Arrhenius
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Preparation of Iodomethane
Iodomethane is widely used as an 'active' methylating agent for the methylation of carbon, nitrogen, oxygen, sulfur and phosphorus. It can also be
used to prepare the Corey-Chaykovski reagent for cyclopropanation and epoxidation. Published procedures generally utilize phosphorus and iodine, which
are generally unavailable to the amateur. I have prepared iodomethane on 400mmol scale in 81% yield by direct halogenation of methanol.
Introduction
Alkyl halides can generally be prepared from the corresponding alcohol by treatment with the hydrogen halide.(ref. 1) The hydrogen halide may be
prepared in situ from its salt and a strong acid.(ref. 2) Phosphoric acid was employed to produce HI (Note 1):
H3PO4 + KI → HI + KH2PO3
While this equilibrium does not favor HI, it can be driven by consuming HI in the following dehydration reaction:
HI + ROH → RI + H2O
This reaction is entropically driven by the removal of the alkyl iodide.
Experimental
**Caution! Iodomethane is a probable carcinogen and is highly volatile!**
All reagents used were technical grade. I combined potassium iodide (66.4g, 400mmol), methanol (120ml, 3mol) and 85% phosphoric acid (175ml, 3mol) in
a round bottom flask and flushed the apparatus with argon (note 2). I performed a simple distillation at a relatively slow pace over a period of
approximately 3 hours (condensor at -5ēC). A clear product with a boiling point of 55-60ēC distilled over when the reaction mixture reached 90-100ēC.
- the potassium iodide was entirely dissolved and the reaction mixture was red. I continued heating until the distillate temperature rose to 70ēC, at
which point the reaction should be stopped. I dilluted the distillate with 30ml of cold water and transferred it to a separatory funnel (note 3). I
removed the lower layer and dried it with anhydrous calcium chloride. A small amount of copper was added to stabilize the product for storage (note
4).
Results
Crude iodomethane (45.83g) was obtained as a light red liquid in 81% of theory based on potassium iodide. Its IR spectrum is consistent with
published spectra.
Discussion
The product obtained in this scaled up procedure is of sufficient purity to use in subsequent reactions. The red color may be avoided by promptly
stopping the reaction when the distillate temperature rises above 65ēC. Fractional distillation of the product would also improve purity. Analagous
procedures may be used to prepare alkyl bromides in excellent yield; procedures are widely available online.(ref. 3)
Notes
1. Sulfuric acid should be avoided in the preparation of hydrogen iodide because it can oxidize the iodide ion.
2. Inert atmosphere is optional, but appears to improve purity and perhaps yield.
3. The obtained organic phase may be washed with sodium thiosulfate and/or sodium bisulfite to remove free iodine from the product.
4. Copper helps to absorb iodine resulting from the slow decomposition of the product. As with other alkyl halides, iodomethane should be stored in
the dark.
References
(1) Sidgwick, NV "The Chemical Elements and Their Compounds" p.1183 (available in sciencemadness library)
(2)Organic Syntheses, Coll. Vol. 4, p.323 (1963); Vol. 31, p.31 (1951). Link
(3) http://www.books-about-california.com/Pages/Experimental_Organic_Ch...
[Edited on 6-7-2009 by Arrhenius]
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CMOS
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nice work, i'm waiting for more.
why condenser is green??
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DJF90
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PC coolant in recirculating system I suspect. Nice work Arrhenius. Perhaps a short fractionation column would help rid the orange/red colour?
[Edited on 6-7-2009 by DJF90]
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Sedit
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I was thinking that it was antifreeze.
Arrhenius is the H3PO4 lab grade or otherwise? I have been looking for an over the counter source but so far either highly dilute H3PO4 or navel
jelly(worthless) is all I have been able to find.
Nice work. I have been toying with a simular synthesis of Bromoethane but using H2SO4 and NaBr with mixed results due to oxidation of the HBr from
H2SO4.
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UnintentionalChaos
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Very nice work. I can get a small quantity of 85% H3PO4 in the near future and will give isopropyl bromide a try by the same route.
Quote: Originally posted by Sedit  |
Nice work. I have been toying with a simular synthesis of Bromoethane but using H2SO4 and NaBr with mixed results due to oxidation of the HBr from
H2SO4.
