[Babcock_Hall]

We are converting 1-bromo-2,2-dimethoxypropane to its iodide form with NaI in acetone.  We see a spot on a TLC plate for the reaction mixture using a phosphomolybdate dip and a cerium dip.  However, the starting material does not show up, either in acetone as a spotting solvent or neat.  Any suggestion about how to visualize the starting material would be welcome.

[Dan]

Maybe try some other general dips, like basic potassium permanganate, ethanolic sulfuric acid or I₂ vapour.

I used to use ceric ammonium molybdate dip a lot as well and it's pretty good, but not as general as those three in my experience. I find there is very little that won't stain in permanganate.

http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

[Doc Oc]

On a slight tangent here, you might have an easier time if you have access to the chloride instead of the bromide.  The NaCl is insoluble in acetone and precipitates out, driving the reaction forward.  This isn't the case with NaBr, so you have to rely on the excess of NaI.

[OC pro]

GC-MS

[Babcock_Hall]

On a slight tangent here, you might have an easier time if you have access to the chloride instead of the bromide.  The NaCl is insoluble in acetone and precipitates out, driving the reaction forward.  This isn't the case with NaBr, so you have to rely on the excess of NaI.

We are seeing some precipitation.  How much molar excess do you typically use?

[Babcock_Hall]

Maybe try some other general dips, like basic potassium permanganate, ethanolic sulfuric acid or I₂ vapour.

I used to use ceric ammonium molybdate dip a lot as well and it's pretty good, but not as general as those three in my experience. I find there is very little that won't stain in permanganate.

http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Thanks, we will try the permanganate next and possibly I₂ or FeCl₃/sulfosalicylic acid* after that.  Do you suppose that the DNP in acid would unmask the ketone (convert the ketal, in other words) and react that way, or is that reaction too slow?  I looked in Stahl's TLC book for acetal/ketal stains but I did not see anything in the index.
* I used to use this for phosphoric acid esters, but I seem to recall that it reacts with other organics.
Stanley, J. Chromatography 16 1964 467
A 0.1 g FeCl3 in 1 mL 1 M HCl, then take to 100 mL with 80% ethanol.
B 1 g sulfosalicylic acid in 100 mL 80% ethanol
Spray first with A then B
some recipes begin with a bromine atmosphere treatment
http://www.sanpontgroup.com/en/view1.asp?id=939

[OC pro]

Why not simply follow via GC-MS? However, such a simple transformation. MeCN, 5equiv. KI, reflux overnight. Done.

[Babcock_Hall]

I was under the impression that alkyl halides could decompose under GC/MS conditions, but I am beginning to doubt this.  With respect to the reaction conditions, I have not used KI in acetonitrile, but I will look for a suitable reference.  Does it work better for converting alkyl bromides (bromoalkanes) or alkyl chlorides (chloroalkanes) into alkyl iodides (iodoalkanes)?

[Babcock_Hall]

Why not simply follow via GC-MS? However, such a simple transformation. MeCN, 5equiv. KI, reflux overnight. Done.

I have a new undergraduate student on this project, so almost everything is fresh.  I was surprised (when I looked up a few Finkelstein reactions) that some people reflux in acetone.  Do you have a protocol for the KI/MeCN reaction?  I am wondering how people typically remove the excess salt (extraction, perhaps?).

[discodermolide]

If there is a solid filter it off.
If not evaporate the acetone or acetonitrile. Suspend the residue in toluene, filter, evaporate toluene.
Purify if necessary.

[OC pro]

Like disco already mentioned most of the salt can be removed by filtration. The rest can be removed via extraction (acetonitrile has to be evaporated before).
@ Babcock_Hall: alkyl iodides can of course eliminate/decompose under GC-MS but sometimes they are quite stable depending on the molecular structure (in fact you didn´t try it although it would have been very straight-forward to inject a small aliquot from the reaction mixture). In my former synthetic times I did several batches of >500g where I converted chlorides and also bromides into the corresponding iodides. Conversion was always >90% overnight. In most cases I could purify via recrystallization. If the stuff was oily I gave the 90% pure material a second shot with 1equiv. KI (will be 10-fold excess!) and it was fine.

[Babcock_Hall]

Potassium permanganate gave a spot for the product but not the starting material.  Nice color background, though.

@OC Pro, Did you remove salt prior to GC/MS?

[Doc Oc]

I was surprised (when I looked up a few Finkelstein reactions) that some people reflux in acetone.  Do you have a protocol for the KI/MeCN reaction?  I am wondering how people typically remove the excess salt (extraction, perhaps?).

I reflux my Finkelsteins in acetone.  Leaving it at room temp takes 24h.  If you heat it, you can often complete the reaction in an hour (can be substrate dependent).  Concentration also helps, I was running my reactions at 100 mg/mL and that took a few hours.  One of my colleagues ran his at 500 mg/mL and his were much faster.  When I concentrated mine to that degree they also went much more quickly.

I extract the salt out because I always wash my reaction with aqueous sodium bisulfite to get excess iodine out.  But remove the acetone first and suspend the residue in another solvent like DCM.

[Babcock_Hall]

Why not simply follow via GC-MS? However, such a simple transformation. MeCN, 5equiv. KI, reflux overnight. Done.

My student does not have access to a GC-MS, although that may change.  What advantages does it have over TLC?
We filtered the solid, removed the acetone, extracted with DCM versus sodium thiosulfate (on the suggestion of an organic chemist here who said it would also remove the iodine, as does sodium bisulfite), dried the organic layer and removed the DCM.  We will analyze it shortly.

[OC pro]

@Babcock_Hall: Sorry had very busy days. Sometimes TLC and GC-MS are a good combination! especially with low-boiling and non-visible stuff like yours or in case of nonpolar aromatic compounds with close Rf. Of course, prior to GC-MS the salts have to be removed. Withdraw an aliquot from the rxn mixture and dilute with water/DCM, withdraw the water layer and you have a nice aliquot suitable for GC but also TLC (very often I saw students withdrawing aliquots for TLC directly out of the rxn without mimicking aq. work-up).