I vacuum dried some pyridinium chloride (AKA Pyridine hydrochloride). This has a melting point of 144C. After drying overnight it stayed as a liquid which makes me wonder if the vacuum drying managed to boil off HCl, leaving pyridine alone.
Dear Jacob Johnston ,
I think I understand your reasoning however you need to keep in mind that the name pyridine hydrochloride is in fact wrong, the correct name is pyridinium chloride.
Historically the name pyridine hydrochloride is as you said yourself used but refers to the salt form of (in this case) pyridine made with the use of hydrochloride:
http://www.chem.ucla.edu/~harding/IGOC/H/hydrochloride.html
So there is no HCl left in the compound.
I suspect that your problem is related to the problem that your synthesis was not anhydrous enough and/or resulted in some impurities. I suspect that recrystallization will solve your problem.
See also here on Researchgate:
https://www.researchgate.net/post/Purification_of_organic_hydrochloride_salt
Hope this helps you a bit further.
Best regards.
Hi Jacob Johnston, it is not possible to eliminate HCl from pyridinium chloride (pyridine hydrochloride) by appling vacuum. It is a rather stable organoammonium salt which, as you mentioned, has a well-defined melting point. If it would decompose, the pyridine would also evaporate under vacuum. My feeling is that the problem here is residual water which prevents crystallization / solidification. Try triturating the oily material with diethyl ether. Upon stirring this mixture with a glass rod, chances are that the pyridinium chloride solidifies and can be isolated by filtration. Good luck!
The peculiarity is it arrived as a solid from a supplier as expected. I opened the lid slightly to allow air in/out, put it in the vacuum oven overnight at 80C, then in the morning it was about half liquid, and half crystals once cooled. It surely can't be from moisture contamination as it was only open to the air very briefly while I moved it in and out of the oven.
Dear Jacob Johnston, this is indeed pretty strange. What I could imagine is that the salt had already attracted some moisture as it is very hygroscopic. Perhaps a small amount of water was already present in the sample received from the supplier. This could explain the fact that is was partially liquid after heating in the oven. What you can do is to dissolve the salt in a minimum amount of ethanol and precipitate anhydrous pyridinium chloride by addition of diethyl ether, in which it is insoluble. One other possibility is to heat the solid with benzene and distill off the last remains of water as an azeotrope with benzene. However, this method is less preferred due to the high toxicity of benzene.
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