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Arrhenius
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Iodomethane prep.
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. Published procedures generally utilize phosphorus trichloride, which is generally unavailable to the
amateur. I have prepared iodomethane on 30mmol scale in 57% 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:
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
I combined potassium iodide (5.02g, 30mmol), methanol (24ml, 600mmol) and 85% phosphoric acid (40ml, 690mmol) in a round bottom flask and performed a
simple distillation at a relatively slow pace (condensor at -5ºC). At reflux, the salt is entirely dissolved and the reaction mixture is red. I
distilled until the pot residue turned clear and no further product came over. BP 50-65ºC. I dilluted the distillate with 10ml of dilute aqueous
sodium thiosulfate, removed the red lower layer with a pasteur pipet and dried it with calcium chloride.
Start
Finish
Product (calcium chloride floating on top)
Results
Crude iodomethane (2.42g) was obtained as a dark red liquid in 57% of theory based on potassium iodide.
Discussion
The product should be distilled to improve purity, but the scale presented here is prohibitively small. I plan to scale up the reaction. This same
procedure may be used to prepare alkyl bromides in excellent yield; procedures are widely available online.(ref. 3)
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_Org...
[Edited on 19-6-2009 by Arrhenius]
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Sauron
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The classical preps of MeI as per Org Syn are
1. The red-P catalyzed elemental iodination of methanol, which has the drawback of requiring a few gams of red P so often unavailable these days
2. Reaction of KI with simethyl sulfate. DMS is nasty stuff.
Your prep sidesteps these neatly so I look forward to your scaleup.
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entropy51
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Another method which requires neither P nor H3PO4 is as follows:
Dissolve KI in MeOH (solubility 1 gm per 8 mL) and gas with HCl while cooling and stirring. KCl, which is less soluble than KI, will precipiate out.
When saturated with HCl, distill off MeI with ice water in condenser. The MeOH that distills with the MeI can be washed out with small amounts of ice
water. The product is normally only slightly tinged with brown, but darkens on storage.
Yield is 50-60%.
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basstabone
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Entropy51: Does the HCl need to be anhydrous?
H3PO4 is very easy to obtain making iodomethane much more available to the home chemist. Thanks a lot Arrehnius!
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Nicodem
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Well done Arrhenius!
I only have two comments. I think you should measure the boiling point (you can use a test tube and a thermometer), just so that your contribution has
some analytical data to confirm product identity. I don't really have doubts that what you got is some fairly pure methyl iodide. But you know the
rules of science...
The other comment is actually a question. Can I move this thread to the Prepublication section? Alternatively, we can wait for you to do a larger
scale experiment with product distillation and all, so you make that one in Prepublication.
Using CaCl2 is a good idea since it removes the last traces of methanol as well. Store your MeI in a glass container with a teflon lined stopper. It
rapidly goes trough the usual polypropylene containers and stoppers. Also, keep it away from any residence places - it is after all a carcinogen to
treat will all due respect.
Some people had success in using alcoholic solutions of H2SO4 to prepare alkyl iodides from KI or NaI. Substantial oxidation to iodine occurs, but the
yields are still satisfactory.
Entropy51, are those your experimental results or do you have a reference for it?
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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Fleaker
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Arrhenius, good work! You will notice that your yield will go up when you scale it up. Also, that dark colour is dissolved I2 (shake it with
metabisulfite and it will clarify) formed because you didn't purge your system with argon.
This is a much better procedure than using 57% HI (which is hard to get and yields a little worse).
You would be wise to distill it again before use.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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benzylchloride1
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Great work, Arrhenius, I store my iodomethane over copper wire to prevent it from decomposing. Iodomethane quickly turns from colorless to a brownish
iodine like color with out the copper wire. Dont use mercury because iodomethane reacts with it to form methyl mercuric iodide in small quantities;
deadly poisonous.
[Edited on 19-6-2009 by benzylchloride1]
Amateur NMR spectroscopist
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Sedit
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Without a bisulfate or thiosulfate wash is one even able to determine the true yeild of MeI with any accuracy when the I2 is dissolved in it since
they only differ in molar weight by a factor of less then 20?
Nice prep Arrhenius can't wait to see more. Most preps using HI seem a little daunting to people such as myself due to the status of Iodine in todays
world so its nice to see that side steped abit
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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entropy51
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basstabone, I have not dried the HCl but rather used it directly from the generator containing NaCl + H2SO4.
