 Malonic acid

Malonic acid is propanedioic acid, a dicarboxylic acid in which two carboxyl groups are separated by one methylene group:Its neutral molecular formula is ${\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_4$ and its average molar mass is approximately $104.061\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.^[ref]

Two of the four hydrogen atoms belong to carboxyl OH groups; the other two belong to the central carbon. Complete neutralization of both carboxyl groups therefore requires two equivalents of a suitable base, not four merely because the formula contains four hydrogens. An original stoichiometric model isFor $1.00\mathrm{mmol}$ acid, the two-equivalent amount of $0.100\mathrm{mol}{\mathrm{L}}^{-1}$ hydroxide solution is $20.0\mathrm{mL}$. The pH at an experimental endpoint also depends on acid-base equilibria and the measurement method; the equivalent count alone does not determine it. Malonate names the corresponding deprotonated system, whose protonation state should be specified.

Heating can promote decarboxylation, producing acetic acid and carbon dioxide. Malonic-acid derivatives underlie the carbon-chain construction described in the malonic ester synthesis.^[ref] For the unsubstituted acid, the stoichiometric balance isAn original ideal mass balance for $1.00\mathrm{mmol}$ converted by only this reaction gives about $60.052\mathrm{mg}$ acetic acid and $44.009\mathrm{mg}$ carbon dioxide from $104.061\mathrm{mg}$ starting material. Following evolved gas and residual material would test conversion, whereas a change in sample mass alone would not distinguish this reaction from evaporation or another decomposition pathway.