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Author: Subject: Separation of thiols and ketones
Baphomet
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[*] posted on 31-8-2008 at 23:19
Separation of thiols and ketones


Hi everyone. Either I am an eccentric genius or blithering idiot. But I like to avoid vacuum distillations (esp. of mercaptans!) and so I want to run something past you all..

The following is an extract from a Google Books entry, reproduced here for research purposes:

Quote:

http://books.google.com/books?id=ZCXPte7PL0EC&pg=RA1-PA421&l...

Thiols are stronger acids than alcohols. Compare, for example, the pKa values of ethanol and ethanethiol in dilute aqueous solution:

Ch3CH2OH + H2O <=> CH3CH2O- + H3O pKa = 15.9

Ch3CH2SH + H2O <=> CH3CH2S- + H3O pKa = 8.5

The greater acidity of thiols compared to alcohols is a combination of two factors. First, S-H bonds are generally weaker than O-H bonds, which facilitates removal of the S-H proton by a base. Second, sulfur (a third-period element) is larger than oxygen (a second-period element), which means that the negative charge on an alkylsulfide ion (RS-) is delocalized over a larger area and is, therefore, more stable than the negative charge on an alkoxide ion (RO-).

Thiols are sufficiently strong acids that when dissolved in aqeuous sodium hydroxide, they are converted completely to alkylsulfide salts:

CH3CH2SH + Na+OH- --> CH3CH2S-Na+ + H2O

pKa = 8.5 pKa = 15.7
(Stronger acid) (Stronger Base) (Weaker Base) (Weaker Acid)

To name salts of thiols, give the name of the cation first, followed by the name of the alkyl group to which is attached the suffice -sulfide. For example, the sodium salt derived from ethanethiol is names sodium ethylsulfide.


This naturally leads me to conclude that separation of thiols and ketones could be as simple as:
- Adding NaOH solution
- Gently heating to drive off water
- Filtering to capture the alkylsulfide salt

Is this right? Thanks




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[*] posted on 1-9-2008 at 02:10


Sulfides tend to under go hydrolysis rather easily I believe. So I would think that heating to drive off the water may lower yields or destroy your product. Also, if its in a basic solution, I'd imagine it hydrolyzing even faster to an alcohol.

I don't know very much about thio chemistry, but I'd imagine this would happen at some point: RSNa + H2O = NaHS + ROH

If you could give us information on what chemicals you are dealing with, and if your wanting to keep both, or just one, we may be able to help you more.

[Edited on 1-9-2008 by kclo4]
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[*] posted on 1-9-2008 at 04:24


Thanks for the info kclo4, I should have thought of that. So heating is not an option...

I'm dealing with a mixture of butanethiol and MEK. The idea is to separate the butanethiol, and if it was possible to form a salt then it's even more convenient. Because then the super stinkbomb can be set off with a small amount of acid :D

Should I try adding a stochiometric amount of NaOH pellets to the mix instead of a solution? Then the water could be dried-off in a CaCl2 chamber..?

I guess the other main thing I wanted to confirm, was that MEK and NaOH will not react.




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[*] posted on 1-9-2008 at 06:57


MEK and NaOH will make a nice aldol condensation, even more so when heated.



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[*] posted on 1-9-2008 at 08:05


Thiolates are very easily oxidized to their disulfides.... Only use cold, degassed condtions, or perhaps a dash of dithionite could you avoid this. Although the smaller chained thiosl might be a little more resistant.

What are the proportions of the MEK and the butanthiol? Is a solution of the thiol in the ketone, or rather equimolar amounts?

Bisulfite adduct first came to my mind, but the bisulfite ion recats with tiolates to form Bunte salts (alkylthiosulfates), and considering, and depending on the pH of the solution, you might have enough to drive the equilibrium forward enough...

I do think a simple fractionnation might be more adviseable. No need for vacuum, but use a bleah wash bottle to avoid any smell, and soak you glassware in bleach or base bath after to remove the smell.

1-butanethiol boils at 98C, and MEK at 79.5C, so shoul dbe easily seperated with a good vigreux. There could be a azeotrope, so it all depends on the proportions of each constituant. I take it you prepared the thiol in MEK? Or is it some kind of commercial solution? You could always recycle the butanethiol-containing MEK for further reactions?




