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DCM+Methanol+10% Pd on Carbon generates HCl?

  1. Jan 12, 2009 #1
    I've been trying to remove a benzyl protecting group of a phenol and also remove a CBZ protecting group on a secondary amine by standard hydrogenation of the material with 10% Pd on Carbon. My protected molecule is very lipophilic and soluble in DCM. The reason I added methanol was because the intermediates generated (the intermediate where the benzyl is removed but the CBZ still present and vice versa) are generally not soluble in DCM, but soluble in methanol. Deprotection of the two groups worked fine, but to my surprise, after working up the reaction, my material is a salt. I definitely know it is a salt because of the broadening of the NMR signals. I'm just not sure where the acid is coming from or even what acid it is.

    I know a good way to dehalogenate material is by hydrogenation with Pd on carbon. Am I dehalogenating the DCM ,and with the presence of methanol as well, generating HCl which is reacting with the amine moities to generate a HCl salt?
  2. jcsd
  3. Jan 13, 2009 #2


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    So you simultaneously deprotected a phenol and an amine, eh?

    Do you see it yet? (you are going to kick yourself)

    BTW, I'm sure that you know that common impurities in methanol include formic acid and that DCM in contact with oxygen and light produces HCL and phosgene provided there is no inhibitor, like methanol, in the DCM reagent. These acids are usually present in only trace amounts.
    Last edited: Jan 13, 2009
  4. Jan 13, 2009 #3
    Yup. The phenol's acidity has never given me a problem with similar analogs before.

    Yup, via NMR, LC/MS, and TLC. 99% pure by HPLC and the mass spec is spot on. You only know that it is a salt because it is obvious in the NMR.

    I don't think that is the problem because there are 1-2 equivalents of acid that are forming salts with my compound. It would take much more acid than trace amounts to form a salt with my compound. The only other thing that I can think of is that the CO2 that is produced from the CBZ deprotection is forming carbonic acid and the molecule is salting with that.

    Now that I think about it, I'm pretty certain that it can't be an HCl salt. Most compounds that have 2 HCl equivalents are incredibly hydroscopic. The salt of my compound doesn't seem to soak up a lot of water from the atmosphere at all if you leave it out.
  5. Jan 14, 2009 #4


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    pKa of phenol is 9.6. pKa of the acid form of methyl amine (ammonium) is 10.6. Secondary amines are stronger bases than primary ones.
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