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Author: Subject: Preparation of Iodomethane
Texium
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[*] posted on 10-5-2022 at 07:11


Quote: Originally posted by Keras  
AFAIK, TsOH has pKa -2.7, so definitely the reaction KI + TsOH → TsOK + HI should be next to complete.
Not sure what’s leading you to that conclusion, considering HI has a pKa of -9.3.

[Edited on 5-10-2022 by Texium]




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clearly_not_atara
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[*] posted on 10-5-2022 at 08:12


Quote: Originally posted by Keras  

I would have to read the article you provide a link to, but from what I gathered from the article in attachment, the equilibrium is not reached unless the temperature is raised. Besides, true orthophosphoric acid doesn’t seem to etch glass. The effect is caused by hot polymerised acid.

Ah, yes, refutation by "I don't feel like reading it", the highest form of argument.

While you apparently don't want to read my articles, I am quite capable of reading yours. Of course, it doesn't say anything about the stability of orthophosphoric acid at temperatures below 300 C. But it does refer to the original source by van Wazer, which you also didn't read. That's okay, because van Wazer just references earlier work by Bell, which is attached. Here's the last sentence of the article:

"No difference in composition was found between strong phosphoric acids prepared by heat and those prepared by the addition of phosphorus pentoxide"

So, as I previously emphasized, you are simply telling people to waste P2O5 for no good reason whatsoever.

While pure orthophosphoric acid cannot be prepared under standard conditions, Bell also finds that (nearly) pure crystalline pyrophosphoric acid apparently can, by dehydrating to just the right composition and cooling. This may have advantages for storing and measuring?

Attachment: bell1948.pdf (564kB)
This file has been downloaded 22 times





[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 11-5-2022 at 04:48


Looking at the strength of HI I now understand the problem of liberating it from an alkaline iodide with acids.

This immediately raises the obvious question of "why not reduce iodine to HI in the presence of methanol". This is essentially the red phosphorus route. The well known problem of obtaining red phosphorus is the reason it is little used. There are, however, other reducing agents such as hydrogen sulphide and possibly organic compounds like ascorbic acid that might work. Hydrogen sulphide certainly does and since the liberated sulphur is not volatile and the product can be easily distilled out of the reaction mixture how about this idea:

Dissolve iodine in excess cold methanol and pass in hydrogen sulphide until the iodine has been decolourised and the mixture turned milky yellow. Warm slowly to reflux for say a couple of hours and then fractionate. The excess methanol can be recovered by simply filtering, neutralize and distill. The low boiling distillate should be neutralized too and then fractionated again.
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Keras
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[*] posted on 11-5-2022 at 05:20


Quote: Originally posted by Texium  
Quote: Originally posted by Keras  
AFAIK, TsOH has pKa -2.7, so definitely the reaction KI + TsOH → TsOK + HI should be next to complete.
Not sure what’s leading you to that conclusion, considering HI has a pKa of -9.3.

Well, the preceding message quoted a pKa of 0, I didn't bother to check (though I must admit I was a bit surprised because of all the halogen acids, HI is reputedly the strongest. But the wording of the message made me think that this theoretical consideration was just, like, theoretical.)

[Edited on 11-5-2022 by Keras]
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[*] posted on 11-5-2022 at 05:27


Quote: Originally posted by clearly_not_atara  

Ah, yes, refutation by "I don't feel like reading it", the highest form of argument.


Lol. No, don’t worry. I was just a bit busy when I answered. And then I moved on and forgot about it. My bad.

Quote: Originally posted by clearly_not_atara  

"No difference in composition was found between strong phosphoric acids prepared by heat and those prepared by the addition of phosphorus pentoxide"

So, as I previously emphasized, you are simply telling people to waste P2O5 for no good reason whatsoever.


Oh, no. Quite the contrary in fact, since 100% ‘phosphoric’ acid would be quite impossible to handle anyway, being solid.

And PS: This time I promise I’ll read your reference!

[Edited on 11-5-2022 by Keras]
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[*] posted on 11-5-2022 at 08:16


Keras: Crystals of H3PO4 can indeed be obtained by various means, as described e.g. by Ross and Durgin 1925:
https://pubs.acs.org/doi/pdf/10.1021/ie50190a031

However, in all cases they are prepared by simply cooling and/or seeding a solution, and when the crystals melt, an equilibrium mixture is quickly established. Phosphoric acid and diphosphoric acid exist as (sort of) pure substances in the solid state, but not as liquids.

Quote: Originally posted by Boffis  
Looking at the strength of HI I now understand the problem of liberating it from an alkaline iodide with acids.

It's true that TsOH is about a million times weaker than HI, and H3PO4 another hundred thousand times weaker still, but this doesn't completely preclude distilling HI for a couple of reasons. For one thing, when you use concentrated acid, the solvent isn't water anymore, and all of the dissociation constants change. For another thing, vapor pressures are roughly exponential with temperature, so HI will be more than a million times as volatile as TsOH.

It should also be noted that the diphosphoric acid, H4P2O7, has a pKa1 of about 0.97, making it more than ten times as strong as orthophosphoric acid (pKa1 = 2.12). Obtained from (using pKa = -log(Ka) / log(10)):
https://cdnsciencepub.com/doi/pdf/10.1139/v54-024

[Edited on 11-5-2022 by clearly_not_atara]




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 11-5-2022 at 09:24


Quote: Originally posted by clearly_not_atara  
Keras: Crystals of H3PO4 can indeed be obtained by various means, as described e.g. by Ross and Durgin 1925:
https://pubs.acs.org/doi/pdf/10.1021/ie50190a031

However, in all cases they are prepared by simply cooling and/or seeding a solution, and when the crystals melt, an equilibrium mixture is quickly established. Phosphoric acid and diphosphoric acid exist as (sort of) pure substances in the solid state, but not as liquids.


