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Author Topic: Monitoring amino acid esterification  (Read 344 times)

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vikram

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Monitoring amino acid esterification
« on: February 13, 2019, 07:18:58 PM »

I am trying to monitor the esterification of a non UV active amino acid.I used thionyl chloride and Methanol as reagents.

As amino group is unprotected, Ninhydrin should give positive result for both reactant and products. However, I believe there should be Rf difference between reactant and product.When I performed TLC and stained with ninhydrin, the reactant was strong brown and one spot at very high Rf value showed pale brown coloration.Now can I think that pale brown spot as my target ester?

I am doubtful because reactant and product both supposed to show strong coloration as amino group is not affected.On the other hand, will just methylating the acid result in high Rf change? Because I feel amino group is also polar, it would not allow spot to raise till high Rf end.
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phth

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Re: Monitoring amino acid esterification
« Reply #1 on: February 14, 2019, 01:57:34 AM »

one must use the correct solvent system there are lots of variations. A good review on some published ways: https://www.researchgate.net/publication/254338234_Thin-layer_chromatography_of_amino_acids_A_review
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Babcock_Hall

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Re: Monitoring amino acid esterification
« Reply #2 on: February 14, 2019, 02:50:52 AM »

It is unclear to me whether or not you ran a standard of the underivatized amino acid.  I would certainly expect a significant difference in Rf values assuming that you have a reasonable solvent system.
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kriggy

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Re: Monitoring amino acid esterification
« Reply #3 on: February 14, 2019, 03:14:44 AM »

Yeah just spot the starting material on the TLC plate so you know where it elutes. Im not sure what is your mobile phase but I would assume the ester having vastly different polarity from the parrent amino acid.

You can also use different stain, I use bromocresol green for that. When used, acidic compounds (I think pKa <5) show as yellow spot on blue background. Helped me a lot when doing troublesome amide formations.

Also, I have reasonable experience with cerium molybdate, which often gives different colors for each of my spots

wildfyr

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Re: Monitoring amino acid esterification
« Reply #4 on: February 14, 2019, 03:02:24 PM »

Phosphomolybdic is a good general purpose stain.
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Babcock_Hall

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Re: Monitoring amino acid esterification
« Reply #5 on: February 16, 2019, 04:57:03 AM »

Bromcresol green has worked well in our hands for free carboxylic acids.
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rolnor

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Re: Monitoring amino acid esterification
« Reply #6 on: February 17, 2019, 12:55:40 AM »

In my experience, the pale brown is your product ester, I have made this reaction many times.
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