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woelen
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Interesting to see that Nicodem's results are positive. The only main differences with my experiment are
1) Nicodem only used 12 ml of methanol, while I used 20 ml
2) Nicodem left the solution stirring for 1 hour before doing the distillation
3) The addition of P4O10 was done in an ice bath, while I did not do that and my liquid became quite hot.
Which of these three could be the essential difference? I have the feeling that the 1 hour stir before attempting the distillation is the most
important, but the heat of the dissolving of P4O10 also could have had negative impact on the experiment.
Good to see that this reaction is possible with P4O10 and MeOH. Maybe I'll try next weekend again in the late evening (it needs to be fairly dark,
because I do this experiment outside and don't want daylight on the reaction mix, it is getting dark here not before 22.30 and good twilight is
reached at 23.00).
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Sauron
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N. already told you that the rapid mixing of the P4)10 and MeOH in absence of cooling is the likely culprit, as evidenced by the discolorayion you
noted.
I'd advise chilling the methanol and mixing slowly in an ice bath.
The question of lag time is trickier. One way to resolve it is to prepare two identical mpxtures, one you react immediately and the other only after
an hou
Yjy not try the method of Stone et al? 90%
32 g MeOH (1 mol) 40 ml
300 g NaI (2 mols) or 333 g KI.
c.300 g 95% H3PO4 (2.96 mols - calculate exactly)
Reflux 6 hours on water bath, I suggest an ice-brine Dewar condenser for reflux and a cold trap to snag any MeI vapor that gets past that.
It ought to be clear that this reaction dies not depend on solvating the iodide in methanol.
[Edited on 23-6-2009 by Sauron]
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Nicodem
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Actually I don't have a clue on why Woelen's experiment failed. For one thing, I do not believe the one hour stirring at room temperature does
anything important. The rule of thumb about kinetics says that the rate of most reactions doubles every 10K, which essentially means 4 minutes at
60°C does the same thing as 1 hour at 20°C. I'm also far from being sure that the overheating of methanol during the P4O10 quench is to blame for
the failure.
Woelen, in my experiment there was no noteworthy oxidation of HI to iodine. This is also kind of logical given that almost no HI is ever present in
the reaction mixture (unlike in the acid catalysed method using H3PO4). Also the reaction apparently starts at a lower temperature, which additionally
means less oxidation. Since MeI does not decompose nearly as easily under the influence of light and oxygen as HI does, you don't have to worry about
sunlight.
I think the yield could be optimized to something like 70%. Up to 10% were surely lost due to mechanical losses from working on a small scale, while
some more could be obtained if I would heat some more.
Here are a few references for other MeOH -> MeI transformations:
de Postis, Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, 223 (1946) 82: uses Zn, I2 and HCl
at 20°C
Vogel, Journal of the Chemical Society (1943) 643: with HI
Dangjan, Zhurnal Obshchei Khimii, 10 (1940) 1671: uses Fe and I2
Dangjan, Zhurnal Obshchei Khimii, 11 (1941) 617 (CA, 1941, 6925): uses Mg and I2 at 40-50°C
Dangjan, Zhurnal Obshchei Khimii, 11 (1941) 1215 (CA, 1945, 4050): uses Al and I2
Fischer, Beckenkamp, Journal fuer Praktische Chemie/Chemiker-Zeitung, 340 (1998) 58-62: uses SO2 and I2 in MeOH
PS: I only now noticed link posted by Entropy to an old thread, where it appears that other forum members prepared MeI via H3PO4 or H2SO4 years ago. I
now remember that old thread, but somehow I did not remember it when I read the report by Arrhenius.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
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woelen
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The more I read and think about this, the more I have the feeling that actually I did make quite some MeI. Given the fact that my colorless liquid was
much less flammable than methanol and that it became milky when water was added I start to think that I actually had quite some MeI, but mixed with
methanol.
I was forced in thinking that the experiment failed, because
1) the thermometer readout of the vapor was between 63 and 64 C.
2) the liquid was perfectly colorless
| Quote: | | Woelen, in my experiment there was no noteworthy oxidation of HI to iodine. This is also kind of logical given that almost no HI is ever present in
the reaction mixture (unlike in the acid catalysed method using H3PO4). |
Arrhenius' liquid was brown and I was indeed also expecting a brown liquid in my receiver flask, but now I think of it, I understand why this need not
be the case when the distillation is done in almost darkness and without acid present.
