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I have a reaction in which pyridin is used as a reagent in excess. The resulting product is water soluble and an anion. Pyridinium there will act as a counter ion. Normally, when the product is soluble in organic solvents pyridine is removed by washing with HCl 1 M or with CuSO4 10%. What is the best way to achieve this when the product is water soluble in comparison? In the moment I would apply these strategies:

  1. Applying vacuum and high vacuum until mass is constant (pyridinium salts can not be removed).

  2. Setting the pH of the water phase slightly basic to pH 8-9 and (pyridinium pka = 5.2) and so multiple extractions with DCM/water.

  3. Applying anionic exchange chromatography (Pyridin is washed through the column. Pyridinium is also replaced as a cation by a cation that is present in the mobile phase).

I am wondering if this is already the best possible approach or if you know of a means to remove pyridine more elegantly?

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  • $\begingroup$ Do you have access to a freeze-drier? $\endgroup$
    – Waylander
    Commented Apr 7, 2023 at 8:16
  • $\begingroup$ What is the composition of the aqueous solution, apart from pyridinium ? I it acid ? basic ? Is there some ethanol ?`some ether ? some inorganic compound ? $\endgroup$
    – Maurice
    Commented Apr 7, 2023 at 11:16
  • $\begingroup$ @ Waylander: Access to liquid nitrogen, high vacuum (10^-3 mbar) and a lyophilization system (1 mbar). $\endgroup$
    – raptorlane
    Commented Apr 7, 2023 at 11:46
  • $\begingroup$ @Maurice: The aqueous phase will contain the product, pyridinium as well as an excess of pyridine. No ethanol, no ether. Basic due to pyridin excess. $\endgroup$
    – raptorlane
    Commented Apr 7, 2023 at 12:00
  • $\begingroup$ @raptorlane I think freeze-drying under high vac is the easiest way to go if the pyridinium salt is an acceptable form. $\endgroup$
    – Waylander
    Commented Apr 7, 2023 at 12:38

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