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peach
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Quote: Originally posted by entropy51  | Methyl iodide can also be made by gassing a saturated solution of KI in methanol with HCL gas.
I've not been able to find the ancient reference in which I found this method, but I have used it and it does work. I'd been thinking that I should
post something on it since it does not seem to be generally known. |
It'd be worth putting that up here, as I expect it works better than the sulphuric idea.
The wiki page needs editing to swap sulphuric for phosphoric, preferably with a note about sulphuric giving a much lower yield and a redirect to the
HI page.
The hydrogen chloride method could also go on there.
| Quote: Originally posted by Sedit | Peach if you do not have phosphoric acid perhaps you could use your Sulfuric and react it with Toluene to make TsOH. This is a non-oxidizing acid that
should work rather well in this synthesis. Its solid so its easy to weigh and handle.
Its funny this thread was just brought up because I just came in from setting up an electrochemical cell with EtOH, NaBr and NaOAc to see if I can do
a Kolbe coupling to produce Methyl bromide. Just another day of me tossing things together and playing around. Once of these days I swear I will take
chemistry seriously  |
Part of the reason for me not immediately trying all of the methods (such as the hydrogen chloride) for methyl iodide was that I was interested in
making all of the iodides rather than looking at just the one - as per the pokemon mantra, catching them all.
So I've been busy making some iodoform today, but I should have sublimated the iodine I recovered from the above post, as the resulting iodoform
wasn't too nice to look at. I'll probably redo that tomorrow.
And methylene iodide is up next. One option being to use HI with the iodoform or the Finkelstein swap with DCM. Got two tubs of KI, but NaI would be
better. I've either got to make some or buy it.
I do have phosphoric and phosphorus, but your TsOH suggestion for methyl iodide is good for those who don't. I started trying to make benzene
sulphonic acid two weeks ago or so, but that needs redoing too as the first try was a mess. I obtained black as my primary product.
[Edited on 3-1-2012 by peach]
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UnintentionalChaos
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Repetition of procedures is always welcome, right?
I ran this prep, but with 500mmol (83.00g) of KI instead of 400mmol and identical amounts of other reagents. Distillation flask was a 500ml RBF.
Condenser and recieving flask were chilled with/in ice water. Recieving flask contained 50ml of ice water from the start to help slow evaporation.
Distillation was slow and took 1hr, 40 minutes. The stillpot was maintained at ~85-90C until the end when it rose to 100C. Distillation was stopped
when the stillhead reached 67C.
A reciever was swapped and for interest, distillation was maintained for an additional 20 minutes. A few drops of extremely dark material collected
under the water. The vast majority of the distillate was miscible (methanol, I would assume).
The biphasic distillate was placed in a 250ml seperatory funnel and a chilled solution of 2.04g of anhydrous CaCl2 in 10ml of distilled water was
added to speed clearing of the aqueous phase. The upper phase was discarded and the lower, pale red organic phase was washed with 50ml of ice-cold
distilled water containing ~1g of NaCl and a spatula tip of sodium metabisulfite. On shaking, the red color immediately vanished and the lower phase
became colorless. The organic layer was transferred to a 50ml RBF and allowed to stand over anhydrous CaCl2 for a few hours in a fridge. Gravity
filtration through a tiny plug of cotton afforded 56.73g of methyl iodide as a dense, colorless, volatile liquid. This is a 79.9% yield from KI.
This is an extremely reliable and easy procedure. The yield is great, provided you take steps to avoid evaporative losses, and it needs no phosphorus
or iodine.
Storage is preferably at -20C or in ampoules. A tiny amount of precipitated copper powder will keep it colorless for months/years at these temps. I
made mine with acidified CuSO4 solution and zinc granules. I've had a considerable amount of MeI evaporate from tightly-sealed teflon-lined vials (at
-20C!) when stored for several months/years. Make as needed to avoid having money evaporate on you...
[Edited on 6-18-14 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Scr0t
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It's even more tolerant than that!