Unfortunately I burned away all of my colorless liquid, because I considered it of no value, so I cannot test it anymore  .
I'm certainly going to retry next weekend, I have plenty of NaI and MeOH and sacrificing another 3 grams of P4O10 is worth the experience I gain from
this. I had the good luck that I could buy 400 grams of NaI for just EUR 5 + shipping off eBay from some one-time seller who only received a single
bid (my bid  ).
[Edited on 23-6-09 by woelen]
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Sauron
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I have the Vogel paper from Nicodem's list, this is a usefulgeneral guide to alkyl halides for amassing a list of constants.
Vogel's general procedure for iodides is on a 500 mmol scale and employs constant boiling HI in qty 300 g and a 6 hr reflux. While the yields with
ethanol and higher are quite good (85-90+%) that for methanol is 60%.
Unless we are to assume some special mechanistic aspect, I suppose the low yield may be due to evaporative losses.
Compare to the method of H,Stone and H,Shechter (J Org Chem, 15m 491 (1950) posted upthread. They make HI on situ from 2 mols KI and 3 mols 95% H3PO4
per mol ROH and reflux 6 hours just like Vogel but trport 90% yield with many alcohols and glycols (but did not do methanol or ethanol). Their main
observation was that 95% H3PO4 worked much better than 85%.
We could prepare HI azeotrope, mix 2 eq of that with 1 eq 95% H3PO4 abd 1 eq methanol and either reflux on a water bath 6 hrs or run the reaction in a
Parr pressure bottle in water bath, then let cool and finally chill, transfer and distill. The latter procedure precludes evaporative loss.
This replicated Stone and Schecter without all that KH2PO4 ppt.
[Edited on 23-6-2009 by Sauron]
Attachment: jr9430000636.pdf (1.3MB)
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Sauron
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Take note of this elegant prep of anhydrous HI from I2 and tetralin at 200 C, 95% yield and 99.5% purity
Flaky solid iodine of 40 g was dissolved in tetrahydronaphthalene (Tetralin) of 160 g charged in a flask of 500 ml at 40°C to prepare a
tetrahydronaphthalene solution of iodine. A flask of 500 ml was charged with tetrahydronaphthalene of 40 g and heated to 200°C. while stirring. The
iodine solution prepared above was continuously added thereto over a period of 2 hours while maintaining the above temperature to react them. Crude
hydrogen iodide gas generated as the reaction went on was introduced into a 10% sodium hydroxide aqueous solution of 1 liter to absorb the whole
amount thereof. A weight change in this aqueous solution was measured with the lapse of time, and the end point of the first reaction was set at the
point where the change thereof was not observed. The yield of the crude hydrogen iodide was 94.6%, and the purity thereof was 99.5% or more.
US Patent 5,693,306
The HI could be bubbled into chilled MeOH - I will try to look up the solubility. Refluxing (if necessary in a pressure vessel) then distilling will
produce MeI (and water)
I do realize that many of us cannot get I2, but I can and this method appeals to me. More so than H3PO4 + KI. I have a Kg or more I2 on hand so
merely need to get some tetralin.
2 I2 + C10H14 -> 4 HI + C10H10
[Edited on 24-6-2009 by Sauron]
Attachment: US5693306.pdf (514kB)
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Sauron
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Another possible route to hydriodic acid from I2 is the Bunsen reaction which
2 H2O + SO2 + I2 -> H2SO4 + 2 HI
As written above the reaction is reversible, and the product inseperable by distillation but if both excess SO2 and escess I2 are used, two layers
form and can be separated easily, the lower layer being HI/I2 or HI3 and higher polyiodides in water. The upper layer is H2SO4.
2 H2O + SO2 + 3 I2 -> H2SO4 + 2 HI3
I assume that the hydrogen polyiodides on heating decompose to HI and I2, an assumption I can check in Mellor.
This reaction is the basis of the sulfur-iodine cycle exploited to generate H2 from H2O but I am looking to instead try to use it to prepare HI, so
wish me luck!
[Edited on 24-6-2009 by Sauron]
Attachment: US4089940.pdf (350kB)
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Sandmeyer
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Of curse, there is also classical preparation in Gatterman's Laboratory Methods of Organic Chemistry on page 95 http://www.sciencemadness.org/library/books/gatterman_1937.p... (IMO, this book, like Vogel's, should be thoroughly read by everyone interested in
organic chemistry).
