Malonic acid

Malonic acid is a three-carbon diacid, so it can undergo two deprotonations, which never take place at the same time:

Successive deprotonations of malonic acid: the monoanion, stabilized by the neighbouring acid group (Ka = 1.5·10⁻³), and the dianion, destabilized by the neighbouring carboxylate (Ka = 2.0·10⁻⁶)

The first deprotonation is the easier one (pKa = 2.83), because the resulting monocarboxylate ion has an electronegative acid group next to it. The second one, in contrast, meets a molecule with a high electron density and is much more difficult (pKa = 5.69): almost a thousand times less favourable.

A carboxylate ion is more stable next to a carboxylic acid group than next to another carboxylate anion.

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