You don't need to monitor temperature and you can just distill until it stops (still pot <=200'C) and it can be distilled more quickly too
with no significant impact on yield i.e. +80%.
It can be done with 75% P2O5 that's solid at room temperature with the same yields.
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Quatro
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Does anyone have experience making MeI by gassing a saturated solution of KI in MeOH with HCL gas?
Or how about refluxing Methanol, Phosphoric Acid, and Sodium Iodide?
NaI + H3PO4 → NaH2PO4 + HI
CH3OH + HI → CH3I + H2O
CH3OH + H3PO4 + NaI → CH3I + H2O + NaH2PO4
(Not sure if that other equation is correct, but the key point is that HI can be made with Phosphoric acid and sodium Iodide. HI and Methanol form
Methyl Iodide)
Anyone have suggestions?
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S.C. Wack
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Sur la préparation des éthers iodhydrique, bromhydrique et méthyliodhydrique.
Par J. E. de Vrij, professeur de chimie à Rotterdam.
Quoique le procédé de M. E. Soubeiran pour la préparation de l’éther iodhydrique publié dans ce Journal, t. XXX, p. 5, donne des résultats
très-satisfaisants, il ne sera peut-être pas sans quelque intérêt de connaître le procédé dont je me sers depuis plus de trois ans, dont le
principal mérite consiste en ce qu’on n’emploie pas le phosphore et qu’on peut préparer des kilogrammes d’éther à la fois sans le moindre
danger. Pour préparer l’éther iodhydrique, on commence par saturer l’alcool de gaz acide hydrochlorique sec. Si l’alcool est privé d’eau,
autant que possible et bien refroidi il peut dissoudre entre 0,5 à 0,6 de son poids de gaz acide hydrochlorique. Après avoir constaté la quantité
d’acide hydrochlorique contenu dans l’alcool, on introduit une quantité déterminée d’iodure de potassium pulvérisé dans une cornue, et on y
verse une quantité d’alcool contenant assez d’acide hydrochlorique pour former avec le potassium du chlorure de potassium. Le lendemain on
distille et on obtient une quantité d’éther équivalente à la quantité d'iodure de potassium employé. Cet éther qui ordinairement ne retient
pas du tout ou très-peu d’iode libre, est ensuite lavé et rectifié comme dans tous les autres procédés. La seule condition pour obtenir un
résultat satisfaisant, c’est de faire absorber par l’alcool la plus grande quantité possible de gaz acide hydrochlorique, ce qui se fait plus
facilement en hiver, ou en plaçant l’alcool dans un mélange réfrigérant pendant qu’on y introduit le gaz.
Éther méthyliodhydique. — Cet éther s’obtient de la même manière et encore plus facilement. Aussitôt qu’on ajoute l’esprit de bois
saturé de gaz acide hydrochlorique sur l’iodure de potassium, la réaction commence immédiatement et le mélange s’échauffe, de sorte que si on
prépare de grandes quantités de cet éther il sera prudent de verser le liquide par parties sur l’iodure alcalin, afin de prévenir une trop
grande élévation de température. L’esprit de bois bien purifié, d’une densité de 0,798, peut absorber 0,6 de son poids de gaz acide
hydrochlorique.
Éther bromhydrique. — Cet éther s’obtient aussi très-facilement en distillant à une douce chaleur 4 parties de bromure de potassium
pulvérisé avec 5 parties d’un mélange de 10 parties d’acide sulfurique concentré et de 5 parties d’alcool de 96° G. L.
Journal de pharmacie et de chimie 169 (1857)
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JJay
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Entirely out of academic curiosity - is it possible to make MeI by reacting aluminum, methanol, and elemental iodine? I assume you'd need a really
cold reflux column.
