I have synthesized some schiff base.It got precipitated from solvent (ethanol). Then I filtered it and washed it with ethanol.
But I observed that, residue starts dissolving when I wash with ethanol.Why it is so?If it is soluble in ethanol then how come it got precipitated from reaction mixture?
Reaction temperature:70oC
From your description the first answer would be, that you have simply reached solubility of the base during the synthesis and the rest got precipitated. (your compound is soluble, but not well soluble in ethanol) After you have added some more fresh ethanol(washing), some of your product simply redissolved in it. Another answer could also be different solubility at different temperatures. Of course there could be even more complicated answers, but try to measure H NMR of solution and the solid part in the deuterated ethanol and you will quickly find the answer.
OK.I tried washing with cold ethanol,as we expect solubility is less.But how at reflux condition it's not soluble?
Every compound has g/mL based solubility. In general it is possible that the quantity of washing solvent used is quite more. Actually when your some part of product expose to bigger quantity of solvent it could definitely dissolve in it. Use max of 1 to 1.5 volume of your solvent according to your compound weight. If you use more, you will definitely loose weight. It is applicable for all compounds not only yours as it is phenomena in crystallisation. If you really want to avoid this use 50:50 (ethanol:hexane) or 70:30 (diethyl ether:ethanol) or IBA. This could take away any impurities.
Schiff bases are susceptible to hydrolysis. If your ethanol is not dry, for example 95 % ethanol, the Schiff base gets hydrolyzed and this changes its solubility behavior.
Dear Vikram Thimaradka for some substances when we decreasing temperature the solubility is increasing.
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