I wonder if you can obtain pure phosphoric acid as a byproduct of, say, bromination of ethanol using phosphorus and TsOH.

Quote: Originally posted by clearly_not_atara  

It's true that TsOH is about a million times weaker than HI, and H3PO4 another hundred thousand times weaker still, but this doesn't completely preclude distilling HI for a couple of reasons.
[…]


As I said before, I tried to make HBr out of NaBr and 85% phosphoric acid and despite heating to circa 200 °C and eliminating a lot of water vapour, I didn't get an ounce of acid, even if it should have distilled at 122 °C.
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[*] posted on 11-5-2022 at 09:53


^Theory is worse than practice :D

But I think you'd have a better shot with TsOH. You can, for example, produce HCl (g) from hydrochloric acid and CaCl2, and the latter is barely even acidic.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 11-5-2022 at 10:45


Quote: Originally posted by clearly_not_atara  
^Theory is worse than practice :D

But I think you'd have a better shot with TsOH. You can, for example, produce HCl (g) from hydrochloric acid and CaCl2, and the latter is barely even acidic.


You mean by dehydrating it?

TBH, when I dismantled the flask I heated, there was a strong indication of some acidic gas floating over the phosphoric acid. It was just too thin to make it to the wash bottle. Do you think a silver mirror made out of silver nitrate and Fehling's solution could protect the glass and allow pushing the temperature higher?

I have small quantities of KI and tosylic acid over here. I can try at least qualitatively to validate the process, using xylene as solvent (for high boiling point).


[Edited on 11-5-2022 by Keras]
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[*] posted on 13-5-2022 at 08:29


I just happened to mix TsOH and KI in a beaker on a back of an envelope today. Experiment was made with max. a dozen of milligrams of both TsOH and KI.
First try in xylene. Bad choice. Neither TsOH nor KI seems to be soluble in it. I was not expecting KI to dissolve, but TsOH stubbornly stayed solid. Nevertheless, the crystals slowly took on a brown tinge, and the xylene a slight pink tinge (dissolved iodine?).

I discarded the xylene and carried on with acetone. TsOH dissolved at once in it. KI didn't, or even-so-slightly, considering my acetone is technical and must contain water. The solution slowly turned orange and then brown. I let the acetone evaporate and was left with a deep ochre goo. I was expecting iodoacetone to form (H⁺ action on the enol form of acetone). There was an unmistakable lacrimatory compound in there, in very small quantities, though I can’t say if it’s simply HI or iodoacetone. But it definitely was eye-watering.

In both cases, I⁻ appeared to be oxidised to some extent. Whether it is by air oxygen or TsOH, I don’t know. But betting on TsOH as a non-oxidising strong acid seems risky.
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[*] posted on 13-5-2022 at 19:26


Interesting. HI is a very strong reductant. It can actually reduce benzene to cyclohexane (which rearranges to methylcyclopentane).

Bis(oxalato)boric acid is very strong and non-oxidizing. It is prepared by combining boric acid and oxalic acid in a 1:2 ratio. This patent says it has a pKa of -0.2 in DMSO, making it stronger than HBr (compare the Bordwell table)

https://patents.google.com/patent/DE10108608C2/en
https://organicchemistrydata.org/hansreich/resources/pka/#ka...

I don't think xylene will be a good solvent for any of this, it is very nonpolar. Acetone, maybe, though self-condensation is still a possibility. I don't know if mesityl oxide is lachrymatory, but it could have formed under your conditions.

I think acetonitrile might be ideal. Even HI has a pKa of just 2.8 in MeCN:

https://d-nb.info/1227454171/34

[Edited on 14-5-2022 by clearly_not_atara]




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 13-5-2022 at 23:33


Quote: Originally posted by clearly_not_atara  

Interesting. HI is a very strong reductant. It can actually reduce benzene to cyclohexane (which rearranges to methylcyclopentane).


Darn, to reduce an aromatic compound, it must be quite strong indeed. And why would cyclohexane rearrange to a five-member cycle?

What I can hardly figure is how you can actually produce HI out of KI and H₂SO₄ given than sulphuric acid’s pKa is actually less than HI’s.

Quote: Originally posted by clearly_not_atara  

Bis(oxalato)boric acid is very strong and non-oxidizing. It is prepared by combining boric acid and oxalic acid in a 1:2 ratio. This patent says it has a pKa of -0.2 in DMSO, making it stronger than HBr.


Interesting. I have both oxalic and boric acid, though not at hand r.n. I might have a crack at it later this month, though.

Quote: Originally posted by clearly_not_atara  

I don't think xylene will be a good solvent for any of this, it is very nonpolar. Acetone, maybe, though self-condensation is still a possibility. I don't know if mesityl oxide is lachrymatory, but it could have formed under your conditions.


You’re right. I discounted aldol condensation, because I considered it being promoted by base rather than acid, but it seems both work as catalyst. The compound clearly tickled the eye and the smell was pungent. That’s all I can say. I will probably order a microscale glass kit later this year, so as to be able to carry out small experiments with more quantitative results.

Quote: Originally posted by clearly_not_atara  

I think acetonitrile might be ideal. Even HI has a pKa of just 2.8 in MeCN.


Then I’ll try with MeCN. Same deadline. I’ll let you know, if you or someone else don’t do it before.

And big thanks for all the reference material I never had heard of, and which is now safe and sound on my hard disk!

[Edited on 14-5-2022 by Keras]
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