Keras
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Methyl iodide (iodomethane) woes (Red P method)
Folks,
I've been giving a crack at the synthesis of methyl iodide as described in the Chemplayer video (using 60 ml of methanol, 75 g of iodine and 15 g of
red phosphorus). Did follow the video step by step, comprising the ice bath. At no point did the mixture temperature rise above 20 °C during the red
P addition.
When I turned to distillation (using a short path apparatus), all went apparently well, with MeI passing at around 40°C as expected (collected a
yellow liquid, but I suppose that is normal). Except that I got only 30 ml out of the expected 70. After the 30th ml or so, the still-head temperature
rose to 60 °C, so I stopped heating in order to minimise methanol contamination.
In the round bottom flask, I'm left with a lot of unreacted red P powder, plus a lot of methanol/iodide mix, I suppose. So what could've gone wrong?
Did anyone already try this path to MeI? If you've any idea, I'll be happy to hear it!
TIA!
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Antigua
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And why did you expect 70ml? That's 0.88 moles and you used 0.3 mol iodine (in the end 0.6mol of methyl iodide could be expected). You got a yield of
0.38mol/0.6 mol which corresponds to 63% yield. That's pretty usual.
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Boffis
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The red P is used in excess, as Antigua says you used only 0.6 moles of iodine which requires only 0.2 moles of P, you used 0.5 moles. The theoretical
yield should have been about 83g CH3I but its density is 2.28 g/cc so that's less than 40ml. So you got a reasonable yield. You can filter off the
excess P and recover it.
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Keras
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Quote: Originally posted by Boffis  | The red P is used in excess, as Antigua says you used only 0.6 moles of iodine which requires only 0.2 moles of P, you used 0.5 moles. The theoretical
yield should have been about 83g CH3I but its density is 2.28 g/cc so that's less than 40ml. So you got a reasonable yield. You can filter off the
excess P and recover it.
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Yeah, I apologise for that message. I thought it over just after posting it, and figured it out (but it was late and I was too lazy to log back here
and delete it). I was carried away because in the video, ChemPlayer's team collects 60 ml 'of a pale yellow liquid'. But they don't use a still-head,
just a bent tube, so I guess they ended up with both MeI and MeOH in their final flask. However, they compute a yield based on that quantity, and
that's why I took for granted it was all MeI.
Chemplayer's method deliberately uses an excess of reactants in order to drive the reaction to completion (so they say). I'll try and recover whatever
phosphorus is left, good idea (even though I still have more than 400 g in stock, I hate wasting material). Any idea to recover the iodine dissolved
in the methanol, btw?
Thanks to both of you for your answers.
[Edited on 24-4-2021 by Keras]
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zed
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Red phosphorus is hard to come by in the USA. I don't know where you are, but you might find it difficult to re-up.
Iodine? Easy enough. Add cold water?
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Antigua
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I'd say - first evaporate as much of the MeOH as possible and then crash out with cold water. Will give better recovery.
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Oxy
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To get iodine from methanol you can simply add water and iodine should precipitate out (as it's water solubility is low and methanol is miscible with
water)
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Keras
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| Quote: Originally posted by zed | Red phosphorus is hard to come by in the USA. I don't know where you are, but you might find it difficult to re-up.
Iodine? Easy enough. Add cold water? |
Alright, thanks to everyone.
No, I don't live in the US, but in the EU (France). But you're right, something has happened with red phosphorus lately. Last time I ordered 250 mg of
it (April 2020), there were no special provisions whatsoever. But checking it right now I see its price has skyrocketed (like $90 for 250 mg, whereas
I paid like $20 a year ago). I'm going to ask what happened. Also you can always order some from Chemcraft ($24 for 250g).
Some countries have weird by-laws. Like, in France, you can't get methanol. However, it's legal to sell it in Belgium. Iodine also is in short supply
over here. I don't know why, and, sincerely, I've stopped trying to figure it out.
[Edited on 24-4-2021 by Keras]
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Texium
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I hope you mean g and not mg! 
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Keras
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Yeah, lol, sorry 
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Keras
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| Quote: Originally posted by zed | | Red phosphorus is hard to come by in the USA. I don't know where you are, but you might find it difficult to re-up. |
By the way, I just watched the NileRed about MeI and he said that filtering off the leftover red P killed his filter.
I have a grade 4 porosity sintered glass filter. Do you think it should escape scot free?