Nicodem, those are experimental results. I have run the prep several times on a small scale, usually 20 gm of KI. I found a mention of it in one of
the 19th century textbooks, but I can't remember which one. I thought it might be Roscoe's Treatise of Chemistry, but it doesn't seem to be in there.
I will post a reference if I can find it, but it was not a detailed prep, just a sentence or two. Sorry for the lack of rigor, but it has worked for
me and I thought it worth passing along, especially since there seems to be no free iodine in the reaction or product.
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chloric1
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I remember one time going into the liquor store buying a fifth of everclear and taking it home and nearly gasing myself with ethyl bromide via the
alcoholic/H2SO4 sodium bromide process. Ah! Those youthfull days of dabbling.
Fellow molecular manipulator
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Arrhenius
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Nicodem: I'll report the boiling point when I scale up and re-distill the product. Or I can do a micro boiling point determination. I chose
phosphoric acid to avoid oxidizing the HI, but at this point I have no idea if it improves yields. I think a slightly more concentrated phosphoric
acid would be useful, but I'm not familiar with the physical properties here. I'll write a 'publication' once i've run it a couple of times. Feel
free to move it now if you'd like.
Fleaker: I'll try metabisulfite.
benzylchloride: yes, I added some copper shot to the product.
I shall take an IR to confirm the structure based on the ~1200cm^-1 band. Everyone satisfied with that? I can't seem to find a literature value for
this... but I'm exhausted. Maybe someone else can look if they're bored.
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Sauron
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The density is a useful constant and easy to take once you have a reasonable qty. A tared volumetric flask is all that is needed and a scale.
Refractive index is nice.
But you may not have a refractometer at hand.
I do not doubt that you have MeI there, what else can it be from those reactants, and is a heavy highly refractive colatile liquid colored by free I2?
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sparkgap
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There's a liquid film IR spectrum of methyl iodide @ SDBS; you can compare what you get with what's there.
Nice work, but wouldn't hot phosphoric acid react with the glass?
sparky (~_~)
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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Nicodem
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I think a constant boiling point taken from the second distillation in your planed upscaling experiment will be more than enough to satisfy the
scientific scrutiny. But if you would still want to take an IR there is a iodomethane spectra at SDBS.
I will keep this thread here so that new ideas are discussed, and you will open a full report in the Prepublication section later when you will have
the opportunity to do the scale up experiment. I have a suggestion for when you do that: use a distillation column, at least a simple Vigreux one or
anything you can get hold of.
The iodine content in the product can't really be relevant since many organic iodo compounds slightly decompose during distillation, but usually only
as much as to give them the dark colour. A small piece of copper or silver wire is indeed used to stabilize many organic iodides and bromides in
commercial products. Organic iodides also must be kept in dark.
Entropy51, thanks for sharing your experience. Gasing with HCl might not be particularly practical, but it is nevertheless a useful alternative.
Another alternative would be to first reflux NaHSO4 in methanol for the time required to form the sodium methylsulfate, then add NaI or KI and
fractionate methyl iodide as it forms. Sodium hydrogen sulfate is easily available, probably even more easily than H3PO4.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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woelen
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This looks very interesting and it is a thing I also would like to try. I have almost 1 liter of 85% H3PO4 (somewhat brown, technical grade). I can
even add a little P4O10 to remove more of its water.
Before I do so, however, I would like to know whether this does not attack the glassware. I have a micro distillation setup with 25 ml flasks and
coolers and I paid a lot for this and don't want to have the glass fogged, because it is attacked by the H3PO4.
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Nicodem
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H3PO4 only starts to seriously etch glass at >150°C (still some etching can be noticed even at 120°C already). Its methanolic solution obviously
can not do any damage to the glass due to the low reflux temperature and dillution. Woelen, instead of repeating this experiment, you could prepare
MeI by using P4O10 instead of H3PO4 (it is always better to make new alternatives than repeating old stuff). It might give a somewhat better yields,
but beware, the quenching of P4O10 by adding it to methanol is highly exothermic and cooling must be used at this stage. But still I would like to see
the comparison of results when using H3PO4 vs. P4O10.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Sauron
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While P4O10 is not particularly dear, it is quitw a bit costlier than tech. orthophosphoric acid. So any increase in yield would be more than offset
by cost, and MeI is already going to be a bit pricey due to the cost of KI or (a bit less) NaI.
This renders the use of the anhydride rather academic, wouldn't you agree? At least on a prep scale, and what else are we talking about here?
