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In article <4faj00$igc@lib107.its.rpi.edu>, farrid@lib107.its.rpi.edu
(Dinah Thiers Farrington) wrote:

> searching for a TLC solvent system, and gotten unusual results. I am
> using silica plates, and I<2 to develop them. Instead of dark spots, I
> see a large white spot  with a rim of darker staining. Absolutely
> nothing in the white spot is picking up the iodine, and I'm quite sure
> this is not something contaminating the silica before the plate is
> developed. Has anyone seen this behavior?  Any explanation  of this, or
> anyreferences describing this would be greatly appreciated. This
> happened so far with solvents of 1:99 ratio of methanol to CH<2Cl<2  and
> 5:95 CH<2Cl<2 to ether.( Whatis the explanation for I<2 marking the compounds,
> anyway?)
>       
>      2. I'm trying to visualize Br-CH<2CH<2CH<2-SH and
> SH-CH<2CH<2CH<2-OH BY this method. Anyone who has separated short chain 
> thiols  could help me greatly by sending me a note about what systems
> they found successful.
>                                         Thank You,
>                                         Dinah Farrington

Your problem may be related to the fact that thiols are very easily
oxidized to disulfides by iodine (which is reduced to colourless iodide).
You should probably try a different reagent for revealing your compounds.
From memory people used to use an azide/iodine stain for TLC of thiols and
disulfides (eg sporidesmin) but I don't know any details. Stahl's TLC book
recommends a sodium nitroprusside spray reagent. Hope this helps,

Chris.