Recrystallizing but not getting any crystals

  1. I am trying to recrystallizing lidocaine out of methanol. I created a saturated solution of lidocaine and hot methanol, waited for it to cool to room temperature, then threw it in a cool water bath. There was no recrystallization. I decided to evaporate the methanol to recover my lidocaine and start over again, but after evaporating the lidocaine in a hot water bath, I was left with a transparent gooey substance in my beaker. Usually when I dissolve something into a solvent and evaporate off that solvent, crystals will form (even though they may not be pure). What happened here? And how can I recover my lidocaine?

    EDIT: Just to be clear, this is lidocaine HCl.
     
    Last edited: Jun 17, 2014
  2. jcsd
  3. DrDu

    DrDu 4,420
    Science Advisor

    Getting an oil instead of crystals is very common, especially on rapid evaporation.
    Are you sure your solution was supersaturated? Sometimes you have to wait for days until crystals form, so you could leave your solution for some time in the fridge and see what happens. You could also add a small amount of crude lidocaine to your solution to act as a seed.
     
  4. Another thing to check out is that certain salts of organic compounds are not always the easiest to prepare and subsequently purify, for any number of reasons. If you just need a pure lidocaine salt, you might want to switch it up and make your life easier if the hydrochloride salt continues to be ill-behaved.

    I presume you checked the suitability of various solvents/solvent mixtures for recrystallizing lidocaine - if you haven't, that's something that should be done as well.
     
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