We know that in amino acids, carboxylic acid groups deprotonate first than the amine groups. For example, in alanine, $\mathrm{p}K_\mathrm{a,\ce{COOH}} = 2.34$, $\mathrm{p}K_\mathrm{a,\ce{NH3+}} = 9.69$ according to this page.

But according to the structures of EDTA as searched from Google, and from this extract from p.325 of Analytical Chemistry, 8e by Christian, et al.:

At sufficiently low pH, the nitrogens can also be protonated and this diprotonated EDTA can be considered a hexaprotic acid. However, this occurs at a very low pH and EDTA is almost never used under such conditions.

the hydrogens of these amino groups seem to be deprotonated first before those of the four carboxylic acid groups. Why is this so?

  • 4
    $\begingroup$ Before you ask "why", ask "if". $\endgroup$ Nov 21 '17 at 5:14
  • $\begingroup$ How many covalent bonds can N form with other elements? $\endgroup$
    – wolphram
    Nov 21 '17 at 8:33
  • $\begingroup$ There used to be a place on the internet where you could enter compounds and things such as acidity constants were calculated, including which proton deprotonated in which order. Alas, I can’t find the site any more. $\endgroup$
    – Jan
    Nov 21 '17 at 14:31
  • $\begingroup$ What do you mean @IvanNeretin? It doesn't make sense to me since carboxylic acid groups are usually more acidic than amino groups in the examples I encounter, which is due to the electronegativity of the protic oxygen, resonance and inductive effects felt by the same oxygen atom. Then I see this hydrogen held by the amino groups getting deprotonated first before the other hydrogens held by the carboxylate groups, so I asked why. $\endgroup$
    – Acnologia
    Nov 24 '17 at 11:54
  • $\begingroup$ I mean you should not blindly trust some random images brought to you by Google image search. Trust your logic; it is sound. $\endgroup$ Nov 24 '17 at 12:02

The carboxylates deprotonate first followed by the amines. The pKa values for the carboxylates ar 0.0, 1.5, 2.0, and 2.66. The amines have pKa values of 6.16 and 10.24. Ref. Harris. Quantitative Chemical Analysis.

  • $\begingroup$ Yeah, that was wrong, I edited it now, but my question still stands. $\endgroup$
    – Acnologia
    Apr 20 '18 at 1:44

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