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I was having some spectacular failure at that same reaction using isopropanol. The quality of H2SO4 I have to use can't be helping either.
smuv must be magical: http://www.sciencemadness.org/talk/viewthread.php?tid=10758
[Edited on 7-6-09 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
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Arrhenius
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All: The condensor is green because I use antifreeze and a circulating cooling bath. It gets to -10C on a good day, and the ethylene glycol won't
freeze. Looks high-tech too
DJF90: I suspect a fractional distillation would work quite well (as stated in the 'discussion'), but I'm not worried by the color. A highly pure
sample will start to turn redish-brown fairly quickly after the bottle has been opened a few times. I'm going to try some O-methylation shortly, I
suppose that'll be a good test.
Sedit: The phosphoric acid is technical grade. I know of one place to get it OTC, which is hydroponics stores. There are pH down products which are
conc. phosphoric with some phosphate salts in it. I highly doubt the salts will interfere with the reaction. As for ethyl bromide, I would suggest
you follow the prep. in the link at the end of my first post. I've used it to make upwards of 500g of EtBr in one run. Using H2SO4:H2O really
mitigates oxidation of bromine. I mean, the reaction is red, but the losses are negligible. I presume NaBr is OTC for you.
Unintentional Chaos: I've made quite a lot of ethyl bromide, and since sodium bromide is OTC (spa bromine source) I would suggest that you use
sulfuric acid and water to do the reaction. See the link above for the prep of ethyl bromide and follow that. Keep in mind that elimination to
propene may be a competing reaction if you heat too much or use too concentrated an acid catalyst. Purity of the sulfuric acid is not really an
issue. I've used brown drain cleaner, no problem.
[Edited on 6-7-2009 by Arrhenius]
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Sedit
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Thank you for the link Arrhenius. Yes NaBr is very over the counter in 52 gram packs for about $1.50 with 99% purity at Wal-Mart. It can be had
cheaper but I have yet to find larger containers with equal purity. I have been trying a slow addition of the H2SO4 in an ice bath to try to avoid
over oxidation but I feel H2O is a must if for nothing else to help dissolve the NaBr and bring it into solution.
As to not lead such a fine threed astray i'll pm you.
[Edited on 6-7-2009 by Sedit]
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Theophrastus_2
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Brilliant work arrhenius!
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Magpie
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Sedit: 58 wt% H3PO4 can be made from OTC materials. See
http://www.sciencemadness.org/talk/viewthread.php?tid=2923&page=2#p...
Arrhenius: In your top picture you show a tube leading to the RBF in the foreground. Is this for the argon? If so, did you use a continuous purge,
or just an initial flush?
[Edited on 7-10-2009 by Magpie]
The average person will go to any length to avoid thinking. -- Thomas Edison
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Arrhenius
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Magpie: Yes, it's an inert gas (argon) line. I purged it before I began heating, but that was it. On the two runs I made, it didn't seem to effect
yield, and perhaps only slightly effected the color of the product. With or without it, I think this is a robust method of preparing good purity
iodomethane. I can no longer do chemistry at home, but I sure would like to see someone take a stab at making allyl bromide *hint hint hint*. PM me
if you're interested.
If anyone repeats this synthesis, I would appreciate hearing about your yield, etc.
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crazyboy
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How easily can methyl iodide be substituted for other methylating agents such as dimethyl sulfate?
Just because you\'re not paranoid doesn\'t mean they\'re not out to get you.
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Sedit
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You are turning methylating reagents into something generic. There are pros and cons of many things and it depends on the reaction at hand you plan on
using it for.
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crazyboy
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| Quote: Originally posted by Sedit | | You are turning methylating reagents into something generic. There are pros and cons of many things and it depends on the reaction at hand you plan on
using it for. |
2,5-dimethoxybenzaldehyde from 2-hydroxy-5-methoxy benzaldehyde.
http://www.freepatentsonline.com/6670510.html
http://www.usdoj.gov/dea/programs/forensicsci/microgram/journal2004...
Just because you\'re not paranoid doesn\'t mean they\'re not out to get you.
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bahamuth
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My workup.
Methyliodide, Iodomethane.
To a 250ml RB flask in a cold water bath there was added 19g Methanol PA grade, 4.0 grams Phosphorus reagent grade.
Then there was prepared a condenser fit for reflux onto the RB flask and through it there were added 50.0 grams elemental iodine in portions so to not
cause boiling, the addition took about 5-7 minutes.
