akcapr
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Bizzare observation about benzylamines- explanation?
Today I reran a standard sample of Benzylamine on a HPLC to use as comparison for analyzing a reaction. Anyway, the chromatogram show two peaks, with
areas roughly 50/50 ratio, separated by retention time of several minutes on a fast 6 minutes gradient. I thought it was some error, so i repeated it,
and saw that same thing. A coworker observed the same phenomenon on TLC for bromo benzylamine. Im curious as to what is happeneing, does anyone have
any thoughts? A pure compound surely should have one peak, and that is esssentially always the case- bizarre!
It is unlikely to be due to degraded reagent as its always worked fine for me, and also since the same thing happened with bromo substituted version.
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Dr.Bob
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We used to QC benzylamines, and they can degrade quite quickly when exposed to air, CO2 and water. They oxidize to imines, which can degrade to
aldehydes, then those can oxidize to acids. Also, they will react with CO2 to form carbonate salts. If the sample is normally kept sealed or is
normally made with dry solvent, and then it was either left open or a different batch of solvent was used, it could have contained some water or even
impurities. So the amine could degrade, your solvent could be contaminated, or maybe you are just seeing double...
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akcapr
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You never saw twin peaks on hplc for benzylamines? Because yeah the samples I prepped did not sit in air or anything, and 50% degradation for a
compound sealed in a bottle is very unlikely. Oh well
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AndersHoveland
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It almost seems that perhaps there might be a second tautomer, though I do not have any idea how this could be.
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Arrhenius
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| Quote: |
A pure compound surely should have one peak, and that is essentially always the case...
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This is not entirely true. In particular, basic compounds can give you two peaks by HPLC if the pH is not low enough. I can't explain to you
precisely why this is, but I've observed it many times with basic compounds when using formic acid as a mobile phase modifier.
| Quote: |
It almost seems that perhaps there might be a second tautomer...
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I would advise you hit the books a little harder. 
[Edited on 5-1-2012 by Arrhenius]
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Dr.Bob
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Quote: Originally posted by Arrhenius  | This is not entirely true. In particular, basic compounds can give you two peaks by HPLC if the pH is not low enough. I can't explain to you
precisely why this is, but I've observed it many times with basic compounds when using formic acid as a mobile phase modifier. | Quote: |
If you have a pH which is not acidic enough (when running HPLC of amines) then you will get an equilibrium of the amine freebase and the amine salt
forming. This can lead to two peaks or just one broad peak, depending on the kinetics and pH. But if the HPLC is set up like most I have seen, it
will have TFA or Formic acid as the modifier, which should provide a good pH. If that is not done, many compounds will give poor peak shape, not
just benzylamine.
My QC work was done mostly with a GC-MS, as benzylamines are hard to QC with an HPLC, due to low UV absorbance, and also poor ionization in MS, due to
low MW, unless you have a MS tuned for low MW, which we did not. But some benzylamines would turn to crap (more than 2 peaks) within a few weeks,
even in a sealed botle, if ever opened enough to get any air in them.
We tested this numerous times to confirm, which helped us to understand why reactions done with "pure" amines where giving lower yields and purity
over time. "Pure" is one of the most overused terms in chemisty, given that any analytical method can only determine within it limitations, and we
found many chemicals that had impurities that only showed up in certain methods, but looked good by others. Just look at the melamine in milk and
adulterated heparin for other examples of an analytical method being "fooled" by certain impurities. |
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