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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Babcock_Hall on December 13, 2021, 11:55:42 AM
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Hi Everyone,
A common subject in forensic chemistry is explosives. One concept that arises is that of oxygen balance. One writes a balanced equation for an explosive compound assuming that all hydrogens are in water, all carbons are in carbon dioxide, and all nitrogens are in N2. One then calculated whether the number of oxygen atoms is less than, equal to, or greater than the number needed in the balanced equation. Based on an example, it seems that O2 is written on the product side with either a + or a - sign in front. If the formula of the explosive is CaHbNcOd, the the oxygen balance is d - 2a - 1/2b (Bell S, Forensic Chemistry, 2nd ed., p. 395; she acknowledges that this method has shortcuts). For TNT the number is -74.0%, and for NG it is +3.5%. These numbers are weight percents. Suzanne Bell wrote. "The oxygen balance gauges are useful for designing explosive formulations that come close to having a next zero oxygen balance."
In Chapter 5 of Forensic Chemistry: Fundamentals and Applications (Siegel J, ed), John Goodpaster also discussed the concept of oxygen balance on pages 189-191. He wrote that an oxygen balance of zero should achieve the greatest explosive power. He noted that one can combine explosives in a mixture to achieve an oxygen balance of near zero. Yet he also wrote that this is not the sole factor that is considered in formulating commercial mixtures of explosives.
Recently in a thread on the explosion of a mesylate in this sub-forum, the question of oxygen balance came up. I have two questions. One is how should other elements be treated in a balanced equation of a compound containing sulfur, for example? Two in evaluating the possibility of an explosion hazard in a novel compound, does the concept of oxygen balance provide any useful information? My very provisional answer to the latter question is that there is limited value.
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This calculations are usefull when you have nitro groups in the molecule, you can not use them for any molecule containing oxygen and carbon. Also if you have peroxide groups it can probably be used. You need a system where moving oxygen from one atom to another releases energy. If you have acetone peroxide, breaking the weak oxygen-oxygen bond and creating two carbon-oxygen bonds instead releases energy.
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Triacetonetriperoxide is an example of a peroxide-containing explosive, but I suspect that you already knew this.
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I missed your reply, please read my
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I think, with your formula, carbon dioxide would be a perfect high-explosive.
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Suzanne Bell wrote (p. 371) "When the oxygen balance is negative and relatively large, CO will form in preference to CO2. In other words the system is under oxidized and lean."
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@Babcock_Hall The oxygen balance is nothing new, but the equation you posted ("If the formula of the explosive is CaHbNcOd, the the oxygen balance is d - 2a - 1/2b") is not quite correct:
https://en.wikipedia.org/wiki/Oxygen_balance
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@Babcock_Hall The oxygen balance is nothing new, but the equation you posted ("If the formula of the explosive is CaHbNcOd, the the oxygen balance is d - 2a - 1/2b") is not quite correct:
Have you read the whole post?
she acknowledges that this method has shortcuts
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Suzanne Bell wrote (p. 371) "When the oxygen balance is negative and relatively large, CO will form in preference to CO2. In other words the system is under oxidized and lean."
Yes, this is my point. You want CO2 to form but if you dont understand that the oxygen should be bound weakly to a hetereoatom like nitrogen in you explosive material or be present as a peroxide bond you will end up with CO2 being a perfect explosive. Its just like saying that H2O would be very flammable. You need to analyze the structure before using this formula or it will be completely useless.
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Neither I nor anyone else that I know of is suggesting that oxygen balance is the only property to examine. Nor did I make the claim that this is a new concept. Let me return to an issue I raised in my opening post. Bell's Table 12.1 provides 34.0 39.2 as the oxygen balance values for ammonium perchlorate and potassium chlorate. Therefore, there must be a way to treat atoms besides C, H, O, and N.
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But you suggested to use this to analyze the di-mesylate and when I opposed to this you went on about the formula? Offcourse there are a lot of other explosives, lead azide etc. that is difficult to discuss in the same context as nitro-compounds but from the beginning it was about mesylates and nitrocompounds, do you understand why these to types of compounds can not be compared, with the formula? Why the formula is useless for mesylates?
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The reason I made a separate thread was because the reaction involving mesylates was beginning to look like an elimination (not a combustion), unless I am mistaken. That was not clear to me at the beginning of the discussion of mesylates.
