Week 13: Ugi Synthesis Collaborative Project

Crystallization:
This week we harvested crystals from those reactions that gave a precipitate and attempted to obtain a solid from those reactions that had not spontaneously formed a precipitate. The two reaction mixtures with 3,4,5-trihydroxybenzoic acid tended to form precipitates the most readily, but not all of them did. The two reaction mixtures with 2,4,6-trihydroxybenzoic acid precipitated very slowly with mostly brown solid. A couple of them had what looked like transparent crystals covered in brown syrup. Several of the vials were dried down with forced air to encourage crystallization. In some a solid was formed, in others a brown syrup resulted. We tried to find a solvent that would dissolve the brown impurities and not the Ugi product or dissolve the Ugi product and not the impurities. This game plan did not work so well. The best idea seemed to be to carefully clean the crystals with methanol in a vacuum filtration funnel. I suppose the best thing to do would be to those that didn’t form a precipitate another week, but that didn’t seem possible, since it’s the end of the semester and all. Is the Ugi reaction slow in forming products or are the products slow in precipitating from solution?

Analysis:
All of the Ugi reactants gave a GC trace. GC-FID analysis of the filtrate gave some non-reactant peaks. However, the Ugi product collected by filtration did not appear to “fly” in the GC. We did IR KBr windows with a few of the Ugi products. We have and archaic system of making the salt windows with a bolt and two screws so the quality of the windows and subsequent IRs is not too good. We do see carbonyl peak and a phenolic hydroxyl peak. The UV spectra of the products are generally useful in distinguishing between different products and seem somewhat different than the reagent UV spectra..

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One Comment

  1. Posted April 23, 2008 at 1:36 pm | Permalink

    We now have some pretty convincing evidence for what is happening with the 2,4,6-trihydroxybenzoic acid run – it is not a Ugi product but is still interesting. I am concerned that your students did not get a white precipitate for this one.
    It is important that methanol is not allowed to evaporate because starting materials will co-precipitate. This may be why you’re getting a brown goo instead of a white crystal.
    Have your students kept an online log and lab notebook so that we can investigate more closely the differences with my group’s results?


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