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Sauron
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Sandmeyer, I am sure all participants in this thread are familiar with the red P catalyzed iodination of methanol, which is described not only in
Gattermann but Org.Syn., Vogel etc. As you well know most members, including me, cannot readily obtain red P and many others cannot get I2 either. So
this method, though timr-honoured, is blocked.
I am looking to avoid the use of H2S if at all possible. I would rather use H3PO4/KI than H2S/I2, and am simply looking for alternatives to both of
those.
Clearly the P4O10/methanol/alkali iodide reaction as exemplified by Nicodem is probably the best route to MeI discussed thus far.
That being sais, you won't mind if I look round for other methods?
I2 and tetralin looks good as a facile means of making HI. As described upthread, the excess tetralin can be reused without removal of the napthalene,
and when the napthalene builds up, distillation or dilution with more tetralin is all that is needed.
[Edited on 24-6-2009 by Sauron]
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PainKilla
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You can make methyl iodide quite easily and in good yield by slowly adding methylsulfuric acid to a concentrated solution of an iodide (sodium,
potassium, ammonium all work) - the original report of this patent was posted on the Hive, where NH4I was used in something like an 80% yield. I
posted this post here.
Not as elegant or interesting as the phosphorous chemistry, but by far more accessible, and the yield isn't shabby at all - I'm willing to bet you can
get 90% with a good setup and good technique.
Here is a procedure I wrote down a while back when I tested this...
Methyl Iodide Synthesis
100g potassium iodide (.60mol) [1]
80ml methylsulfuric acid (.96mol) [2]
60ml water
In a 500ml flask equipped with magnetic stirrer and prepared for distillation, 100g (.60mol) of potassium iodide is dissolved in 60ml water. This is
stirred until the full dissolution of the KI, and then heated on a water bath until the internal temperature measures 80*C. To this solution,
methylsulfuric acid is SLOWLY added via an addition funnel, and the methyl iodide distills almost immediately. [3] After about three hours, no more
methyl iodide was seen to distill. The receiving flask contained 50ml of saturated NaCl solution, cooled by an ice bath. [4] The methyl iodide is
separated from salt solution and dried with magnesium sulfate.
Yield: 23ml (52.4g) (61%) [5]
Notes-
[1]– Any alkali halide salt works well, the ideal salt being NH4I (ammonium iodide), due to the the formed sulfate salts' high solubility in water
(this is speculative).
[2]– Methylsulfuric acid was prepared via US Patent 3,047,604. This entailed mixing equimolar ratios of sulfuric acid and methanol, followed by
addition of anhydrous sodium sulfate and cooling the mixture to a temperature below 32*C. This was then magnetically stirred for 4 hours. The mixture
is allowed to sit until the solids settle, and the methyl sulfuric acid decanted in near quantitative yield. Based on the original post, you can use
H2SO4/MeOH 1:1 without going through this sort of purification.
[3]– If added too quickly, hydrogen sulfide gas is generated, and eventually sulfur. Hydrogen Sulfide is toxic. Perform this reaction only under a
hood, or outside. IHL-HMN LC50 800 ppm (5 min). Thus, add acid slowly, making careful note of the rate of distillation.
[4]– Storing methyl iodide under water is not necessary, but it makes the purification easier and also reduces losses from evaporation. Note
however, that its solubility is 14g/L (it is less in brine solution).
[5]– The yield can very easily be improved by adding methyl sulfuric acid very slowly, as excess acid seems to oxidize the potassium iodide to
iodine. Additionally, if a good condenser is employed, the shortest possible distance to receiver is preferable, as the methyl iodide otherwise
condenses back into the reaction mixture and eventually decomposes. This was noted to be the cause of lower yields at the time this procedure was
written.
References: CS164707 (patent)
Methyl Iodide Physical and Chemical Properties
Molecular formula: CH3I
CAS No: 74-88-4
Appearance: colorless to pale yellow liquid with an ether-like odor
Melting point: -64 C
Boiling point: 42 C
Specific gravity: 2.28
Solubility: 14/g L-H2O (20C)
Stability: Stable. Incompatible with strong bases, strong oxidizing agents. May discolor on exposure to light. Moisture-sensitive.
Toxicology: VERY TOXIC, AND CONTACT MAY BE FATAL. May cause cancer. Possible teratogen. Vesicant. May cause harm to the unborn child. Readily absorbed
through the skin. May cause sensitization. Severe irritant. Narcotic. Typical PEL ca. 5 ppm. Possible long term damage, toxicity may be delayed.