This thread is intended to be a more general consideration of oxygen balance, with either practical or pedagogical implications. The title is a decent summary of what I thought was a good idea to discuss; this thread was never intended only to be about mesylates. I teach a smattering of combustion chemistry in my forensic chemistry course, and I was considering whether or not to bring in oxygen balance. The issue with mesylates is probably whether or not moving oxygen atoms to carbons to create some other form of sulfur is or is not thermodynamically favorable. Provisionally, I accept that this is not favorable for sulfur, but it would seem to be favorable for at least two oxides containing chlorine.
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Thanx, I am happy with that.
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I think, with your formula, carbon dioxide would be a perfect high-explosive.
Both Suzanne Bell (p. 372) and John Goodpaster (p. 189) indicated that one starts by writing a balanced combustion equation, then calculating oxygen balance. The equation CO2 :rarrow: CO2 is not meaningful; therefore, one should not calculate the oxygen balance at all.
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Both Suzanne Bell (p. 372) and John Goodpaster (p. 189) indicated that one starts by writing a balanced combustion equation, then calculating oxygen balance. The equation CO2 :rarrow: CO2 is not meaningful; therefore, one should not calculate the oxygen balance at all.
Agree, but the "combustion" is wrong word here.
2 C7H5N3O6 (TNT) → 3 N2 + 5 H2 + 12 CO + 2 C
Where is the oxygen ? I don't see any combustion here.
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Orcio_Dojek,
John Goodpaster used the word "decomposition," and Suzanne Bell used to word "combustion." The equation you wrote For TNT is different from the formalism presented by either author. Please refer to my opening post.
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As for formalism (for purpose of calculations) - OK. The oxygen balance assumes complete combustion (or decomposition if the oxygen is released) (C converts to CO2, H to H2O and so on...). Real detonation is the other thing.
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I think, with your formula, carbon dioxide would be a perfect high-explosive.
Both Suzanne Bell (p. 372) and John Goodpaster (p. 189) indicated that one starts by writing a balanced combustion equation, then calculating oxygen balance. The equation CO2 :rarrow: CO2 is not meaningful; therefore, one should not calculate the oxygen balance at all.
When you wright like this I am not sure that you understand what I mean. Do you understand what makes one compound a high-explosive and the other not? You dont need a formula to see if a compound potentially can be a explosive. Maybe I expressed my self a bit stupid with the CO2-example.
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rolnor,
I think that the CO2 example was a bit of a distraction, but I welcome all attempts to work toward a better understanding of this topic. Goodpaster defines explosives as "materials that undergo a rapid and energetic transformation into more stable substances, liberating large quantities of heat and gas." This is concise and covers all of the points that I can think of. I don't see the need to subdivide on the basis of low- versus high-explosives in this context; can you clarify? It might be worthwhile to subdivide explosives into categories based on the type of chemical transformation.
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I dont know, maybe high-explosive is just a word for me, I have not seen the term low-explosive. I am not a native.
Yes, it can be pedagogic to group the compounds in categories.
Its different for nitro/nitrat and peroxy compounds compared to heavymetal azides for example, the azides dont involve redox processes really.
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High- versus low-explosives are differentiated on the basis of the speed of the reaction front, specifically whether it is greater than or less than the speed of sound. One can imagine reactions which involve little or no internal redox chemistry (Loss of a carboxyl group as carbon dioxide is technically a redox reaction in that the carbon is going from +3 to +4), but which generate lots of gaseous products. Maybe azides fall into this category, but I have not looked into them in any depth. One could differentiate those from reactions involving nitro groups or peroxides, in which redox chemistry is important.
EDT
Perhaps one way to differentiate substances is whether or not bonds to oxygen are forming to new atoms in the balanced equation. This might differentiate nitro-type or peroxide-type reactions from a hypothetical reaction involving loss of carbon dioxide (or some other gas). The latter might (in the universe of possibilities) create much gas, but the oxygen atoms are staying bound to carbon.
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OK. I think in the case if peroxides, the oxygen atoms with broken oxygen-oxygen-bonds will form new bonds with carbon so there is some sort of redox. Its probably complicated to seee exactly what happens. It will be water forming also so its a mix of reactions I guess.