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[*] posted on 1-9-2008 at 09:44


I have a feeling that adding a base to that mixture wil give you a thioketal.

On the other hand, if you add the right base you might be able to do this. Thiols get their alternative name, mercaptans, from their reaction with mercury compounds. OK, I'd rather distill mercaptans than play with lots of mercury salts, but I think you might be able to separate the thiols as copper or zinc salts. With a bit of luck and experimentation you might get a salt that's nice and insoluble so you could just precipitate the thiol salt, filter it off and then distill the MEK to clean it up. Adding acid to the salt and distilling should regenerate the thiol.
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[*] posted on 1-9-2008 at 17:19


Thanks for the info guys. I will try NaOH in a cold solution, hopefully it will not form thioketals. Maybe I can add a dessicant like CaCl2 directly to the mixture, and if there is not too much water then it can be trapped as water of crystallisation.



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[*] posted on 1-9-2008 at 17:27


Don't add CaCl2, it can react with MEK as it can do with acetone. In any case, it will promote aldol condensation of MEK, again just like acetone...

I would really go for a standard atmospheric fractionnation.




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[*] posted on 1-9-2008 at 18:36


If you're so dead against distillation (why? You can, as Klute said, avoid the smell by using simple bleach!), extraction might work. Dissolve in ether, DCM, whatever. Basify with dilute NaOH (1-2 M, maybe), the thiolate should go into the aqueous and you can extract the ketone. Then acidify and extract the thiol.

You shouldn't have a major issue of ketalisation or aldol reaction if it's kept at RT or below. Work relatively quickly, though, just to be sure.

Once again, I vote with Klute - distillation would surely be the easiest and most certain method for this job!
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[*] posted on 1-9-2008 at 19:53


Will that work? MEK is soluble in water to some extent...

Thanks for the pointer, but have you ever smelled a low-mw thiol?! ;)




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[*] posted on 2-9-2008 at 01:56


I work with thiols fairly frequently (although I'm focusing on some non-sulfur work at the moment, which is nice!)

And you'll need to extract a few times (once won't be enough) to get all the MEK out of there. Same will apply when you come to extract the thiol - many of them are somewhat water soluble, too. But ultimately, the vast majority WILL move into the organic layer, as it is preferentially soluble in that.
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[*] posted on 2-9-2008 at 05:47


Well, I won't go insisting anymore than necesarry, but :)

From my experience with thiols, even in solution they smell BAD. I'm sure you will be much more exposed to the smell while extarcting, acidifying, etc, than just dumping the whole mess in a distn setup, distilling most of the MEK with a bleach wash bottle connected to the vacuum inlet, swapping receivers when temps goes up, discarding the MEK in dilute bleach, collecting your thiol, stoppering the receiver, and quickly immersing the whole setup in a tub of dilute bleach (or directly rincing it with bleach, setup closed).

I'm sure your product will contain much less MEK than by extracting, and you will have to remove your solvent at one point and perform a distn, so why noy just do everything in one step and in a closed setup? :)

BTW, did you make the butanethiol yourself?




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[*] posted on 3-9-2008 at 02:23


OK well.. probably distillation would be easier. But I'm interested in other methods anyway. It's not just the end result but the learning process that matters. I'll try distill but have thought of another harebrained idea to try as well - apparently PbO can form a complex with thiols which then falls out of solution and can be recovered with acidification.

Plus there is the added benefit that the thiol will already be in a salt form. Where with a distill I will still need to convert to a salt afterward.

Yes I made the butanethiol myself. From rotten fish and garbage juice ;)




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[*] posted on 3-9-2008 at 05:57


Don't got throwing lead salts on to people now :)

So you extarct rotten fish and garbage with MEK? Nice weekend distraction, I'm sure your relatives must like it :)

Let us know hwo it goes!




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[*] posted on 7-9-2008 at 08:06
Butanethiol


BUtanethiol is toxic to males; you could get sterile (donot know if this is reversible). On the other hand, by no means basic treatment could lead you to a separation without entering in aldol condensations. You may think perhaps in oxidizing the thiol to the disulfide, which should be solid or at least have a high BP and then distill your ketone before reducing again the disulfide.
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