Hi Jin,

I'm not familiar with Lawessons reagent but will give suggestions from my little experience in organic synthesis.

If you spot the crude product and the reagents on same TLC and develop, you will see if there is any corresponding spot with Rf similar to the starting material.

If there is no spot corresponding to any starting material from the crude, probably you've got side reactions and the impurity could come from here. You may wish to check possible side reactions in this case.

If however only one spot, it's possible you've got a 100% completed reaction with no impurities or its possible the impurity is not UV active (since you are using UV light). You may wish to use TLC sprays here if you suspect a UV inactive impurity.

Impurities of organic origin are easily spotted in 1H NMR. Since you've done the NMR, you could examine the spectra for impurities. This may also help in identifying the particular impurity. If it's coming from a solvent, you'll see the solvent peak on the spectra.

If the baseline spot still remains at the baseline even at 100% of your polar solvent, you may need not worry cos it is probably gonna get stuck in the column after your product is eluted.

Sometimes, the product could have an affinity on purification solvents. Ethyl acetate particularly is a bit hard to get rid of. Solvent of crystallization is also a possiblity for crystalline products, and may increase the weight of the purified product. Dry the product in vacuum desicator overnight. You may use vacuum oven if your product is thermally stable.

Ideally, percentage yield is not expected to exceed 100%. From my experience, it's mostly due to solvent of purification, and proper drying of product helps.

I hope this helps.

Kavnen,

Many thanks for sharing your experience.

There are no significant signals of any solvents but CHCl3, which is from chloroform-d in the NMR spectra.

And I developed Lawessons reagent and the reaction mixture on the same TLC plate. The Lawessons reagent was on the baseline and my product had a Rf of ~0.3. But the impurity shouldn't be LR because I quenched the reaction with sat. NaHCO3(aq) and LR can't survive such condition.

The baseline spot was still there even after silica gel chromatography.

I suspect the impurities should be some inorganic compounds but I don't know exactly.

Hi Jin

Sometimes when you quench a reaction, the starting material is converted from a reactive form to an unreactive form and the bottom line spot could be the unreactive form of LR or persistent LR that wasn't quenched (probably insufficient quench reagent ie NaHCO3 in this case). You may wish to explore the quench mechanism (ie between NaHCO3 and LR) to predict the unreactive or degraded form of LR.

The Rf value is small and It's possible you have a co-elution of the bottom line spot and your product. The particle size of silica gel used in flash column is not same with that on TLC plate so it's possible to have a co-elution from column even though there's good separation on TLC.

You could do this side experiment: increase solvent polarity to make the bottom line spot from your purified product to move. If it's moved Rf corresponds to LR, it's an indication that the impurity is LR. If there is a difference in Rf, it's possible it's from the unreactive form or degraded LR.

Also, if it's not gonna be too much hassle, I recommend you run a 1H NMR of the LR and your purified product separately. Stack the two spectra together (use a spectra processing software) and examine if there are corresponding peaks with your product and LR... Such peaks will have integral values significantly different from the integral value of one proton in your compound. Impurity peaks could be small hence zooming may help. If the impurity is from degraded or unreactive form of LR, you'll see peaks which do not correspond to either your product or starting material.

Thank you very much for your advice, Kavnen.

Sometimes you may spot too much sample or your sample contain water!?

4-Methoxybenzenecarbothioic acid will be the by-product of the Lawesson's reagent after the functional conversion of the carbonyl group (C=O) to the thiocarbonyl group (C=S). That might be the spot you are seeing at the baseline of your TLC and this by-product is also UV active which is in agreement with your observation.