While looking for the NMR spectra for MDMA, I came across this interesting tidbit:

Quote

. dH (CDCl3): 6.74 (d, J ¼ 8.0 Hz, 1H), 6.69 (d, J ¼ 1.6 Hz,1H), 6.63 (dd, J ¼ 7.9 and 1.7 Hz, 1H), 5.92 (s, 2H), 2.73 (hextet, J ¼ 6.5 Hz, 1H),2.66–2.49 (m, 2H), 2.39 (s, 3H), 1.05 (d, J ¼ 6.2 Hz, 3H); dC (CDCl3): 147.76,146.05, 133.39, 122.27, 109.63, 108.27, 100.92, 56.57, 43.32, 34.17, 19.80. Solids formed after storage of this compound in a stoppered ?ask overnight, as the amine absorbs atmospheric CO2. Elemental analyses (C, H, N) were carried out, but results did not correspond to the free amine on this account.

I guess this is a pretty common phenomenon as there is a lot of literature on the subject.  The amine reacts with CO2 to form an ammonium carbamate (not carbonate) species.  Does anyone have any experience with this?  Is the process fully reversible upon addition of acid?

Heat. R2NCO2H => R2NH + CO2

Carbamic acids are derived from amines: R2NH + CO2 => R2NCO2H

Carbamic acid is about as acidic as acetic acid. Ionization of a proton gives the carbamate anion, the conjugate base of carbamic acid:

R2NCO2H => R2NCO2- + H+


Carbamate http://en.wikipedia.org/wiki/Carbamate

all that time i thought it was carbonic acid reacting with the free amine.
news to me.

If water + CO2 is present. When dissolved in water, carbon dioxide exists in equilibrium with carbonic acid: CO2 + H2O <=> H2CO3

The hydration equilibrium constant at 25 °C is called Kh, which in the case of carbonic acid is [H2CO3]/[CO2] = 1.70×10?3: hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO2 molecules.

Carbonic acid. http://en.wikipedia.org/wiki/Carbonic_acid

The freebase reacts with the carbonic acid, H2CO3, forcing the equilibrium to the right.

i remember seeing pictures of a person creating carbonate salts from mescaline by bubbling co2 into the organic solvent containing mescaline freebase.  the same person claimed that mescaline freebase was very reactive and that after they let mescaline freebase set out for awhile it reacted with co2 in the air and formed mescaline carbonate.

being a fairly similar molecule, at one point i was concerned if mdma would do the same thing... form mdma carbonate by letting the freebase sit out.  i've never seen mdma freebase form the carbonate salt by just sitting out but in those circumstances, mdma freebase never sat out for more than a few hours anyway.

that's really all the more i can add to the subject