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Amos
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| Quote: Originally posted by JJay | | Entirely out of academic curiosity - is it possible to make MeI by reacting aluminum, methanol, and elemental iodine? I assume you'd need a really
cold reflux column. |
Actually, I have done this, and it would've been much more successful if I had taken more steps to prevent evaporation or MeI escaping from my reflux
apparatus. I got about a 25% yield but was able to recover over half of the starting iodine, so I can probably attribute the biggest drop in yield to
problems with the equilibrium, such as not using anhydrous reagents. If you want to know more, Chem Player on YouTube has a video on making ethyl
iodide in a pretty good yield using ethanol instead of methanol. This is what I was basing my methyl iodide synth on.
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JJay
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| Quote: Originally posted by Amos | | Quote: Originally posted by JJay | | Entirely out of academic curiosity - is it possible to make MeI by reacting aluminum, methanol, and elemental iodine? I assume you'd need a really
cold reflux column. |
Actually, I have done this, and it would've been much more successful if I had taken more steps to prevent evaporation or MeI escaping from my reflux
apparatus. I got about a 25% yield but was able to recover over half of the starting iodine, so I can probably attribute the biggest drop in yield to
problems with the equilibrium, such as not using anhydrous reagents. If you want to know more, Chem Player on YouTube has a video on making ethyl
iodide in a pretty good yield using ethanol instead of methanol. This is what I was basing my methyl iodide synth on. |
I saw Chem Player's video... its pretty interesting. I was considering making some EtI or EtBr last week. The yields in the video aren't that bad but
are too low for my purposes given the high cost of iodine.
Using anhydrous reagents and a drying tube is often the difference between a low yield with lots of side products and a quantitative yield... but I
don't think the yields would be that much higher than Chem Player's even if everything was anhydrous.
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alimonium
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I'm curious why such an excess of phosphoric acid is used ? I'm thinking the extra ethanol helps the iodide distill instead of sitting at the bottom
of the flask, but why so much acid? Would yields suffer if stoichiometric amounts were used ?
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Chemi Pharma
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I have a study that claims the better way to produce alkyl iodides from alcohols, ethers or even alkenes is using an alkaline iodide
(sodium/potassium), with 95% phosphoric acid, giving 90-95% yield.
Since 95% phosphoric acid isn't an easy reagent to find and buy at the market, the study teaches how to produce it from the comercial and cheap 85%
phosphoric acid and P2O5.
You know that P2O5 added to phosphoric acid increase it's concentration up to more than 100% and means the way to produce even polyphosphoric acid,
widely used in organic cyclizations and acylations.
Here's the paper:
Attachment: Iodides from clevage of ethers or iodination of alcohols and alkenes with KI + H3PO4.pdf (328kB)
This file has been downloaded 1404 times
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NaK
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| Quote: Originally posted by alimonium | | I'm curious why such an excess of phosphoric acid is used ? I'm thinking the extra ethanol helps the iodide distill instead of sitting at the bottom
of the flask, but why so much acid? Would yields suffer if stoichiometric amounts were used ? |
I tried it stichiometricly and almost didn't work at all. I got like 5 grams from expected 60 grams of iodomethane.
That makes sense as there are two equilibrium reactions going on:
KI + H3PO4 <-> KH2PO4 + HI
HI + CH3OH <-> CH3I + H2O
The more water and the less phosphoric acid, methanol and iodide are in there the slower the reaction will proceed, to a point where it's basically
just methanol coming over.
I didn't have a flask big enough matching the destillation setup i wanted to use so instead of having a large methanol excess I added magnesium
sulfate as drying agent to the mixture. Removing water will also drive the equilibrium. Worked pretty well and I got liquid coming over at 50C.
That also implies to use the highest concentration of phosphoric acid you can find, 85% is common. As concentrated phosphoric acid is pretty
hygroscopic having a concentration even higher might improve the yield.
A good indicator how well this reaction works is the temperature of the distillate. If it rises to 70C too early you can be pretty sure that something
is wrong.
