Sciencemadness Discussion Board

Pyridine from Pyridine hydrochloride

nimgoldman - 7-10-2018 at 13:28

I have some wet pyridine hydrochloride (Py.HCl) which I would like to turn back to pyridine base.

I guess this procedure is simple:

- dissolve Py.HCl in water
- add molar excess of dilute base (e.g. 10% NaOH)
- extract pyridine with pet ether or ethyl ether
- distill off solvent
- dry pyridine with NaOH, filter
- distill fresh pyridine over NaOH

Is this the correct procedure?

Sigmatropic - 7-10-2018 at 13:37

Why wouldn't you just skip all steps and distill over excess NaOH? The salt is free based and dried simultaneously.

fusso - 7-10-2018 at 13:38

Why not use conc/sat NaOH? Less water means less pyridine would dissolve in it, and less pet/diethyl ether needed.

JJay - 7-10-2018 at 14:28

Why wouldn't you dry the pyridine before distilling off the solvent? That way there would be less mechanical loss from product getting caught up in the drying agent.

nimgoldman - 7-10-2018 at 14:45

Quote: Originally posted by Sigmatropic  
Why wouldn't you just skip all steps and distill over excess NaOH? The salt is free based and dried simultaneously.


Are you sure the molten pyridine hydrochloride will react with NaOH and the reaction won't be too vigorous?

I guess the side product would be NaCl which will stay dissolved in water, but using just molted pyridine.HCl could result in caking of the NaCl precipitate.
Quote: Originally posted by JJay  
Why wouldn't you dry the pyridine before distilling off the solvent? That way there would be less mechanical loss from product getting caught up in the drying agent.


Yes I would definitely dry the solvent containing pyridine and also wash the drying agent with fresh solvent to prevent losses.

I believe pyridine would have to further dried with at least a little NaOH/KOH as it is so hygroscopic.

[Edited on 7-10-2018 by nimgoldman]

AvBaeyer - 7-10-2018 at 17:55

It is relatively difficult to extract pyridine from water even if the aqueous solution is basic. The best thing to try is to add your HCl salt to 25-30% potassium carbonate solution (a few ml per gram of salt) in a sep funnel. Add ether and shake vigorously (cautiously). A second ether extraction may be needed. Dry the extracts over potassium carbonate, remove the ether and distill the recovered pyridine. This is a method I have used over the years to isolate amino acid esters from their HCl salts and works quite well. I cannot guarantee it will work for pyridine but I would try it.

AvB

nimgoldman - 8-10-2018 at 09:12

Quote: Originally posted by AvBaeyer  
It is relatively difficult to extract pyridine from water even if the aqueous solution is basic. The best thing to try is to add your HCl salt to 25-30% potassium carbonate solution (a few ml per gram of salt) in a sep funnel. Add ether and shake vigorously (cautiously). A second ether extraction may be needed. Dry the extracts over potassium carbonate, remove the ether and distill the recovered pyridine. This is a method I have used over the years to isolate amino acid esters from their HCl salts and works quite well. I cannot guarantee it will work for pyridine but I would try it.

AvB


Thanks. I was thinking about salting out the pyridine from water. Potassium carbonate should work better than sodium chloride. I will try it.

nimgoldman - 16-10-2018 at 18:10

Just reporting the extraction was successful with a good yield.

Next time I have to be more sciencey and weight everything to actually quantify the yield...

[Edited on 17-10-2018 by nimgoldman]

Tsjerk - 17-10-2018 at 11:18

At least tell us what you did

nimgoldman - 17-10-2018 at 11:47

Quote: Originally posted by Tsjerk  
At least tell us what you did


It's in the OP.

The only modification is that Py.HCl is dissolved in 25% K2CO3 solution instead of plain distilled water.

Etereal extract have been evaporated in a rotavap at atm. pressure. No ether came over up to 75 °C, leaving behind clear liquid of the characteristic pungent smell of pyridine.