Sciencemadness Discussion Board

Stability of amines

dextro88 - 26-5-2020 at 02:29

Hello can I ask a little question, I’m having some very old methylamine aq solution which have obvisiosly darkened and some old pyrrolidine bottles, they seems to have been stored well out of sunlight at low temperatures, my question is are they still ussable they are 2-3 years old and was thinking for example to purificate them via acidification,recrystallisation and was thinking is the efford adequate or they had decomposed ?

Dr.Bob - 26-5-2020 at 06:19

Aq. methylamine will degrade slowly, likely oxidizing and forming carbonate salts (amine + CO2). The challenge is that it cannot be easily distilled, so it will be hard to purify, other than trying to distill off the amine into some non-organic solvent and react with HCl to make the salt, which is stable, and then use it in the future.

The pyrrolidine is less stable, it may have formed all sorts of stuff, as it is not as stable as simpler amines. But it can be distilled easily, and that would give you a quick way to purify it. But I would just do that when you are ready to use it, as amines don't keep well, unless completely sealed air tight, which most bottles leak a little at the cap, so only ampules, and very good bottles will seal that way (even better if air is blown out with argon or the like before sealing.)

dextro88 - 26-5-2020 at 07:19

Thank you very much for the suggestion, actually they are in theyr original botles and air seled on the caps with lots of isolirband to prevent any gass leaks so they have been well defended from air, with minimal air in them and have stayed like that in relativly room temps, I probably will react the methylamine with naoh and buble in Hcl/Methanolic solution, evaporate and will recrystalize from boiling ipa and hope to recycle most of it, pyrrolidine botles some of them arent open even, probably destilation will be used, do you have in mind some test I could do to test the actual purity and decomposition level of the pyrrolidine?

[Edited on 26-5-2020 by dextro88]

draculic acid69 - 26-5-2020 at 18:11

I made some pyrrolidine a while back and it didn't keep well.it either discolours to a pale yellow or in the one that had some caustic soda in it turned black.just distillation it before use

dextro88 - 28-5-2020 at 10:40

thank you for you response, i cant even see the color as the botles are dark and are right from the supplier, but are the decomposition products with lower bp or with higher boiling point, i can apply vacum and destill it but woud it carry any byproducts that will act as amines and create nasty byproducts in my reactions ?



[Edited on 28-5-2020 by dextro88]

Dr.Bob - 28-5-2020 at 15:11

Almost all degradation products are higher BP and higher MW. That is why distillation works so well, as if you heat slowly and stop once the temp goes up, you get pretty clean distillate. I have just learned that it makes the most sense to purify things right before you need them, as time almost always will make the purest things less pure, unless sealed in ampules under argon, even then, some compounds degrade. Compounds can isomerized, get light activated radical chemistry occuring, and even undergo self reactions, like redox chemistry (cannizzaro reaction is one example.) But even those reactions are minimized by the original purity being high, which avoids traces of acid, base, water, etc, which tend to catalyze those reactions. But if you just purify things when ready to use, the storage is not a big deal.

morganbw - 28-5-2020 at 16:43

Thank you Dr. Bob, I do enjoy a tidbit of wisdom.
Hopefully I will have listened.

draculic acid69 - 28-5-2020 at 19:31

Quote: Originally posted by dextro88  
thank you for you response, i cant even see the color as the botles are dark and are right from the supplier, but are the decomposition products with lower bp or with higher boiling point, i can apply vacum and destill it but woud it carry any byproducts that will act as amines and create nasty byproducts in my reactions ?



[Edited on 28-5-2020 by dextro88]


With my stuff the impurities were higher boiling.all colours stayed in the flask.

dextro88 - 28-5-2020 at 22:47

Really i know the amines arent stable but i was forget abaut these bottles years ago and now when chemical suppliers take 6+ weeks to deliver i thinked for them otherwise they woud be destroyed, i dont like to use suspisous chemicals and and i am very thanksfull, you guys helped me throw this hard moment :)