[Edited on 16-4-2020 by NaK]
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Cou
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very nice i might try this x)
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Tsjerk
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Very nice and easy preparation! I'm running it at the moment. I did notice however there are copious amounts of methanol coming over with the
distillate even when running at such a pace the distillation head is kept at around 43 degrees. The product is coming over with a drop every 2/3
seconds at this temperature. Okay... I don't use a fractionating column and the ambient temperature at the moment is 31 degrees, so little to no
reflux going on, but still.
Why I noticed is the following: I'm collecting over a column filled with CaCl2 and the methanol is dissolving the CaCl2. In the collection vessel it
is salted out (solubility of CaCl2 in methanol is 20/100 gr/gr at 0 degrees...). Not a problem, as it will be easy to separate, but be aware. I'm
preparing the MeI for a grignard, in which methanol would be quite a pain.
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NaK
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That's why you need such a large excess of MeOH
[Edited on 1-9-2020 by NaK]
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DReed123
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I'll just mention that the linked study in the above post regarding 95% phosphoric acid is legit. In it the authors found insane increases in yield
with the 10% increase in phosphoric acid concentration. This follows from thermodynamics because water is a product of the reaction. It's probably a
good idea to toss a dessicant like mag or sodium sulfate in the reaction vessel to help drive the reaction towards the right as well.
[Edited on 4-9-2020 by DReed123]
[Edited on 4-9-2020 by DReed123]
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clearly_not_atara
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I think the phosphoric acid is the desiccant. Using an excess of phosphoric acid makes sense -- you want to maximize the efficiency of utilizing
iodide, which is far more expensive than the phosphoric acid or your alcohol, most likely.
What I don't understand is why we are concentrating 85% phosphoric acid by adding P2O5 rather than just heating it. It will eat away at glass but it
should be fine to dehydrate phosphoric acid in stainless steel.
Sulfates, I think, would not be good desiccants here because they are (weakly, but still) basic and a little oxidizing.
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DReed123
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Phosphorus Pentoxide is the anhydride of phosphoric acid. You can safely add it in small portions if you put the phosphoric acid in the freezer first.
Toslyic acid would likely be ideal as well. Good point on using the sulfates. That's actually why phosphoric acid is used instead of sulfuric
acid..too oxidizing for iodide. For some of the alkyl halides the authors found yield doubled in switching from 85% to 95%, which is pretty insane.
[Edited on 8-9-2020 by DReed123]
[Edited on 8-9-2020 by DReed123]
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Mateo_swe
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Is it possible to use NaI instead of the KI (same mol%) in the original procedure (first post) without changing any other parameters?
If one adds some phosphorus pentoxide to OTC 75% phosphoric acid, can the increased concentration of the phosphoric acid be determined by some simple
means like density measurement or must a titration be made?
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SuperOxide
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| Quote: Originally posted by Mateo_swe | | If one adds some phosphorus pentoxide to OTC 75% phosphoric acid, can the increased concentration of the phosphoric acid be determined by some simple
means like density measurement or must a titration be made? |
Yeah, you should be able to. If you add too much then it just starts to turn into PPA (Polyphosphoric Acid), which I could be wrong but I don't think
that would be too big of a deal. PPA just turns back into phosphoric acid once you add some water to it.
You could get pretty close to 100% just by adding the proper molar amount though. That's what I would do (then maybe just use density to verify the
concentration, roughly)
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Keras
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IINM, 100% H₃PO₄ is a solid with m.p. around 40 °C.
It cannot be obtained from commercially available grades (75, 85%) because the acid ‘polymerises’ before being entirely dehydrated. In any case,
it boils away before reaching the anhydride form.