Toxicity data:
ORL-RAT LD50 76 mg kg-1
IHL-RAT LC50 1300 mg/m3/4h
Disposal Information: Slowly hydrolyzes to methanol and iodine. Dilute hydroxide solution can be used to speed this process along. Environmental
hazard. Note low boiling point when hydrolyzing.
[Edited on 25-6-2009 by PainKilla]
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Sauron
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I believe I posted that patent recently in connection with the preparation of diethyl sulfate, for prurposes of making ethyl centralite. But never
mind. Note that it is a USN patent and ocviously they were and are interested in stabilizers for nitrocellulose.
The reaction between methyl hydrogen sulfate (methylsulfuric acid) and an iodide is closely (intimately!) related to the Org.Syn. procedure for methyl
iodide from KI and dimethyl sulfate. Methyl hydrogen sulfate is after all the half ester of H2SO4 and methanol, DMS is the full ester.
The Org Syn prep is on a 5 mol scale and gives >90% MeI or about 500 g from 900 g KI. Note that when domethyl suflate reacts methyl hydrogen
sulfate is the other product besides MeI; the reaction is between 4.8 mols KI and 5 mols DMS, a 4% excess.
This begs the question of whether methyl hydrogen sulfate can be recovered from the DMS reaction and used used to prepare a further equivalent of MeI.
The other issue is how toxic is the half ester? Obviously it is an alkylating agent, else it would not work in this reaction. The prudent practice is
to handle it like DMG, with all the respect a carcinogen deserves. This may have a chilling effect for some members. DMS of course is a human BRAIN
carcinogen. Brain tumors are no fun or so my younger sister told me before she died of gliablastoma multiform.
[Edited on 25-6-2009 by Sauron]
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PainKilla
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Based on the rates of hydrolysis, methylsulfuric acid is probably as dangerous as sulfuric acid, but no more.
"Dimethyl sulfate hydrolyzes slowly in cold water but rapidly in warm water and acidic solutions. The hydrolysis occurs stepwise, initially forming
methyl sulfuric acid, then sulfuric acid and methanol. It can be calculated that DMS hydrolyzes to methyl sulfuric acid with 99.9% completion as
follows:
...
Complete conversion to sulfuric acid occurs at a much slower rate. Salts of methyl sulfuric acid are formed when DMS is hydrolyzed in aqueous alkali."
source
Strongly suggests methylsulfuric acid is a terrible methylating agent (mainly because the proton will hop off far faster than the methyl).
[Edited on 25-6-2009 by PainKilla]
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Sauron
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That's encouraging.
So
KI + Me2SO4 -> MeI + MeSO4K
Can't we liberate the MeSO4H and isolate it to exploit with more KI?
If not no big deal as MeOH and H2SO4 are almost free (relative to the cost of KI anyway).
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PainKilla
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The reaction occurs pretty much instantly, and at relatively high temperatures with a concentrated ionized solution, so the addition of methylsulfuric
acid to this hot mixture probably causes significant destabilization of the solvated iodide ions, possibly allowing for increased nucleophilicity of
the iodide towards the methyl group while the acid itself is only 'locally' dissociated during the very brief time during which methyl iodide is
formed. Subsequently the very volatile methyl iodide leaves solution, returning some stabilization towards solvation of the remaining
sulfate-bisulfate ion... Subsequent addition of acid continues the process. This is conjecture of course, but from what I remember the reaction occurs
only at higher temperatures, and with concentrated solutions, maybe suggesting that this reaction occurs due to unique conditions...
This is also roughly the protocol I use for making methyl/ethyl halides, and it works very well for methyl bromide/iodide and ethyl bromide/iodide,
although naturally for the bromides the oxidation is a non-issue.
Given the strange behavior of the reaction, I wonder if it can really be described so simply as CH3HSO4 + KI --> CH3I + KHSO4; the fact that both
iodine and hydrogen sulfide can be produced if not carefully performed might suggest there is more than meets the eye, and that this reaction is very
sensitive to competing reactions (energetically speaking, procedure-wise just requires slow addition). Or rather it is to say, the reaction is only
possible because of a very finely controlled set of conditions.
Regardless, I think the acidic conditions are important, which is probably why the dimethyl sulfate / iodide reaction does not go to completion
normally...