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Oxy
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Personally I would use a Buchner and filter paper. Cleaning glass sinter from red phosphorus can be very hard
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Keras
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| Quote: Originally posted by Oxy | | Personally I would use a Buchner and filter paper. Cleaning glass sinter from red phosphorus can be very hard |
Yeah, that seems reasonable. I’m not sure how to clean red phosphorus stuck in the sintered glass disk? Piranha solution?
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njl
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A funnel clogged with red p sounds like a pain... the phosphorus might slowly react to various oxides, which react with water to form soluble acids.
Reflux condenser?? I barely know her!
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Keras
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| Quote: Originally posted by njl | | A funnel clogged with red p sounds like a pain... the phosphorus might slowly react to various oxides, which react with water to form soluble acids.
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Apparently, adding iodine dissolved in methanol to form PI₃ then flushing with water should do the job, as PI₃ reacts with water to form soluble
HI and H₃PO₃.
Uh, I just read that PI₃ can reduce sulphoxides to sulphides. I wonder if I could use PI₃ to make dimethylsulphide from DMSO.
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Keras
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| Quote: Originally posted by njl | | A funnel clogged with red p sounds like a pain... the phosphorus might slowly react to various oxides, which react with water to form soluble acids.
|
I did recover a bit more than 10 g of red phosphorus. Filtered on the pump, giving the sintered glass a red tinge. However, I read in another thread
that pretty much everything can be washed off using bleach. Tried this, works like a charm.
However, my product disappeared :/ I mean, I had that slightly yellow MeI.
Washed it with sodium bicarbonate and a few crystals of sodium thiosulfate. It became clear. Then I washed it several times, collecting the lower
layer, but somehow I found the liquid sticky (it tended to stick to the glass, leaving blobs of it at each funnel drain). So I decided to back extract
it from the aqueous layer using DCM. At the end I mixed the DCM with the initial product and set up for distillation. But, eh, what a nitwit I was. I
forgot that MeI and DCM have about the same boiling point, so now I’m left with a solution of MeI in DCM. Any idea to separate them?
Thanks!
[Edited on 10-5-2021 by Keras]
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Triflic Acid
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Maybe do a fractional distillation with a really tall column, maybe like 4 or 5 stacked? Or, you could try adding a chloride salt, then boiling the
mix so that the chloromethane turns into a gas and condensing the DCM/Methyl iodide. Then do a finkelstiein on the chloromethane in acetone.
There wasn't a fire, we just had an uncontrolled rapid oxidation event at the power plant.
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Keras
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| Quote: Originally posted by Triflic Acid | | Maybe do a fractional distillation with a really tall column, maybe like 4 or 5 stacked? Or, you could try adding a chloride salt, then boiling the
mix so that the chloromethane turns into a gas and condensing the DCM/Methyl iodide. Then do a finkelstiein on the chloromethane in acetone.
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Thinking about it, I’ll attempt a fractional crystallisation. MeI should be solid in an acteone/dry ice bath, whereas DCM is still liquid at -77
°C. With a bit of luck, I can get MeI crystals, then separate them, and wash them with cold toluene (liquid at -77 °C, too). I would then be much
easer to get MeI from toluene, which have quite separate boiling points.
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Tsjerk
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Pure MeI is solid at those temperatures, but your MeI is in solution. I don't think you will be able to separate, have a look at melting point
depression.
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Keras
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| Quote: Originally posted by Tsjerk | | Pure MeI is solid at those temperatures, but your MeI is in solution. I don't think you will be able to separate, have a look at melting point
depression. |
Well – it did work to some extent. I definitely saw tiny purplish crystals. But it was very hard to get the DCM solution to cool, even when the
surrounding acetone/dry ice bath had reached -75 °C. I had to directly dump more dry ice into the DCM solution.
However, this turned out to be a total dud. Of course, I had refrigerated the Buchner somewhat in order to keep the crystals, but I obviously didn't
do it the right way (not enough, or I waited too long before filtering). Once poured over the sintered glass, the DCM boiled all its dissolved carbon
dioxide and the crystals were gone. Worst, at this very moment my vacuum aspirator decided to spit water out of the bucket, I jerked in surprise and
the Buchner ended up splashing all its contents on the ground. Bye bye MeI :/ (Hopefully I work in a very well ventilated old barn, so the clay ground
absorbed the liquid and it was all gone when I turned back from getting a coffee.)
Anyways, not dramatic but I’m a little grumpy because I lost 75 g of iodine. I still have a quite a lot in stock, but I hate losing reagents this
way.
Next time, I will be more clever and do MeI back extractions using toluene. Anyone out there, don’t extract MeI with DCM, lol
[Edited on 11-5-2021 by Keras]
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