So you can probably reach 100% phosphoric acid by addition of phosphorus pentoxide, but be warned that:
1. P₂O₅ is hard to work with, especially because the phosphoric acid that forms tends to form a syrupy layer covering the unreacted material and
preventing it to react further
2. The resulting material will be solid and very difficult to stir
Now, I’m a bit skeptical about this, because lately I tried to get HBr out of a mix of phosphoric acid and NaBr that I heated. Any evolved gas was
led into a bubbler full of water. I heated quite a bit (but not high enough to endanger the glass), got dew on the flask, and quite a lot of bubbles,
but they turned out to be only air, since the water in the bubbler was only very slightly acidic when I stopped the ‘reaction’ (pH ~ 6). So far,
for me, phosphoric acid is not strong enough to evolve HBr in appreciable quantities from NaBr. Now I agree in this experiment the few molecules of HI
produced are consumed right away in the substitution reaction, which drives it forward, but I’m still surprised it works so well.
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Boffis
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Do you really need such strong acid? If the acid is merely a catalyst and the product is exceedingly volatile you can probably us a large excess of
rather dilute phosphoric acid and K or NaI and add a small excess of methanol, heat to 70 C and continually fractionate off the methyl iodide. See
Fery's thread on the reactive distillation of ethyl formate. I have tried this and found that you can distil ethyl formate out of the mixture until
the formic acid content is only 5%!! The only advantage of high strength acid is that it increases the reaction rate. If you distil it very slowly you
can get ethyl formate at even this low concentration of acid as I report in Fery's thred.
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clearly_not_atara
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| Quote: Originally posted by Keras | IINM, 100% H₃PO₄ is a solid with m.p. around 40 °C.
It cannot be obtained from commercially available grades (75, 85%) because the acid ‘polymerises’ before being entirely dehydrated. In any case,
it boils away before reaching the anhydride form. |
Unless there's some way to stabilize it that I haven't
heard of, any phosphoric acid exists in equilibrium with its oligomers:
2 H3PO4 <> H2O + H4P2O7
H3PO4 + H4P2O7 <> H2O + H5P3O10
etc, see for example https://link.springer.com/content/pdf/10.1007/BF00648228.pdf
This equilibrium is the same regardless if the phosphoric acid is dehydrated by heating or by adding anhydrides. I stand by my original supposition
that adding P2O5 to phosphoric acid is just wasting P2O5. But I think Boffis may be right about the concentrated acid being unnecessary.
You could also use TsOH so that you never worry about damaging glassware.
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Keras
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| Quote: Originally posted by clearly_not_atara | Unless there's some way to stabilize it that I haven't heard of, any phosphoric acid exists in equilibrium with its oligomers:
2 H3PO4 <> H2O + H4P2O7
H3PO4 + H4P2O7 <> H2O + H5P3O10
etc, see for example https://link.springer.com/content/pdf/10.1007/BF00648228.pdf
This equilibrium is the same regardless if the phosphoric acid is dehydrated by heating or by adding anhydrides. I stand by my original supposition
that adding P2O5 to phosphoric acid is just wasting P2O5. But I think Boffis may be right about the concentrated acid being unnecessary.
You could also use TsOH so that you never worry about damaging glassware. |
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.
I wonder if protecting the glassware with a silver mirror would be an effective treatment against etching, both in hot phosphoric acid and fused NaOH
cases, such as in the decarboxylation of benzoic acid to get benzene.
Also this free article.
Attachment: Phosphoric Acid on Glass.pdf (321kB)
This file has been downloaded 376 times
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Boffis
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I have been thinking about this a bit more and I checked up on the pKa values for various acid. For practical purposes the value for HI is 0 but the
first base of phosphoric acid is 2.15 and pKa2 is 7.2 so the concentration of free HI in the mixture of KI and phosphoric acid is going to be very
low. The only way this reaction is going to work is to continually remove the methyl iodide as it forms and this may also explain why the acid
concentration needs to be so high. Tosylic acid is a much stronger acid but if you were to use it to replace the phosphoric acid you would have to add
more than one molar equivalence to the KI use to liberate the HI as the reaction proceeds.
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Keras
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AFAIK, TsOH has pKa -2.7, so definitely the reaction KI + TsOH → TsOK + HI should be next to complete.
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