I agree though, it makes little sense to use DMS to make MeI when this method is more economical and far safer to boot... DMS can handle less reactive
substrates than MeI AFAIK, so it's just easier to make it this way and save DMS for those purposes anyway...
[Edited on 25-6-2009 by PainKilla]
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Sauron
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There's no arguing the safety issue, but if one has a hood, gloves, etc and a wash bottle of conc ammonia at hand, DMS is really not very daunting,
especially considering the target MeI which shares much of its hazards.
[Edited on 25-6-2009 by Sauron]
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PainKilla
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That's 230ml or 525g @ 62% (methyl iodide has a density of 2.28g/ml) for a kilogram of potassium iodide. It makes no difference if you scale up or not
- the limiting reagent is KI with either procedure, and the yield is near quantitative (according to the other posts using this general procedure)
so... I'd rather take the 'safe' one, unless methanol and sulfuric acid are more expensive for you than iodides are - they aren't for me.
I do agree that safety is relative, but for the average person, making methyl iodide from methanol, sulfuric acid, and an iodide is *far* more
accessible than using DMS - which is useful in and of itself and a shame to waste, irrespective of price, on something often inferior and otherwise
obtainable via ubiquitious reagents.
And dimethyl sulfate isn't that easy to make (under amateur settings at least) - I've tried extensively and could not get yields anywhere near the
claimed industrial 80-90%... 40-50% is the best I was able to get (using a vacuum distillation ~5-10mmHg).
[Edited on 25-6-2009 by PainKilla]
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Sauron
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My mistake.
Obviously, then, methyl hydrogen sulfate is indeed quite competitive with DMS in this reaction.
OT question: will MeSO4H react with KSCN in analogous fashion to give MeSCN?
[Edited on 25-6-2009 by Sauron]
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PainKilla
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I don't know much about the chemistry of that anion but:
http://books.google.com/books?id=zBpJAAAAMAAJ&pg=PA486&a...
(Note that Ca(CH3SO4)2 + KSCN yields CH3SCN upon distillation... However, I recall reading that Ca(CH3SO4)2 liberates (CH3)2SO4 upon distillation, so
this might just be an situ way of generating DMS... I could never get the metal salts of methylsulfuric acid to yield DMS though, as patents and old
literature claimed.)
Citation in question:
http://www3.interscience.wiley.com/journal/112330280/abstrac...
----
Maybe it's possible, I don't think anyone has ever tried it. If thiocyanates are stable under acidic conditions (doesn't look like it, see attached
paper), and those conditions promote S-alkylation over N-alkylation, then you might be OK... I would imagine the reaction would need to be run at a
higher temperature however, since methyl thiocyanate is less volatile... It would be interesting to try at least, since iodides would normally be
oxidised instantly by straight sulfuric acid, but still manage to react with methylsulfuric acid without extensive oxidation... maybe the thiocyanates
will also do so - they certainly appear soluble enough that a concentrated solution would react favorably, if heated to a high enough temperature to
allow for the distillation of MeSCN.
Attachment: ACTION OF SULFURIC ACID ON THIOCYANATES.pdf (114kB)
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[Edited on 25-6-2009 by PainKilla]
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Sauron
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Vogel states that alkyl hslide metathesizes in ethanol with KSCN.
The lit, has barium thiocyanate reacts with DMS to give MeSCN and so that is the basis for my analogy, See the methyl thiocyanare thread for refs and
details.
If this does work, I mean the half ester and KSCN, it saves a step and the expense of KI.
If it does not fly, then the half ester makes MeI and MeI then makes MeSCN.
[Edited on 25-6-2009 by Sauron]
[Edited on 25-6-2009 by Sauron]
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Nicodem
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It is interesting that PainKilla posted about his experience with MeI preparation using methylsulfuric acid, because yesterday I just set another
experiment based on in situ methylsulfuric acid formation and iodide methylation. Too bad it didn’t live up with the expectations, quite
possibly due to the side reaction problem PainKilla described.
