Analysis of a mix of anhydride and its dicarboxylic acid

[lo_stefano]

Hello to everybody.

I have a problem in the determination of the purity of an anhydride.

I am trying to check this parameter usign a GC with a DB-5MS column. I obtain a good peak of the anhydride but I am not sure if there are impurities, in particular I am not sure if with this method I can see the probable presence of the dicarboxylic acid.

I dissolve the anhydride into pure acetone and after the addition of water no other peaks appear in the chromatogram.

Have you any idea about a solution of this matter? I was thinking about the use of a FFAP column but I have never used it.

Thank you for your support!

Additional information:
Boiling point of anhydride = 327°C
Boiling point of dicarboxylic acid = 408°C

[Don_Hilton]

How are you disolving the anhydride to analyze it? Are you using an anhydrous (and aprotic) solvent?

You note that you find no additional peaks after adding water to an acetone solution - but I expect there to be sufficient water in acetone that you might not need to add any water to completely hydrolize an acid anhydride anyhow. So, if you are using acetone, you may be looking at the dicarboxylicacid already.

[lo_stefano]

Thank you for your answer!

I am using acetone with 0,1% of water. So, you could be right!

Anyway, the odd thing is that there are hundreds of chinese reviews where they use Hplc systems with wayer:methanol as eluent...

The anhydride is particular, the name is cantharidin, look to the structure for better considerations.

Thank you again!

[Don_Hilton]

It looks like you have an anhydride that does not like to hydrolize. With the propensity of this compound to be stable in the presence of water, and I would wonder if it would revert to the anhydride in the vapor phase in the GC inlet. I would get a sample of the dicarboxylic acid and run in on the GC - you might find an identical peak.

[lo_stefano]

It looks like you have an anhydride that does not like to hydrolize. With the propensity of this compound to be stable in the presence of water, and I would wonder if it would revert to the anhydride in the vapor phase in the GC inlet. I would get a sample of the dicarboxylic acid and run in on the GC - you might find an identical peak.

So, do you think that this anhydride is stable? I thought to get the acid standard, but if I find the same retention time I have the beginnig problem...

[Peter Apps]

Cantharidine is a natural product - the active ingredient in Spanish fly and also found in several other beetle species - so it must surely be stable in the presence of water.

Years and years ago I analysed it by GC after extracting it from beetles - the analytical detials escape me.

Peter

[Don_Hilton]

When I saw the compound I googled "cantharidine hydrolysis" and saw a number of papers, all of which indicated that cantharidine is pretty stable. On paper contrasted the rapid hydrolysis of norcantharidine with cantharidine.

I'd be willing to bet lunch that the dicarboxylic acid dehydrates to the anhydride in the GC inlet. If that is the case, to analyze for the dicarboxylic acid in the presence of the anhydride, you probably need to go by LC or a spectroscopic technique. If you are working with bug extracts - the spectroscopic techniques are not likely to be useful to you. If you are working with synthesis products, I'd go with NMR.

[MSCHemist]

or derivitize the dicarboxylic acids.