chemrox
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MeI thread revisited
http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0399
gives a way to make MeI using a special apparatus. It looks like the purpose is to wash the I2 crystals down with condensing MeI. One of the
reagents is P.
My two questions are:
1- wouldn't a soxhlet do just as well?
2- isn't this an insitu HI prep where the HI could be made as outlined and the MeOH added from an addition funnel per ususal procedure?
I haven't checked Vogel's yet .. sometimes I think I need a home copy and a lab copy
[Edited on 25-2-2007 by chemrox]
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chemrox
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additional information
Adams, Johnson & Wilcox, Lab Experiments in O Chem, 5th ed., Experiment 16d, Ethyl Iodide:
in a 200 ml RB place 2,5 g red P and 30 ml EtOH (95%). Over 15 minutes add 25 g pulverized I2. During addition swirl the flask and cool with cold
water bath. After addition allow mixture to stand 4 hrs with a reflux condenser (500-600mm) attached. After at least 4 hours, heat very gently for 2
hrs. (How was this figured out?). Set the apparatus for distillation and collect the EtI in a flask cooled over the top with ice water. (Seems like
a vacuum adapter and hose would be called for here.) Use a lowly luminous flame to drive over the last EtI.
The procdeure called for steam, water baths and finally flame...it's an old book .. seems like a mantle could handle all of it.
[Edited on 25-2-2007 by chemrox]
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Sauron
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Didn't a Russian member just give us a novel procedure for MeI using MeOH, I2 and Al foil?
Requires no red P.
If that post was not a hoax, this ought to be a decent route.
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Nicodem
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| Quote: | Originally posted by chemrox
My two questions are:
1- wouldn't a soxhlet do just as well?
2- isn't this an insitu HI prep where the HI could be made as outlined and the MeOH added from an addition funnel per ususal procedure?
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1 – No. A Soxlet apparatus is designed to flush the extract all at once. This would result in a sudden addition of lots of iodine to the reaction
mixture. Since the reaction is quite exothermic you would end up with a runaway reaction (especially if working on an 16 mol scale like in the Org.
Syn. example). The possible secondary effect of such carelessness would be death from cancer resulting from the exposure to methyl iodide vapors
blowing out of the condenser. Rather use an addition funnel with methanolic iodine solution instead, so that you can control the addition rate
manually.
2 - Not exactly (note that there is no water present). The iodine reacts with phosphorous forming phosphourous(III) iodide which on turn reacts with
methanol yielding methyl iodide and phosphorous(III) acid. However, the reaction between PI3 and MeOH is not so simple mechanistically and actually HI
is also an intermediate and forms as well, but you can not say that the whole preparation is based on in situ HI formation as that would be
some sort of misinformation and oversimplification. In addition, the resulting H3PO3 can also react with I2 and MeOH forming HI, MeI and H3PO4. And
there are also other possible pathways in the overall reaction. That is why you don't want to look for a specific reaction pathway of such
preparations, but consider only the overall redox reaction: 6MeOH + 3I2 + 2P –> 6MeI + 2H3PO3.
…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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PI3 can be puchased here while P cannot. However PI3 is expensive. Much cheaper (here) to buy MeI or make it another way like dimethyl sulfate (but am
not sanguine about using one brain carcinogen to make another brain carcinogen.)
The Org.Syn. procedure also includes that alternative route in detail.
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garage chemist
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The reaction of HI with alcohols does not occur as cleanly as the one with HBr. The problem is that HI can react with formed alkyl iodide, forming I2
and the alkane. This is the basis of ephedrine reduction using P and I2.
Therefore, PI3 is the preferred reagent, and HI is an intermediate but it does not react with the alcohol, but with the intermediate trimethyl
phosphite.
I have recently conducted the substitution of OH by Br on an alcohol which cannot be reacted with HBr because of its very high tendency to yield the
olefin by dehydration, giving the olefin as the main product.
PBr3 on the other side worked like a charm, giving the alkyl bromide in high yield with negligible amounts of olefin.
This confirms that the use of phosphorus halides is not simply an insitu preparation of hydrogen halide. There is a reason why these more expensive
reagents are used.
MeI can be prepared by reaction of KI (1 mol), 85% H3PO4 (1 mol) and methanol (excess) by constantly distilling out the formed methyl iodide so that
it does not have time to be attacked by more HI. This gives acceptable yields without the use of elemental P, but as expected, it gives considerable
amounts of I2 as byproduct (which can be recycled after reaction and converted into KI).
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chemrox
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| Quote: | Originally posted by Sauron
Didn't a Russian member just give us a novel procedure for MeI using MeOH, I2 and Al foil?
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He's usually a reliable source of information. Before I posted I did a search for, "methyl iodide" and didn't find the one you allude to .. would
like to read it .. must get better at searching the forum..
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chemrox
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Found it, thanks! Looks good too...
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