Hello, what does "C++" tag do in this question?

Hi, some mistake which I didn't pay attention :)

Hi,

I've worked a lot with pyridinium and similar organic salts, from my experience the separation in the salt state is almost never possible ( stright phase is mostly off limits for these salts, reversed phase chromatography works only in rare cases where the lipophilicity of the compounds would differ greatly -which is not the case for You, crystallization usually is not an option either, precipitation by forming metal complexes also usually fails and zwitterionic compounds are even worse). Thus the most straightforward way is to synthesize and purify the compound when it is not a salt and only in the last step to quaternize /hydrolyse and basify it.

Cheers,

Kaspars

Kaspars,

I was afraid of such an answer cuz for me solving that problem also seems a difficult (or even impossible) task.

Maybe someone want to add something more to it?

Thanks

MD

Please try fractional recrystallization.

Separating salts is indeed tricky. Fractional crystallisation as suggested by Rosario Algodon Salas might work.

One other thing that could also be successful in your case is exploiting the differing acidities of your compounds. What you basically want to do is neutralising your compounds stepwise, and then extract them with an organic solvent from the aqueous solution.

The idea here is that the ionic (salt) form of your compound will be preferrably soluble in water, while the neutral form is more soluble in the organic solvent.

In your concrete example, I would start in basic conditions, so that everything, including your pyridinium salt, is deprotonated. The former pyridinium salt should now be a neutral organic compound and can be extracted with an organic solvent, while the compound with the acid functionalities remain ionic and stay in the aqueous pahe. Then add increasing more acid until the compound with the single COO–-function is neutralised (the acid group should normally be protonated before your pyridin nitrogen – things get a bit more complicated if you have zwitterionic forms), and extract that. Only the last compound should be left in your aqueous solution at this point.

After separation you can then re-adjust the pH of your solutions to regain your compounds as salts.

Thanks for the answers

The problem is that I am working with just miligrams of the mixture of salts so it would be a bit difficult to do fractional crystalization. On the other hand nitrogen atom in pyridinium salts I am working with is quaternary (not protonated) so is always (+) charged - that's why I cannot try different acidities in this case as you suggested Simon. And additionally acidity of the carboxylic groups should be more or less similar as they are attached to the same moiety.

I think that any chemical transformations to solve this problem won't work - rather some instrumental methods, but still didn't find any.