[Urbanium]

This really simple procedure for preparation of dithiolanes mentioned here on the bottom of the page 446 (start of the Experimental section):

onlinelibrary.wiley.com/doi/10.1002/mrc.2764/abstract

says that the product is purified by distillation after the reaction is stopped. Tried it twice and in both cases I got nothing from distillation (although 1H NMR shows clearly that the product is formed).

I was considering isolating the product by extraction with pentane or hexane. The original reaction mixture contained 1,2-ethanedithiol (0.04 mmol), acetaldehyde (0.044 mmol) and small amount of p-tosylic acid (50 mg). Could extraction be a good alternative procedure for isolation?

[pgk]

You have probably preceded the distillation under vacuum and after the end of distillation you have quickly open the air valve. Thus, atmospheric oxygen violently penetrated in the warm (hot ?) distillation flask and quickly oxidized the so prepared 2-methyl -1,3- dithiolane to a mixture of the corresponding 1-oxo- and 1,3 –dioxo derivatives.
(Remark: Serious fires might start by similar procedures,  in bulk quantities ( > 1 ton). Why?).
Alternative procedure 1: Slowly opening the air valve after the end of the vacuum distillation.
Alternative procedure 2: After the end of the vacuum distillation, gradually increase the pressure by slowly introducing N2, at low rate and monitor the pressure by the manometer.
Alternative procedure 3: In acidic aqueous medium (TsOH), 2-methyl -1,3- dithiolane will be hydrolyzed and ring opening will occur. So, you can proceed the extraction with a saturated mixture of NaHCO3, followed by washings with water until neutral to litmus.
PS: Boiling point of 2-methyl -1,3- dithiolane 175 °C at 760 mmHg that roughly means near 70oC at 14 mmHg. (If you operated a distillation at normal pressure at ≈ 180oC, it is even worse, regarding sulfur oxidation.)

[Urbanium]

Not sure if we misunderstood, but the problem with distilling is that I get absolutely nothing out of the mixture. Nothing is condensed in the condensate-collecting flask. BTW I'm trying to prepare 1,3-dithiolane, the non-methylated product. Authors say its bp is 42°C at 2 kPa. I warmed up the system up to approx 70 °C but nothing goes out. And it was done under reduced pressure provided by the common aspirator attached to a water line, don't know how much does it lower the pressure.

[Urbanium]

Alternative procedure 3: In acidic aqueous medium (TsOH), 2-methyl -1,3- dithiolane will be hydrolyzed and ring opening will occur. So, you can proceed the extraction with a saturated mixture of NaHCO3, followed by washings with water until neutral to litmus.

I was planning to extract with hexane or something other nonpolar, if 1,3-dithiolane is going to pass from DCM to it. What you wrote means that I should do the extraction with NaHCO3 solution instead or that I should wash the hexane extract with that?

[Dan]

42 °C at 20 mbar is about 150 °C at ambient pressure (incidentally, this is very close to 1,2-ethandithiol). If it was me I would attempt a pentane or ether extraction from aqueous NaOH (ethanedithiol should go into the aqueous at pH 12, and dithiolanes are base stable). Concentrate at 0 °C.

Your aspirator may not be hitting 20 mbar, but if you're still not seeing distillate at 70 °C that implies your aspirator is not pulling below about 70 mbar (I am getting these values from a nomograph). Do you have access to a vacuum gauge to check the actual pressure of the setup?

[pgk]

Just a minute. Sorry, but I am missing something. How do you prepare 1,3-dithiolane, the non-methylated product by adding acetaldehyde to 1,2-ethanedithiol? Do you mean paraldehyde?
Anyway, if you are planning to use DMC, just use aqueous NaOH at pH = 12, as Dan advices, because bicarbonates form strong emulsions with chlorinated solvents, during extractions. And then, neutralize the organic phase by repeated extractions with water, until the aqueous phase, will be neutral to litmus.
In addition, you can get read-off of acethldehyde (or formaldehyde?) by further extracting he organic phase with NaHSO3 (saturated solution of sodium metabisulfite).

[Urbanium]

Yes, I meant paraldehyde.

Thanks a lot for all the suggestions.

[pgk]

The right nomenclature is paraformaldehyde. Anyway, paraldehyde is a nomenclature that is widely used, too.
According Aldrich, the boiling point of 1,3-dithiolane is 183oC. So you need a distillation temperature higher than 70oC at 2kPa.