I was trying to determine whether I could synthesize methyl iodide by combining methanol, potassium iodide and sulphuric acid together... The way I see it, the sulphuric acid will protonate the OH instead of the I forming a very stable leaving group of water. Then the iodine ions would donate an electron pair to the carbon. You would be left with the salt Potassium bisulfate.... Then to remove my methyl iodide, just distil.

Will this work?

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    $\begingroup$ Probably yes, in combination with something like a Dean-Stark apparatus to remove excess water. $\endgroup$
    – Jori
    Aug 25, 2015 at 15:44

2 Answers 2


I know in theory the reaction of methanol with hydrogen iodide will form methyl iodide and water. This is part of the Monsanto process for acetic acid. But the problem is that back reaction will also occur.

Also if you try to use concentrated sulfuric acid to stop the back reaction (water plus methyl iodide to form methanol and HI) being a problem. But then you will form HI from the KI, the HI tends to decompose into hydrogen and elemental iodine. This will make the reaction very messy.

The Dean-Stark head will be useless for this reaction as methanol (methyl alcohol) and water are totally misicble. A better dehydration reagent would be trimethyl orthoformate. If you were to mix methanol, trimethyl orthoformate, sodium iodide and sulfuric acid I think you would have a better chance of forming methyl iodide. But the purity would not be good as it would have methyl formate mixed with it.

The easy way to make methyl iodide is to react a solution of sodium iodide in water with dimethyl sulfate. If you put the sodium iodide in a pear shaped 50 ml flask, add a stir bar and then the dimethyl sulfate you can then heat it up and the methyl iodide will distill out as it forms. I think that this reaction has much more promise for a methyl iodide synthesis.

Dimethyl sulfate can be formed by the reaction of sulfur trioxide and methanol, so in industry it is a good and cheap methylation reagent. This could then be reacted with sodium or potassium iodide to form the methyl iodide.

I tend to put a still head on top of a 1 inch long fractionation column and then use a liebig condensor to collect the methyl iodide. The only reason a chemist does this is if you want to make I-131 containing methyl iodide. I have found that if you add I-131 as sodium iodide to the synthesis before you add the dimethyl sulfate that you can make radioactive methyl iodide.

When I do this I work in a very good fume hood, while wearing nitrile gloves to handle every object. It is important with this reaction to pay attention to both the radioactive and the non radioactive hazards. The dimethyl sulfate is a very nasty toxic reagent (carcinogenic). I tend at the end of the reaction to allow it to cool. I then add some diethylamine to the still pot and then distill the diethylamine through the glassware (by this time the methyl iodide has been placed inside a glass vial over copper wire). The idea of the diethylamine is to make sure that no dimethyl sulfate remains in the glass equipment.

I then allow it to cool, I then wash out all the glass ware into a plastic waste bottle. I then put the glass equipment inside plastic bags. The glass is allowed to stand in a fume hood for weeks while the radioactivity decays away. The waste is allowed to stand. I have always used I-131 while has a half life of 8 days, but I am sure the work could be done with a longer lived radioiodine.

  • $\begingroup$ Instead of Dimethyl sulfate, can I use Trimethyl sulfoxide Bromide? I am quite comfortable working with it and I guess NaI will replace the I with Br and DMSO will become free again and CH3-I will be created? I have a Bromine recovery system at site and I could use the formed DMSO to re use in the reaction. Or maybe I could use HI. I am comfortable with HBr so HI should be easy too $\endgroup$ Jun 10, 2022 at 20:45
  • $\begingroup$ In princible you could combine Me3SO Br with iodide and then heat it to decompose it. Many years ago as a second year undergraduate I did a synthesis project where I reacted methyl iodide with DMSO to make Me3SO I. I found that it did not have a melting point, instead if I heated a little in a melting point tube it eventually decomposed to DMSO and methyl iodide. But if I was you I would not want to do things with methyl iodide outside of a fully equipped chemistry lab. You need a fume hood to use it safely. $\endgroup$ Jun 12, 2022 at 4:46
  • $\begingroup$ I work in an Industrial Lab and I make copious amounts of Me3SO Br daily (like about 500kg daily) so I am pretty comfortable making it. We got a recent order for Methyl Iodide and though they gave us their own proprietary process, I was thinking of recycling my DMSO for this use. $\endgroup$ Jun 12, 2022 at 17:12

So what I said in the comment still holds: it could work (particularly in combination with a Dean-Stark apparatus to remove excess water), but there is no way to know for sure unless someone tries it out.

The more usual (and early) method is described in Organic Syntheses and utilizes the reaction of methanol with iodine in the presence of phosphorous

MeI synthesis from methanol

I don't know the exact reaction mechanism (if anyone has a clue, please feel free to edit!) but what happens is that $\ce{PI_3}$ is generated in situ from the elemental iodine and phosphorous and reacts with methanol to yield the methyl iodide and phosphorous acid

$\ce{3 CH3OH + PI3 -> 3 CH3I + H3PO3}$

Phosphorous acid does not act as nucleophile and so the problem of a competing backward reaction, which you would have in your proposed synthesis, is largely prevented.

I'd also like to add some caution: the reaction described in Organic Syntheses is not suited for beginners or in amateur setting. The chemicals involved are nasty: highly toxic, highly flammable, alkylating and pyrophoric. Don't try this at home :)

  • $\begingroup$ I actually ended up finding something about the proposed reaction. The sulphuric acid ends up being reduced by the Iodide so the methyl iodide is only an intermediate state. I saw results for this describing a 20% yield for methyl iodide (under cold conditions). On the other hand, using phosphoric acid in place of sulphuric acid works with very high yield. $\endgroup$
    – jakerz
    Aug 26, 2015 at 14:37
  • $\begingroup$ To use a Dean Stark head you need to have water in a solvent which it is not miscible such as benzene or toluene. The idea of the Dean Stark head is that the water will evapourate with the hydrocarbon solvent and then collect in a condensor the water forms a seporate layer which is trapped in the head rather than returning into the flask. If you attempt to use a dean stark head on a mixture of methanol and water then you will never get the two layers. I guess you could use a DS head to remove methanol from hetane however. $\endgroup$ Jun 12, 2022 at 4:38

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