When the addition of the iodine were complete, the flask with the condenser still attached for reflux and cold water running through it, was left
stirring for about an hour or so before it was set on a hot water bath for gentle reflux for another half hour or so, before it was taken off the bath
to reach RT.
Quickly the flask was set up for simple distillation with the same condenser , with the distillate coming over into a 125ml sep. funnel cooled in a
Griffin beaker with icy cold water, again to ensure that no product escapes. The distillation was ended when no more came over to the sep. funnel.
The crude distillate, brown, milky, from iodine and water brought over by the MeI, was washed twice with 0,1M NaOH, top layer removed by decantation,
so it was clear and colorless, and once with RO water before it was transferred to a clean, dry RB flask with about 5-10 grams of anhydr. Calcium
chloride prills. This was left for 10 minutes or so to ensure all water was removed by the calcium chloride, and then set up for a final distillation
on hot water bath, with a brown preweighed flask for receiver. The RB flask was distilled to complete dryness for, yet again, ensuring no loss of
product. The product was perfect clear with no color.
This procedure should yield over 80%, my first try I got ≈47.5 grams which is ≈84.9%.
My second run, with three times the amounts listed above, yield rose to ≈89% (149.3 grams), as to why I do not know, perhaps a larger system
gives better yields.
Finally the product was stored over around .5 grams powdered Cu metal to act as stabilizer in a brown black painted bottle with Teflon lined cap.
From 200 grams of iodine, 197 grams of MeI came over, with ≈223.7 grams as 100%, giving an overall yield ≈88%.
BTW, red phosphorus can be obtained from vendors who sell "bulk" crystal iodide on ebay, ask if they got a website...
[Edited on 3-11-2009 by bahamuth]
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DJF90
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Interesting work up. Why do you wash with NaOH? This seems strange to me; I would work up the reaction using a sodium thiosulfate wash followed by a
brine wash. This should remove all the residual iodine, and most of the water in your product. Drying can then be commenced over CaCl2, although I
would probably use potassium carbonate myself. But nice prep, and nice yeild.
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bahamuth
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I washed with 0.1M NaOH because I found this old student assignment on the net somewhere, cannot remember where now though, but here is the original
writeup:
129. Preparation of Methyl Iodide from Methyl Alcohol and Phosphorus Iodide (Sections 49, 204).
- In a 200 cc. round-bottomed flask place 15 grams of methyl alcohol and 3.2 grams of red phosphorus.
Have ready a reflux condenser with cork attached.
Place the flask in cold water and add in small portions at a time 38 grams of iodine;
the addition should take about 10 minutes.
If the contents of the flask begin to boil, attach it to the reflux condenser;
when reaction ceases add more iodine.
Finally attach the flask to the condenser and let it stand for at least 4 hours (preferably over night).
Distil through a condenser from a water-bath, as long as any liquid passes over.
The receiver should be placed in cold water as methyl iodide is very volatile.
Wash the distillate by decantation with a dilute aqueous solution of sodium hydroxide
until the lower layer is colorless, and then once with water.
Separate the methyl iodide carefully from the water using a separatory funnel,
and transfer it to a small distilling flask.
Add about 10 grams of anhydrous calcium chloride.
Stopper the flask, place a cork over the end of the side-arm, and set aside until the liquid is quite clear.
Place a thermometer in the flask, connect the latter with a condenser, and distil from a water-bath.
Note the boiling-point and weight of the methyl iodide.
Calculate the theoretical yield from the iodine used (why iodine?),
and the percentage of this obtained.
Methyl iodide boils at 44°, and has the specific gravity 2.27 at 15°.
The yield in the experiment should be about 80 per cent of the theory.
Note. - An excess of the alcohol is usually taken in preparing alkyl halides by the method illustrated above.
The phosphorus and iodine are used in the proportions necessary to form phosphorus tri-iodide, PI3.
Also did this workup synthesizing propyliodide with a little less yield, I think, could not find my papers just now to confirm.
[Edited on 3-11-2009 by bahamuth]
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entropy51
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Nice job bahamuth, but for some of us that would be a waste of iodine and phosphorus, both of which are watched, especially on ebay. MeI can be made
using NaI and H3PO4 or gaseous HCl, which are more accessible and safe to order or make. http://www.hms-beagle.com was also selling MeI recently.
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Arrhenius
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That procedure using NaOH is in Reference 3 from my initial post. In my opinion, the phosphoric acid method still represents the safest, most
economical way to prepare iodomethane.
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