[AlphaScent]

Has anyone ever a carboxylic acid that when solvent removed due to impurities the solid formed is more of an amorphous solid.  Consistency of peanut butter.  I have tried a number of washings and solvents.  I am next going to try and recrystallize from 0.5 N HCl.  Anyone have this trouble before??

Compound is a straight chained, conjugated alkyne/alkene with 1-carboxylic and 11-hydroxy moiety.
No lactone formation. 

[discodermolide]

Try methyl ethyl ketone I would not use HCl due to the unsaturation.

[opsomath]

What are the impurities? Have you already passed it into weak base, extracted, then precipitated to remove neutral impurities?

Some things are too oily to recrystallize well and that's all there is to it. You may have to do a column or distillation/sublimation depending on the MP/BP.

[orgopete]

Recrystallization has been a potent purification technique, but one in which impurities can affect the result differently. I was separating isomers, but it was reported in the literature that a very small amount of one isomer would cause a large drop in the melting point of the other. I suspect that may be the case here. The alkene/alkyne structure seems ripe for possible isomer presence or air oxidation.

Since it is being reported the solid is amorphous, then I suggest that as the solution is becoming saturated, it should be filtered to remove any cloudiness (impurity less soluble than product) that may form. At this stage, you should use a filter aid for removal. If the solvent is non-polar, silica gel can absorb the more polar impurities to remove them. If a polar solvent is used, then charcoal has the opposite effect.

If crystal formation is occurring from a crystal clear solution, then amorphous solid is occurring because the product was not collected before the impurities began to precipitate. A magnifying glass is helpful to determine what is happening during the recrystallization.

A recrystallization is simply an experiment in differential solubilities. We could be more helpful if a more complete description of each result were provided. What was done and what was the result?

[408]

Also possible is making a salt of it and purifying the salt, then acidify and extract to get the carboxylic acid back.   Even convert it all into the crude sodium or potassium salt, dissolve in methanol and diffuse ether into it.  Generally this works to get perfectly pure crystals. 

[AlphaScent]

Hey guys,

Thanks for the ideas.

I worked like this:

1) Saponified the ester with a 10% KOH in MeOH.

2) Washed the basic solution with hexanes (3x)

3) Brought the solution to a neutral pH with 2 N HCl.

4) Extracted with cold ether (3x)

5) Then added 1x volume of ether and stirred, slowly adding 2 N HCl until a pH of 2 was reached.
      There was a THP group that needed to be cleaved so I did it here, this brought the acid/alcohol into the ether solution

6) Washed the organics with cold 5% NaHCO3, Brine, dried and filtered

7) removed solvent i.v. and allowed it to sit.  started to crystallize with in 5 min.  It was a nice, solid piece,  not a like peanut butter. 

recrystallized from 50/50 EtOAc.Hexanes to yield 100% pure material by NMR!!!


Thanks again guys, the brain storming helped me a lot

Cheers.  KCCO