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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Babcock_Hall on March 31, 2025, 08:57:58 AM

Title: Purifying a thiol by silica gel chromatography
Post by: Babcock_Hall on March 31, 2025, 08:57:58 AM

We need to purify a thiol for the first time.  It is in the form of a diethyl ester bearing a phosphonate group.  Obviously, we need to be mindful of the possibility of oxidation and work quickly.  Are there any other problems that we might encounter, such as streaking?  It occurs to me that an additive such as 0.5% HOAc might be helpful, but I am not sure.

Title: Re: Purifying a thiol by silica gel chromatography
Post by: rolnor on April 03, 2025, 02:48:21 AM

I don't think it so critical, are you afraid of disulfide formation? If you are going to alkylate it, you can do this on the raw product, then purify the sulfide i guess.

Title: Re: Purifying a thiol by silica gel chromatography
Post by: Babcock_Hall on April 03, 2025, 07:35:08 AM

Yes, disulfide formation was on my mind.  In water, it happens more quickly at higher pH (more thiolate), but perhaps disulfide formation is slow under the conditions of chromatography.

Title: Re: Purifying a thiol by silica gel chromatography
Post by: rolnor on April 04, 2025, 01:14:26 AM

I have worked with 1,3-propnedithiole and even a very old bottle is pure, I dont think this happens very quickly unless you have an oxidizer present. I made phenyl disulfide a long time ago, I remember I used base and iodine as oxidizer. All this chemistry really stinks...

Title: Re: Purifying a thiol by silica gel chromatography
Post by: wildfyr on April 04, 2025, 05:32:48 PM

Work quickly and under acidic conditions to limit disulfide formation.

adding acetic acid sounds like a good idea as long as you won't struggle to remove it after.

Title: Re: Purifying a thiol by silica gel chromatography
Post by: Babcock_Hall on April 07, 2025, 06:11:23 PM

We ran the column and obtained three fractions.  One is a nearly pure thiol, the second is the thiol with some of the starting material (still bearing the S-acetyl group).  We were not able to find solvent conditions that allowed for much separation between the two; therefore, this result was not surprising.  If we have to perform this step again, we will look for ways to improve the separation.  Formic acid might be a little easier to remove than acetic, owing to a slightly lower boiling point.  We also saw a late-eluting compound that might be the disulfide.  The disulfide might have arisen from the deprotection reaction, which is performed under alkaline conditions.

Title: Re: Purifying a thiol by silica gel chromatography
Post by: rolnor on April 08, 2025, 12:47:58 AM

But the disulfide should be less polar? At least a little? Nice work!