njl - 7-4-2021 at 06:24
The Hunsdiecker reaction looks like a very useful reaction for the synthesis of alkyl halides. Typically, the silver salt of a carboxylic acid is
treated with a halogen (I2 for example). Silver iodide precipitates, trapping the unstable pseudo-iodonium ion in solution. The I+ forms an unstable
O-halogenated intermediate which decomposes via a radical pathway to CO2, an organic radical, and an iodine radical which combine to form the alkyl
iodide. This mechanism of generating a cation from a salt is rather unique in my opinion and I find it to be very interesting. However, silver salts
are obviously expensive relative to similar reagents. Does silver itself play any role in this reaction other than forming an insoluble precipitate?
If not, Couldn't other metal cations like lead be used? I also don't completely understand the need for a precipitate to form. I understand that it
allows for the equilibrium to favor I+, but even if the resulting iodide is soluble shouldn't the I+ intermediate exist at least transiently? What
would happen if, for example, solid sodium propionate was gassed with chlorine? Why wouldn't this also generate the same intermediate?
My interest here stems from the fact that a free radical reaction is taking place with simple and benign reagents without any initiator (uv,
peroxides, etc.).
Texium - 7-4-2021 at 07:11
I think the insolubility is a key factor in allowing this reaction to proceed at an observable rate. Even if the “I+” existed transiently in
equilibrium, it may not exist for long enough to undergo the next step of the reaction. It’s also likely that the reaction proceeds best with
iodide, followed by bromide, and then chloride, since the larger halogens form more stable radicals. If you’re going to experiment with
unconventional conditions, you should probably stick with iodide. Perhaps lead would work in place of silver. I have my doubts though, if only for
the fact that if it did work, that’s probably what people would use.
njl - 7-4-2021 at 09:00
I would be sticking with iodine simply because it's much easier to handle. Looking around I've found papers referencing the use of Pb(IV) but no
concrete answer. Here is something I hesitate to call a paper on using Hg (II).
Finally, below is a book on acyl halides that also covers the acyl hypohalites (intermediates in the reaction). Note that on pages 483-484 the author
mentions using alkali metal carboxylates to successfully prepare alkyl iodides. Page 485 discusses the formation of perfluorinated acyl hypoiodites
from the free acid and a source of I+ (tert-butyl hypoiodite in their case). I would still like to know what to expect from the reaction of dry sodium
carboxylate with chlorine (or reactions in that vein).
edit: the file is too large to attach so here it is from libgen.
[Edited on 4-7-2021 by njl]