Rabeya Bosri

Sometimes the oily compound resulted from presence of some impurities . try to extracted your product with a proper solvent . also you can dissolve the impurities with a solvent then decantation to get solidified compound.

Since amino acids contain both acid and amine group, pH also plays important role. So while work up proper care has to be taken to rid of all traces acidic/basic reagents used in reaction in order to get solid product .

sometimes you do not get solid because the product is very hygroscopic and the energy cost of losing the water is too high. I had the same issue with a product that would not crystallize unless all the solvents I used were freshly dried.

Also regarding the impurities hypothesis. Some impurities are not UV active ( like TEA which is used in many reactions). In those cases, you would not be able to see them on TLC ( unless you stain the TLC plate).

As I deal with those compounds, I found that the best way of dealing with them is to precipitate them from aqueous solution by dissolving your compound in large amount of acidic or basic solution, then try to adjust the pH to neutral.

Sometimes you have to improvise. Some compounds would be easier to prepare as hydrochloride salt, some as a sodium or potassium salt and some as zwitter ions. Explore all avenues!

Dear Rabeya, thanks for sharing this interesting technical problem on RG. In fact, this is a very common problem which virtually all organic chemists face from time to time. Countless crude products are first isolated as oils. I think that Kashif Ameer already provided a rather helpful and comprehensive advice. However, without knowing exactly what type of compound you are dealing with, it is a bit difficult to suggest to you the right method. For example, in carbohydrate chemistry it is quite normal that even rather pure products are oily or syrup-like. The first thing I would do in your case is heat the product under high vacuum at slightly elevated temperatures (e.g. 40-60 °C) in order to remove residual solvent as much as possible. Then you could e.g. disolve it in methanol and precipitate it again by adding diethyl ether (of course the solvent combination depends on the nature of your compound). That way you should be able to obtain a solid material.

Good luck with your experiments and best wishes, Frank Edelmann

Hi Rabeya,

When TLC is used, the developing agent with less polarity is selected to separate the raw materials and products. Or choose the appropriate solvent for recrystallization.

Thank you everyone for your suggestions.

Dear Rabeya Bosri just in case that your product is temperature-sensitive and you don't want to employ elevated temperatures, I suggest that you try a vapor diffusion technique which works at room temperature. You can dissolve your product in a higher-boiling and more polar solvent (e.g. THF, chloroform etc.) in a small vial. Then place the vial in a larger glass container which contains e.g. some n-pentane. Then close the container and let it stand at room temperature for a few days. The n-pentane will then diffuse into the solution, and chances are that nice crystals will form. Alternatively you can carefully layer a solution in a high-density solvent such as dichloromethane or chloroform with n-pentane. Crystals will then form between the two layers.

For comprehensive information on various crystallization techniques please have a look at the attached very useful tutorials.

Good luck with your experiments and best wishes, Frank Edelmann

Dear Frank T. Edelmann

Thank you so much for your important comments and suggestions. I will try as you suggested. I hope it will work this time.

Best wishes

Rabeya