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Author: Subject: Why are sulfuric acid ratios the same for nitrate salt and nitric acid?
aromaticfanatic
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[*] posted on 22-12-2020 at 09:44
Why are sulfuric acid ratios the same for nitrate salt and nitric acid?


I have been perplexed by this for years. Why is it that sulfuric acid amounts are similar or the same for nitrations using nitric acid as well as nitrate salts.

Shouldn't the nitrate salt require more sulfuric acid to form the nitric acid in situ and protonate it than if the sulfuric acid just has to protonate the nitric acid in a typical mixed acid bath?




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Antigua
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[*] posted on 22-12-2020 at 10:49


Are you looking at molar rations or gram ratios?
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aromaticfanatic
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[*] posted on 22-12-2020 at 13:17


Quote: Originally posted by Antigua  
Are you looking at molar rations or gram ratios?


molar and gram

for ETN people often use 15g erythritol and 100-120mL of sulfuric acid no matter if nitric acid or nitrate salt




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Fyndium
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[*] posted on 14-1-2021 at 11:42


I'm also curious about this. I consider reagent saving a big factor in reaction efficiency, and there seem to be some fluctuation in sulfuric acid amount used.

Some synthesis appears like it is used in well excess, as high as 35-fold in volume, but a better question is, what is the minimum amount that can be used without detrimental effect on yield?

I never liked the idea of whipping stuff together in order to obtain something desired. Instead, I love perfected processes where everything is used just the required amounts, and even better if some parts can be recycled, if worthy.

[Edited on 14-1-2021 by Fyndium]
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Sulaiman
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[*] posted on 14-1-2021 at 15:16


I assume that a major part of such a reaction is the capture of water by the sulphuric acid,
if you add a nitrate salt some sulphuric acid will be required to convert it to HNO3
If you add nitric acid then it is 31% w/w water, that needs to be 'removed' by the sulphuric acid.
I'll let others do the math.
As commercial sulphuric acid is cheap, excess is used.

I often use excess of one reactant, usually the cheap one to ensure that all of the more expensive reactant is used.

If you use stoichiometric quantities of all reactants,
the reaction will slow down as it nears completion due to dilution / molecules of reactants rarely meeting.




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Fyndium
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[*] posted on 15-1-2021 at 04:57


So it is practically always more economical to use nitrate salt, as nitric acid requires basically double step consumption of sulfuric acid.

If overhead stirring is used, can it be too vigorous? Considering the salt method tends to form a paste over the course.

[Edited on 15-1-2021 by Fyndium]
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stamasd
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[*] posted on 16-1-2021 at 01:44


Don't forget that the resulting salts (sodium or potassium sulfate/bisulfate, depending on which initial nitrate is used and in which proportions) also have an assisting effect in trapping water - especially the sodium one. They are not completely inert as to the nitration reaction and can help driving it forward by providing an amount of dehydrating effect.



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Boffis
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[*] posted on 16-1-2021 at 06:37


They aren't and you can't generalize aout such things but you are probably looking at ill designed and poorly tested procedures. There are many issues that affect the ratio of sulphuric acid to nitrating agent. Sulphuric acid acts as a solvent, dehydrating agent and as the generator of the nitrating species. One of the advantages of alkali nitrates is that the reaction with sulphuric acid generates sodium hydrogen sulphate and hydrogen nitrate; no water, so higher nitration species concentration (nitronium ions?). On the other hand the problem of the separation of the sodium salt from the mixture can be a problem as I reported recently in another thread. It was not a problem when nitration was carried out at higher temperatures because of increased solubility of the salt but this may not be an option in many nitrations. Your options are then to increase the amount of solvent (sulphuric acid) or turn to nitric acid. You have to design you nitration mixture to match the needs of the conditions you bare trying to achieve. This is a bit of a dark art but there is plenty of quality literature on the subject available (probably more than for any other procedure!!). Generally however, I find that unless you have a quality procedure to follow you have to do a bit of small scale trial and error.

Overhead stirrers certainly help with heterogeneous mixtures and in general the more vigorous the better. I recently posted on the nitration of p-dichlorobenzene with sodium nitrate; here an overhead stirrer with a propeller type stirrer is essential. In the thread on the nitration of bromobenzene I found that the addition of dichloromethane, more sulphuric acid and partial use of nitric acid in place of some of the sodium nitrate can be used to make the nitration mixture stirrable by a magnetic stir-bar which in my case is essential as the reaction is carried out under reflux, hence enclosed equipment. (I don't a sealed stirrer gland for the overhead stirrer).
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Fyndium
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[*] posted on 16-1-2021 at 06:46


Reflux does not mean enclosed, it just means you need a longer stirrer shaft. :P

Yes, there are number of variables here, and likely there are some generalizations of what dehydrates and how effectively.

Maybe I haven't had time to read enough of the forum, but there appears to be more of surface-scratching than actual studying and documentation of different procedures. This reminds me of when I was younger and into energetics - I was more interested in the results, not the kinetics - if x + y gets z, it will suffice. A library of studies and documentations would be invaluable to any chemist whatsoever, as tried and true methods make working and developing new things a lot more efficient. Here, it would mean trying out different amounts of reagents, different salts of nitrates (sodium, potassium, calcium, etc) and measuring if more solvent is needed or not, or does it affect the yield, etc. I'm not sure if there are inert solvents apart from sulfuric acid (inert may be poor choice of words per se) which would liquefy the reaction.
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Pyro_cat
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[*] posted on 19-1-2021 at 18:31


Isnt HNO3 kind of a nitrate salt itself ? HNO3 or KNO3 its still one atom stuck to an NO3
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katyushaslab
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[*] posted on 19-1-2021 at 18:42


Quote: Originally posted by Boffis  
They aren't and you can't generalize aout such things but you are probably looking at ill designed and poorly tested procedures.


A LOT of the nitration procedures for polyols/etc (think: mannitol, erythritol, pentaerythritol, glycerin) seem to be basically "folk knowledge" levels of optimisation/design, where one of two things happens:

1. some poster on some forum gets it working via trial and error with a decent yield and it gets copied around as gospel forever.

2. its the procedure found in "some book", gets posted/used, and becomes gospel. Often with the original reference lost. This may feed into "1".

As kind of an aside: with some nitrations, mixed-acid works better than straight-HNO3 not only due to dehydration effects, but also sulphonation of the thing being nitrated as an intermediate step. With others, the amount of sulphuric acid used is larger than needed because stirring/solvency...

[Edited on 20-1-2021 by katyushaslab]
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Fyndium
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[*] posted on 20-1-2021 at 13:03


There are some scientific research papers where ETN was prepared by various means, and they have descriptions of synthesis, so there should be legitimate sources for ratios and methods available with somewhat professionally optimized results.

Although, when performed in mmol scale, they tend to use huge excess of solvents mainly due to ease of handling. The actual productive scale would limit the amount of solvent a lot.
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