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ItalianXKem
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you remember acetone and bleach (sodium hypochlorite) = chloroform ?
that synthesis can be simile
iodomethane is CH3I
we have acetone (propan-2-one) CH3COCH3
2 NaIO + CH3COCH3 = Na2CO3 + 2 CH3I
but sodium hypoiodite is rare and to be have lot of money to buy this salt
but , iodium tincture (methanol , ethanol , KI and I2) is very easy to find and buy
first need to separate I to iodium tincture
second make te necessary compound of I
third reaction between compost of I and a alkylating agent
now I don't know
good luck !
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Sauron
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That reaction proces CHI3 not CH3I
I see you have not learned any chemistry since you were last here.
You cannot do a partial haloform reaction, the reaction will proceed as long as there are alpha hydrogens to replace.
Hypoiodite is never bought but is made in situ from Ki and NaOH
But you will never get anything but CHI3 - never CH3I.
[Edited on 20-6-2009 by Sauron]
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Nicodem
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Quote: Originally posted by Sauron  | While P4O10 is not particularly dear, it is quitw a bit costlier than tech. orthophosphoric acid. So any increase in yield would be more than offset
by cost, and MeI is already going to be a bit pricey due to the cost of KI or (a bit less) NaI.
This renders the use of the anhydride rather academic, wouldn't you agree? At least on a prep scale, and what else are we talking about here?
|
Well, since H3PO4 is industrially made by quenching P4O10 it would be a bit unusual for it to be any cheaper than its precursor. Actually the major
chemicals dealer S***a sells H3PO4 for 2.9 Eur per mol of P and P4O10 for 2.6 Eur. I think this fairly reflects the cost of hydration. But who cares
about the price difference? What does it have to do with the topic? Not to even mention that phosphorus pentoxide is an ubiquitous chemical (used as
general desiccant for vacuum desiccators) in every lab, while H3PO4 is a bit less common (though in many parts of the world its diluted solutions can
at least be bought in stores). But not even its availability is an issue here. The point is in that this forum is about art and science so
researching is the goal. And since Woelen has both phosphorus pentoxide and the motivation to do experiments, then why would he care about economics?
Woelen, keep in mind that if using P4O10 you must not use such a huge excess of methanol/acid as in the original post. You should use a 4 : 15 : 1
molar ratio of KI(NaI) vs. MeOH vs. P4O10. One mol of P4O10 can activate 6 mols of MeOH, 4 for sure, the rest depending on the temperature (because of
thermodinamic uphill due to the low pKa2 of phosphoric acid). So, using 4 eqivalents of the iodide would assure its complete conversion while using a
minimum of 15 equivalents of MeOH would make the reaction mixture less of a slurry and the quenching of P4O10 less dramatic (I would actually suggest
to use even more of MeOH to make the slurry easier to stir and the iodide salt more soluble). The reaction should occur at lower temperatures since
the methyl (poly)phosphates surely react faster with the iodide.
Arrhenius, next time you do this, if you have a two neck flask, please take temperature measurements of the reaction mixture as well, so that we can
see at which point the reaction proceeds optimally with MeI distilling over. If you don't have the required flask, then at least follow the oil bath
temperature.
Edit: Corrected the suggested KI(NaI) vs. MeOH vs. P4O10 ratio. Previously I forgot we are dealing with iodides and not HI.
[Edited on 20/6/2009 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Sauron
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If you want to make 25 g is matters a little, if you want to make 2,5 Kg it matters a lot. Picking an overpriced supplier like Sigma and then
normalizing on price per mol of P is rather arbitrary. Tech grade H3PO4 probably originatess from rock phosphaates industrially, aand you can buy a 20
L pail of it for the price of a Kg of lab grade P4O10. You know as well as I do that it used as rust remover snd is sold fr that in every hardware
store. Sigma doubtless is selling ACS grade. So who are you kidding?
I am osten criticized for being a spendthrift who buys reagents rather than make his own. Well, when I do make my own I don't want to spend more than
I could buy the same for.
There are lots of members who know how to squeeze a nikle till the buffalo shits, yet here you are telling me that being concerned with cost is
somehow cocnstrart to the forum's principles?
Really, that is the least tenable position I have ever seen tou take!
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Sauron
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I just looked at P4O10 on Acros and prices for various grades run from $61 a Kg for lab grade to $125 Kg for ACS analytical. Illogically 2.5 Kg of the
lab grade was $197 which is a lot more than 2.5 x $61, go figure.
Anyway what matters is the stoichiometry of the respective reactions and their yields, and only then can the cost of a mol of product per whatever
number of mols of the reactant in question were reqiored, be calculated and compared.
