I've done a synthesis reaction with 3 precursor. the product has shown different physical appearance compared to those three precursors, but when it comes to TLC, the stain just won't separate by any combination I tried. should I try more of combination or do you have any suggestion about the eluent selection?
Physical appearances mean nothing. Some metal catalysts, like Pd catalysts will turn black over the course of the reaction, it does not mean the product is formed.
You need to provide more info for anyone to help you. With 3 precursor, are all 3 forming the skeletal backbone, or are some just reagents? What sort of solvent combination have you tried? Compounds with certain functional groups may need to add some acid or base. Are all the spots stuck on the baseline, middle, top?? There are so many variables and too little info.
You can also try HPLC or LCMS to confirm that some product has actually form.
Hi, you did not mention your compounds, it could be your TLC systems are not correct. Please search the TLC conditions in Camag TLC data base (for your compounds). Kind regards
http://www.camag.com/en/tlc_hptlc/customer_magazine_cbs/ccbs_database.cfm
Different physical appearances could be due to several factors including formation of variant ligands from the positioning of particular metals, and ofcourse, the presence of impurities as you fear.
I would suggest trying several solvent systems (gradients) on TLC, exhaust them as reasonably as possible.
- Check the reagents being used (preparation/ expiry date, etc)
- Check the plates
f this fails, you may check the structure again using spec. to confirm if it really is the one.
- Lastly, should all fail, if you can, redo the synthesis
thank you for the answers Mr Yang, for this synthesis I used succinimide, 4-methoxybenzaldehyde, and urea, all are expected to form skeletal backbone, until now I have used dichloromethane : n-hexane, n-hexane : ethyl acetate, I use acetone to dissolve my products with some white precipitates formed (even after I filter it the TLC results no difference). After the elution the spots (4-methoxybenzaldehyde and product) have the same Rf, only the product spot formed tailing and there are some left on the bottom.
thank you for the suggestion, Mr Indrayanto, Mr Jameel, Mr Mtews and Mr Marvelous, but is it okay to check the NMR without knowing its purities? I'm quite sure that there some byproducts or some unreacted precursors upon the synthesis.
I am not familiar with the product you are trying to synthesise, but it seems to be a type of cyclic condensation product.
I highly doubt the 4-methoxybenzaldehyde will have a similar Rf to the product. 4-methoxybenzaldehyde is quite non-polar, if the product spot is very close then you will probably need to try hexane:ether or pentane:ether combinations rather than EA or DCM. I think you need to dilute further and run the TLC with a co-spot. In addition, aromatic aldehydes should show up very well with staining.
Tailing can usually be resolved by adding either a small % of base or acid.
NMR on crude mixtures can be done, and are done routinely by some labs. For example, you can look for whether the aromatic aldehyde signal is still present, this will indicate whether 4-methoxybenzaldehyde is still present. But for this, LC-MS will probably be even faster if available.
Yes, it is a Mannich reaction. Thank you for your suggestion, Mr Yang, but is there no possibility that the acid or base will react with the product or cause the product formed to change or dissociate?
You could try to re-develop of the spot on tlc, but firstly find the suitable mobile phase for the spot to be migrated at rf 0.30-0.35. My opinion for mobile phase resembles with Dr Eugene. You could also try hexane:chloroform (ex:20:1, 30:1) or with toluene combinations. For tailing spot, try to dilute the product, just wondering the spot might be too concentrated. Kind regards.
Thank you, Mrs. Sabandar for your suggestion, that is very considerate of you.
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