Experiment on the synthesis of MeI from MeOH using NaHSO4
In a two neck flask (100 ml) equipped with a thermometer, a suspension of 10.4 g NaHSO4 (80 mmol) and 6.35 g NaI (40 mmol) in 65 ml methanol (1600
mmol) was first purged with argon, closed and left stirring for 18 hours. The color changed to pale brown and HPLC analysis (UV detection at 220 nm)
showed the presence of some MeI. A small Vigreux distillation column (1.5 cm wide, 8 cm long) was set on the flask and the reaction mixture heated and
stirred on an oil bath. The first drops of distillate came over at 37.5°C while the reaction mixture temperature was 65°C. The temperature of the
distillate slowly increased to 58-60°C and remained such for the next hour or so, but the reaction mixture boiling temperature stayed at 66°C. The
distillate was collected till 62°C. This colorless liquid (18.7 g) was added to 40 ml of water. A white milky emulsion formed, therefore 20 ml of
brine was added. After standing for 30 min the few droplets of heavy oil that settled at the bottom were pipeted out (1 g) and dried with CaCl2
granules which floated on top. This gave 0.95 g of a nearly colorless oil (16%). The IR spectra was identical to that of methyl iodide.
A strong smell of H2S was obvious during the cleaning up of the reaction mixture remains, so apparently some reduction side reaction goes on, though
it is strange that no iodine coloration is present.
PS: About MeSCN synthesis via methylation of tiocyanates with methylsulfuric acid... Unlike the iodide anion, the tiocyanate anion is a weak base (pKa
of HSCN is 4) while the methylsulfuric acid is a relatively strong acid. Mixing them together means MeOSO3H would get deprotonated (forming a much
less electrophilic MeOSO3(-) anion) and the thiocyanate being protonated (forming a much less nucleophilic and unstable HSCN). Thus the reactivity of
both reagents would drop severely and a competing HSCN decomposition reaction enabled.
A compromise would be to react a methylsulfate salt with a thiocyanate salt. Methylsulfates are still somewhat reactive methylating agents even though
several magnitudes less electrophilic than dimethyl sulfate of methyl sulfuric acid (examples can be found in other forum threads where references
about phenoxide and nitrite alkylation with sodium or potassium salts of ethyl- or methylsulfuric acids are given). The lower reactivity of MeOSO3(-)
would mean a higher reaction temperature would be required. Thus to a methanolic solution of methylsulfuric acid (prepared by the usual method from
NaHSO4) would have to be neutralized with K2CO3, then KSCN added and distilled to dryness to hopefully give a methanolic solution of MeSCN. A
relatively simple experiment to verify.
…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
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Sauron
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There's a fair amount of material on methylsulfuric acid in Sartori's book even though it is not a "war gas" because of ots relationship to dimethyl
sulfate and the fact that it was present in some French and German "war gas" mixtures as a byproduct of manufacture of DMS from methanol and
chlorosulfonic acid.
Heating methylsulfuric acid to 170 C at reduced pressure causes dissociation to H2SO4 and dimethyl sulfate. I guess this was the prep of DMS Painkilla
alluded to.
Per sartori the lab and industrial preps of dimethyl sulfate are from methanol and chlorosulfonic acid followed by this thermal disproportionation.
He offers as alternatives, methanil and SO2Cl2 or, methylsulfuric acid + methyl chloroformate.
Painkilla, have you tried the SO2Cl2 route to DMS?
I did a thread on chlorosulfonic acid a while back and will have to review it but I think I found ways to make it other than SO3 + HCl (g).
Of course we know how to make methylsulfuric acid from MeOH and H2SO4 per the US Navy patent as well as from bisulfate.
Anyone working with methylsulfuric acid needs to keep that thermal disproportionation in mind because no one wants to generate DMS vapor accidentalltm
do we?
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PainKilla
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Yeah, what's what I figured you meant, I just wanted to see if I could find any mention towards the use of any methylsulfate-based derivative...
Nicodem, are you sure the pKa of HSCN is 4? That was the first thing I looked at, and according to several publications it is a strong acid with a pKa
between -1 and 1... This goes along well with the 6-8 pH range cited for potassium thiocyanate solutions, I would expect at least that under hot and
concentrated aqueous conditions thiocyanate isn't very much worse a base than iodide.
I would not expect methylsulfuric to retain such a low pKa as plain sulfuric either, so it is bound to be in a similar range (-1 - 1)... In this
respect, it might still be worth trying at least, especially if this reaction occurs because of higher-order solution chemistry. Of course, since the
pKa's ultimately are much closer, it does seem likely that the yields aren't going to be as good if the reaction does work. On the other hand, if the
distillation of the MeSCN occurs fast enough, it might be able to compete with other side reactions, thereby driving the reaction to completion...