The reaction between H3PO4 and KI produces potassium phosphate and 3 moles of HI and the HI then reacts with methanol
The reaction between methanol and P4O10 produces I suspect a mixture of methyl phosphate esters and these esters react with KI to produce MeI and
potassium phosphates.
Both reactions appear to me to be sequential, anyway.
I will be surprised if the P4O10 reaction goes cleanly to O=P(OMe)3 asI would predict a sigmificant amount of the mono and di methyl esters so this is
going to complicate things isn't it?
[Edited on 20-6-2009 by Sauron]
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Nicodem
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| Quote: Originally posted by Sauron | | I will be surprised if the P4O10 reaction goes cleanly to O=P(OMe)3 asI would predict a sigmificant amount of the mono and di methyl esters so this is
going to complicate things isn't it? |
I never said that trimethyl phosphate must form. The only thing that is needed is to make the methyl group in MeOH electrophilic enough for it to
react with the iodide anion (which is pretty easy given the strong nucleophilicity of the iodide). Whether this is done by protonation (using H3PO4 as
reaction medium) or by phosphate ester formation (like when using P4O5) matter only to the reaction conditions and not in regard to the reaction
products. I can't see how the formation of a mixture of mono-, di- and trimethyl phosphates instead of just trimethyl phosphate could ever matter for
the reaction outcome. All methyl groups in all the esters are similarly electrophilic and all the esters are in dynamic equilibrium among themselves
and H3PO4.
| Quote: | | The reaction between H3PO4 and KI produces potassium phosphate and 3 moles of HI and the HI then reacts with methanol |
You can not obtain 3 mols of MeI from 1 mol H3PO4. The H3PO4 must be used in a huge excess as to be able to protonate methanol and the inorganic
product can not be any other phosphate but KH2PO4 (check the pKa1 to pKa3 of H3PO4). Note that Arrhenius used 23 equivalents of H3PO4 vs. KI. That was
not a bad idea given he used 85% H3PO4, but even with 100% H3PO4 you can not get beyond the theoretically allowed 1:1 ratio.
[Edited on 20/6/2009 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Arrhenius
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Sparkgap: Thanks for the reference. My IR doesn't use salt-plates or pellets, so I doubt I can reproduce that spectrum, but I'll see
what I can do. Would be fun, ya?
Nicodem: Thank you for the notes. For the first part of the reaction I had the thermometer extended into the reaction mix. The
solution temp is above 75ºC, probably 100ºC if I had to guess at this point. The oil bath was too hot to touch. I'll use two thermometers next
time. I had used a large excess of methanol because it does co-distill to an appreciable extent. When the product is worked up from
thiosulfate/metabisulfite the methanol is largely removed.
Sauron: I buy my reagents. However, concentrated phosphoric acid is available in some stores in the US. I seriously doubt KI is,
but this synthesis should be accessible to most amateurs who are un-paranoid 
enough to buy KI. My goal is only to beat the price of $80 for 50g of MeI. This is the price from the only place I could find it (that would sell it
to me). I think phosphoric acid can work pretty well here.
Woelen: Try following the OrgSyn prep a little more closely; the only caveat being that the temperatures you have to work with are
considerably lower. Let me know how it works. I would suggest atleast a 3x scaleup from what I did above. The amount of phosphoric acid and
methanol I used was arbitrary. I used technical phosphoric acid, and I doubt the brown color in yours will cause any problems.
P.S. I particularly like the catalytic hydrogenation used to prepare hexanediol in the OrgSyn prep. Can't wait to have a hydrogenation bomb that can
do that!! haha!
[Edited on 20-6-2009 by Arrhenius]
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Sauron
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All right, Nicodem, H3PO4 is cheap enough to employ in such a large excess. Particularly the technical grade is. The lab grade is not so bad, the ACS
grade is 2X lab grade and not called for here IMO. So
H3PO4 (excess) + KI -> KH2PO4 + HI
CH3OH + HI -> CH3I + H2O
And we have Arhennius' yield and it is still pretty clear that despite the large excess of acid the major cost is the KI as long as one ddoes not piss
money away using ACS acid..
So, can you propose stoichiometry and any excess if required for the reaction using P4O10 instead of H3PO4?
In particular what is molar ratio between P4O10 and MeOH?
Will preformed trimethyl phosphate react with KI to produce MeI and inorganic phosphate?
[Edited on 20-6-2009 by Sauron]
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