[goldy]

Hello,
I am in trouble because I could not seperate compounds. My compounds are oily and UV inactive(may be essential oils but tried the methods given in different books for essential oils and didn't work.) So I want to isolate them by crystallization. Please suggest me the possible solvents and method for crystallization as soon as possible.
Thank you very much.

[orgopete]

This is the barest amount of information to make suggestions upon. Many mixtures can be oils. I am presuming that a lack of absorption in the UV indicates a lack of functional groups. If someone were to ask which compound had a greater intermolecular bonding forces, propionic acid, isopropanol, or diethyl ether, they would know it was the acid. Consequently, the boiling points and melting points also follow that trend.

If someone were to ask you whether the mp of a compound without any functional groups, whether it will he high or low, they would say it would be low. If you wanted to crystallize isopropanol from diethyl ether, I wouldn't say it would impossible, but I would advise you that it is likely to be quite difficult unless you were accustomed to operating at very low temperatures.

Tell us about the problem. How many components are present? Are they isomers? What is the MW(s)? What elements are present? You should be able to get more information about this mixture from an IR, GC, and MS. If column chromatography is ineffective, distillation may be a suitable option.

[ardbeg]

If your compounds are oils at RT then crystallization is pretty much out of the window.  If the mix is oily and your product is a solid, you may want to add cold pentane to try to triturate and precipitate your product.

[discodermolide]

Why don't you post the structures, it may then be easier for us to assist!

[Smrt guy]

The fact that you're mentioning the UV inactivity suggests you do not want to do a column (or that you think you cannot).  Using a phosphomolybdic acid dip you may be able to see the spots on TLC.  Otherwise you would probably have to distill.

[goldy]

Why don't you post the structures, it may then be easier for us to assist!

Actually the structure of the compound is unknown. This is the hexane axtract of the medicnal  plant. Also I didn't find the reference for this work. This must be the lipid. The HNMR of the compound shows that this is the very simple compound without any important coupling(eg duplet,trplet etc.).
The most important thing is that I could not seperate the pure compound. I only can use silica open column because so I could not get the satisfactory result.
Please suggest me the best way for the seperation of this kind of compounds.

[orgopete]

There aren't any magical forces between molecules. Those with relatively strong forces, like hydrogen bonding, are more likely to be higher boiling, melting, interact more strongly with stationary phases that or are soluble in solvents that participate in hydrogen bonds. If not too polar, silica can be used for chromatography, but increasingly you will see reverse phase chromatography being used.

If your compound does not have groups that interact as strongly, then simple distillation and vapor phase chromatography can often be used. Reverse phase chromatography might also be used, but you may need to have an alternate detector.