panziandi - 26-7-2008 at 16:20
When phosphorus trichloride reacts with 3 eq of an alcohol the first reaction to occur is the formation of the trialkyl phosphite and 3 eq of HCl. The
HCl can rapidly react with the phosphite to form phosphorus acid and the alkyl chloride. If however it is performed in the presence of a base the HCl
is trapped and the main product is the phosphite.
Now I'm wondering whether refluxing a trialkyl phosphite with potassium iodide would yield the alkyl iodide? For example i'm thinking:
(CH3O)P + 3KI => 3CH3I + K3PO3
Does anybody know if this would work or have a better suggestion?
Klute - 26-7-2008 at 17:05
Why not use directly HI rather than precious PCl3? There are lots of ways of preparing alkyl iodides without PCl3..
panziandi - 26-7-2008 at 17:49
Ah sorry I was giving people an introduction to trialkyl phosphites in my first paragraph... I already have trimethyl phosphite and was wondering if
the conversion to iodide was possible.
Sauron - 26-7-2008 at 19:33
In the reaction of Cl3P with alcohol, the product will be primarily the RO3P as long as you stir well and don't allow the HCl to build up. A minor by
product might be dialkyl hydrogen phosphite,
If you replace PCl3 with PBr3, you will get no RO3P and only alkyl halide and phosphorous acid, unless you take special measures to prevent the HBr
from reacting with the trialkyl phosphite initially formed.
And I3P is even more aggressive at returning only the alkyl iodide and phosphorous acid.
A complication particularly with Me3P is that alkyl halides react with Me3P even in trace quantity to give the Michaelis-Arbusov rearrangement,
producing methylphosphonate diester depending on the alkyl ligand.)
As this is a nasty acetylcholinesterase inhibitor I suggest you do NOT do this with your Me3P.
As to your question about KI, I am not sure but I would suspect the answer is yes. Trimethyl and triethyl phosphite are powerful alkylating reagents
just like MeI and Me2SO4.
But MeI is a lot easier to come by than Me3P so I suggest you save the phosphite and prepare your MeI some other way.