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When I conduct a synthesis, after some workups, I usually evaporate large amount of solvent using rotovap, recover compounds sticked to the wall of the flask to the glass vial using small amount of solvent, and finally evaporate all residual solvent using vacuum oven (usually at low temperature about 20~30 ℃ and apply vacuum gradually overnight). After the solvent is completely evaporated, I usually get gooey gel-like compound that is sticked to the bottom of the vial.

sample vial

I have two questions:

  1. I have never obtained powder-like sample after evaporating the solvent. But I have a few experience in organic synthesis, so does the formation of gel-like form rather than powder form depend on the characteristics of the compounds, not the evaporating method? Actually, I have once obtained gel-like sample containing large amount of unreacted starting materials, even though that material was weighed as powder form at the beginning. How can I get the powder form rather than gel-like form when I evaporate the solvent? Does the evaporating method matter?

  2. After I evaporate all the solvent, I try to redissolve the sample with the same solvent, but it is not readily redissolved although I use similar amount. Usually it takes some time and heat. But why? It was readily dissolved in the workup stages. Is this because that the sample is sticked to the bottom of the vial?

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    $\begingroup$ Hard to tell without knowing what exactly is being dried. Unless you are dealing with ionic liquids, I'd expect powder from the salt, not necessarily the organic substance itself. You might want to try alternative methods such as lyophilization (takes talking with biochem people), or use vapor diffusion crystallization (takes hours (if very lucky)/days/weeks). $\endgroup$
    – andselisk ♦
    May 31, 2022 at 4:52
  • $\begingroup$ @andselisk I wondered if my evaporating method is generally not appropriate for obtaining powder form of the sample. Solvent I use is common organic solvents like toluene, dichloromethane, ethyl acetate, etc. $\endgroup$
    – Krang Lee
    May 31, 2022 at 4:53
  • $\begingroup$ @andselisk Do you have any opinion about question 2? I have almost always faced that problem. $\endgroup$
    – Krang Lee
    May 31, 2022 at 4:59
  • $\begingroup$ One of the common reasons for a lack of crystallization is impurities / trace impurities, including water, salts, can cause these problems. Again for crystallizations for solids is to dissolve in a warm solvent with a minimum volume. Once it is dissolved, very slowly adding a solvent which does not dissolve it (usually polarity based choices) and slowly cool it. In such cases, often pure crystals can be obtained. $\endgroup$
    – AChem
    May 31, 2022 at 5:03
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    $\begingroup$ If your compound is brand new, then you have no clue whether the compound is supposed to be a solid or a liquid and its color is not known either. Let us assume that you were expecting a solid, based on known and similarly structured compounds. The meaning of purity is quite different in different branches. What is pure for an organic chemist (on the basis of NMR etc.) is quite impure for an analytical/physical chemist. So purity is the key to successful crystallization. $\endgroup$
    – AChem
    May 31, 2022 at 5:04

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