Although, the bisulfate remaining would also promote formation of HSCN increasingly as the reaction went along (unless the pKa was significantly
altered due to the concentrated ionic solutions and temperatures) - I think it's at least worth an experiment.
Interesting to see that NaHSO4/methanol work, albeit it poorly. This further confirms the idea (to me at least) that this is some sort of
solution-chemistry phenomenon (with respect to the potentially good yields of the original reaction); at the very least, water appears to be necessary
for good yields.
Heating methylsulfuric acid under vacuum was indeed what I referred to Sauron, but the yields on the reaction are low, although they do increase as
you consistently increase the vacuum so perhaps a 0.5 - 1mmHg vacuum would give the yields cited in the literature. I've never tried using SO2Cl2; it
seemed like a very roundabout way of making DMS - SO2Cl2 seems useful for other things, even if I were to make it...
I don't think the disproportionation would occur under any conditions used in this reaction - I've tried heating plain methylsulfuric acid to high
temperature without a vacuum, and IIRC no DMS was obtained - and that was at ~160-200*C, not the 80*C used here. Though naturally, due caution is
necessary either way, since methyl iodide is after all, at least as dangerous as DMS with respect to volatility.
I'd try this reaction again in order to improve yield since when I wrote it down initially, but unfortunately I can't do amateur chemistry under my
current settings.
Forgot to add:
"In the course of some work on the thiocyanates it became necessary to know the strength of aqueous thiocyanic acid. Ostwald's earlier conductivity
experiments indicated that this acid is almost as strong as hydrochloric, but more recently Latimer estimated from thermodynamic calculations that it
is a weak acid with an ionization constant of approximately 1x10^-4. The experiments summarized in Table I prove that thiocyanic acid is a strong
acid. The third column gives the pH calculated from the relation, pH = -log (H+), where (H+) is the concentration of the hydrogen ion in moles per
liter for complete
dissociation of the acids. In a private communication Latimer has informed us that soon after he published his book he realized that thiocyanic acid
is a strong acid." (source attached)
Attachment: Strength of Aqueous Thiocyanic Acid.pdf (247kB)
This file has been downloaded 839 times
[Edited on 25-6-2009 by PainKilla]
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Sauron
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The alternative routes to chlorosulfonic acid are:
100% H2SO4 + POCl3 (3:1) gives chlorosulfonic acid and H3PO4, chlorosulfonic acid can be distilled out. POCl3 is made by heating P2O5 and NaCl in an
autoclave at 400 C.
Per a US patent, 100% H2SO4 and SO2Cl2 + vatalytic HgSO4 in an autoclave at 160 C 8 hrs gives quantitative chlorosulfonic acid.
I am not suggesting that using chlorosulfonic acid is required to make methylsulfuric acid, but it is well worth keeping in mind.
As I mentioned, Vogel says that alkyl iodides in alcohol react with KSCN (or NaSCN) to give alkyl thiocyanates.
My source for the reaction of dimethyl sulfate and barium thiocyanate is Merck Index, but annoyingly although this is under Literature References,
they give no citation. Shit! I suppose the virtue of Ba(SCN)2 is that the resulting BaSO4 will be insoluble. But this salt is made per Inorg.Syn. from
NH4SCN which is expensive, also expensive to make since you start with CS2 and NH3, obtain ammonium dithiocarbonate and thermolyze that to NH4SCN, S
and H2S (see Ullmann's on thiourea.)
On the other hand since I opened up cheap CS2 from S and acetylene, this is not so costly, just laborious.
Anyway I am hoping that DMS will also work with KSCN and save me the bother.
And if methylsulfuric acid can replace DMS in this so much the better.
[Edited on 25-6-2009 by Sauron]
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PainKilla
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I don't think you even need (anhydrous) methylsulfuric acid for this reaction, as Antoncho demonstrated in the original excavation of this reaction
that just plain methanol + sulfuric acid work well; I've use methanol/sulfuric acid this way (without doing the sodium sulfate purification) for other
alkyl halides to very good effect, so it seems probable that despite the water, a mixture of methanol/sulfuric acid is already almost entirely
methylsulfuric acid.
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Sauron
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Painkilla if you are wanting DMS/DES but having mediocre results with disproportionation of the alkylsulfuric acids, you might try making SO2Cl2 and
esterifying that with 2 eq alcohol.
Sources are divided over whether that reaction is slow, or violent, so proceed with caution.
Sic gorgeamus a los subjectatus nunc.
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