[HP]

Hi,
Recently i red that the aminoacids dont give ammonium salts (RNHCH2COO:-NH4+ for example) by reacting with ammonia solutions and by boiling their hydrochloride salts with aq ammonia the corresponding free ammino acid and NH4Cl are formed...The same procedure with treating amino acid salts with aq NaOH give the corresponding amno acid Na- salt and i was wondering why not the same and with NH4OH as knowing that acetic acid react with NH4OH to ammonium acetate?
Hope someone have the answer nearby, ths

[kiwi]

have a think about what functional group is two bonds away from the carboxyl group

[HP]

I know this is primary amino and in strongly acidic media it is protonated as hydrocloride ammonium salt (RNH2+Cl-  CH2CH2COOH). Then, when adding NH4OH, NH4Cl is formed and at some pH where the amino acid isoelectric point a bethainic form of the amino acid is formed (cyclic RNH2+ CH2CH2COO:- ). Well in the ammonia solution there is may be enough of HO:- anions and NH4+ cations which make the solution strongly basic and i am still wondering why amino acid ammonium salt (COO:-NH4+) is not formed as similar the sodium salt is formed by the action of aq NaOH? Is this something with the nature of the NH4+ cation which is a weak acid... Please help me with comprehensive explanation if possible.

[kiwi]

you're on the right track, forming the NH4+ COO- salt does occur, but if you try to concentrate that you are going to promote NH4+ COO- -> COOH + NH3(g). once you've protonated the carboxylate, you will form the final zwitterionic form of the amino acid (and that amine isn't volatile, so the same situation doesn't occur). this isn't just a theoretical situation either, it finds great utility in preparative anion exchange chromatography, where you elute with ammonium bicarbonate (or triethylammonium bicarbonate), then freeze dry down your fractions. the salts break down under vacuum to the amine, water and carbon dioxide (all volatile under these conditions), leaving your (involatile) compound.

[HP]

Hi, and sorry for the late reply...
Well can i think that once the zwitterionic form of the amino acid is formed (at and above the isoelectric point) then ammonia cant further deprotonate it by ion exchange reaction with COO:NH4 salt and H2O formation (may be the zwitterionic carboxylate COO:- is much stable than COO:NH4 which is in equilibrium with NH3 gas). Let me formulate my question so: does in aqueous solution of ammonia and amino acid, amino acid ammonium salt exist or is formed and by heating and evaporation finally the zwitterionic formed stay nad gaseous NH3 live ? Can enybody give me explanation by mean of the two stabilitet constants of the zwitterionic and ammonium salt forms in comparison with Na- salt ?
Ths for discussion

[AWK]

Compare pK values for ammonia ans amino groups of aminoacids.
I think aspartic and glutamic acids form salts with one molecule of ammonia.

[HP]

Yah, aspartic and glutamic acid are examples for amino acids having two COOH groups - one of them in zwitterionic form with the amino group and the other free. So this free COOH can freely form the ammonium salt but again ammonia shouldnt disrupt the zw structure with the other...I myself still dont have the exact answer why

[AWK]

Amino groups of aminoacids are more basic than ammonia. You can obtain unstable salts of aminoacids with ammonia, but only with huge excess of ammonia, ie in liquid ammonia

[HP]

Cool, now i grasp it! It is really a very useful method for purification and isolation of amino acids i can share and alredy can explain true
Thanks