I have an aqueous solution of N,N-dimethylphenylalanine methyl ester hydrochloride and I need to crystallise it as a salt. I'm afraid of increasing the temperature or the concentration of acid due to the possibility of ester hydrolysis. What crystallisation method would be the most efficient in this situation? Any help would be greatly appreciated.
If you have no solid to test solubility, my first attempt would be to use an anti-solvent (which you'll need to determine empirically). The anti-solvent for your hydrochloride salt should be miscible with water, so for ease, I would suggest starting with simple alcohols; methanol, ethanol, 2-propanol. See which gives you some solid precipitating out of solution, check purity and then work from there on concentrations or temperature if required.
This molecule is an ammonium anion with an ester side chain as a chloride salt.
The first option you can choose to undertake is the vacuum evaporation of the solvent at room temperature or 30 degrees should prevent any heat degradation. I do understand your point about increasing the acid because in effect you have a buffer solution.
Your second alternative is to partition into a warm amine-ophilic non-aqueous/organic layer and crystallize from there.
You can read more about that here: https://books.google.co.uk/books?id=YY3pCAAAQBAJ&pg=PA8&lpg=PA8&dq=partitioning+of+quaternary+organic+salts+into+solvent+layer&source=bl&ots=O0iyoLglNF&sig=jQb5hDP0SvhA2kg7CXGTXsIE4bg&hl=en&sa=X&ved=0ahUKEwjAydrc5pjaAhXCIMAKHQI4ANgQ6AEITjAF#v=onepage&q=partitioning%20of%20quaternary%20organic%20salts%20into%20solvent%20layer&f=false>