[Durlag]

Hello,

At the lab I'm evaporating, to dryness, a solution of 0,1N HCl containing various amino acids (plus NH₄Cl). I then re-dissolve in DMF for a SN2 reaction.
 
I have a few questions regarding this.

It was my understanding that all the amines in there would form ionic bonds with the Cl- upon evaporation. So in my dry residue I would have: amino acid hydrochlorides and NH₄Cl.

But then, if I add DMF, do the salts dissolve? Are the ion - dipole attractions between DMF and Cl- / DMF and protonated amines strong enough to break the ionic bonds?
I've read sources that say "ionic solids don't dissolve in organic solvents" and others that say "DMF dissolves many salts" so I don't know what to think.

I then heat my DMF solution, would that cause dissolution of the salts, if it haden't happen before?

In practice, when evaporating to dryness, I observe salts. But there is no residue upon addition of DMF. They seem to dissolve readily. But maybe it's just a suspension of tiny particles?

P.S.: Various questions:
I'm curious to what happens to the carboxyle groups during evaporation. In what form will they be when the is only the dry residue? Can they bind to the ammonium cations?
If they are in the form COO- (I don't even know of this is possible!), that means they can form ion - dipole bonds with the DMF, so the amino acids will still be soluble. Right?
Could they reprotonate in the end of the evaporation? By the way, what happens to the pH of the solution as you approach dryness?
If Cl- ions are bound to protonated amines, does that mean you evaporate less HCl molecules than you had in solution in the beginning?

Thanks!

[Babcock_Hall]

At 0.1 N HCl, I would expect that the carboxylate groups of amino acids (most of which have a pK_a1 value of 2 or slightly higher would protonate.  What is your putative nucleophile?

[Durlag]

Thanks for your reply.

I think so as well.

I'm doing a silylation, which is analogous to SN2. The nucleophiles are the carboxyl and amine groups on the amino acids. More details (from Moldoveanu and David, 2002):


My biggest interrogation is: in what states will be these groups when all of the HCl is evaporated?
Most carboxyl groups will be protonated. What about those that aren't, if there are any (e.g. histidine's carboxyl group has a pKa of 1,7)? Will they bind to NH₄⁺? (By the way, will NH4⁺ "prefer" binding to Cl^- or COO^-)?
Amines will be protonated, and will bind to Cl^-, forming salts. Will the DMF then break this attraction?

Even if it does, protonated amines are not supposed to be nucleophiles.

But I  do observe my amino acids by GC-MS, and the MS spectra correspond to amino acids that have their carboxyl AND amine groups silylated. So, if evaporation does yield hydrochloride salts (protonated amines plus Cl^-), silylation still seems possible. Despite the fact that DMF could not be able to dissolve the salts (as in break the ionic bond), and that even if it does, protonated amines are not supposed to be nucleophiles.

Also, a vocabulary issue, can the DMF "dissolve" the salts, as in, put them in solution (no solid salts anymore), without breaking the ionic bond between protonated amines and Cl^-? Would that be called dissolution? I guess that could happen thanks to the carboxyl groups. Both unprotonated carboxylate groups and protonated carboxylic acid groups would be soluble (ion - dipole bond and dipole - dipole bond, respectively).

I'm considering that the amine and carboxyl groups don't show acid - base behaviour in DMF? Since it's an aprotic solvent. Which means, when you add DMF, they will stay in whichever state they were in the dry residue. Is this right?

[Babcock_Hall]

I am going to confine myself to some very general comments for now.  The first comment is that when one has a salt in an organic solution (assuming that the salt is soluble), the salt may form an ion pair, in which the two ions are still in close contact, which is not how we usually think of ions in water solution.  The second comment is that one should always follow a published protocol when doing these sorts of reactions, if one is available.  The third comment is that ammonium (protonated amine) groups are certainly not nucleophilic.

[Durlag]

1) So the salt may, or may not, dissolve, and it may, or may not, dissociate. My observation is that when I add the DMF to the vial, the salts do dissolve. But that doesn't tell me if they dissociate.

2) I did follow a published protocol, except that I should have added a desalting step (on ion exchange column) prior to the pre-column derivatization.

3) Ok. But then, how can they be silylated in DMF? Could they deprotonate in DMF? It was my understanding that this wouldn't happen in DMF.

:-)

My ultimate hypothesis was that Cl^-, being a nucleophile, interferes with the silylation and reduces my yield of silylated amino acids. But if the hydrochloride salts don't dissociate, then I guess another mechanism is responsible for my lower yield.