalyst that sensitizes them. It is the sensitive materials, that run away from mild initiation, that we must fear and those at least start their misbehavior with reactions having intelligible mechanisms. Though we should never regard mechanistic theory as the truth, it is a guide to thought that may allow us to predict risk. If you can draw a plausible intramolecular mechanism leading to weakening or rupture of the energy containing bonds, you are more likely to face a risk than if not. Considerable investigation of reaction mechanisms has been made where commercial, or even more, military explosives are concerned and it behooves us to know something of them. They are applicable more widely. A recent report that also shows the value of safety testing, inspired these reflections.1 During scale-up of a pharmaceutical synthesis, it was found that a mononitronaphthalene, 4-methy-8-nitronaphalene-1-sulfonyl chloride was shock sensitive and also capable of runaway to deflagration from around 40°C. Even mononitrobenzenes, with far less padding, are not that sensitive; indeed we generally assume
that a mono-nitroaromatic is safe. Also, the system is too deactivated to form polysulfones, so heating itself initially. I freely admit I would not have predicted this sensitivity, yet I really should have been able to do so. Draw it out. The name does not tell you, but the nitro and sulfonyl groups are close in space and displacement of chloride by one of the oxygens of the nitro group looks plausible. That gives you a salt, but this can lose a proton from the methyl group to give a species where the concealed nitro group has a quinimino structure, with a C ⫽ N⫹ unit. Now it is strongly suspected that such species are on the road to explosion for most nitro-explosives. Thus, TNT may well deflagrate if washed incautiously with alkalis, that are strong enough to deprotonate its methyl group, allowing a quinimine structure. Nitromethane salts, that have lost a proton and can have the aci-nitro structure, are dangerous; until we reach tankcar quantities, nitromethane itself is not. Both TNT and nitromethane are dangerously destabilized by contamination with the more basic amines. However, we should never assume that molecules have read our textbooks. It is difficult to see how a nitrocubane can have an aci-structure, yet heptanitrocubane still has a very acid proton and deflagrates on contact with pyridine, to which the unprotonated, but even higher energy, octanitrocubane is immune.2 References 1. Miller, R. A., et al., J. Org. Chem., 2000, 65(5), 1399. 2. Eaton, P. et al., Angewand. (Int.), 2000, 39, 401.
© Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Science Inc.
35